Gastrointestinal

Immune checkpoint inhibitor colitis: diagnosis and management

Clinical Overview and When to Suspect ICI Colitis

— Any-grade diarrhea: ~30% with CTLA-4, ~15% with PD-1/PD-L1, up to 45% with combination ipilimumab + nivolumab

— Clinically significant colitis: ~10% CTLA-4, ~1–3% PD-1, ~13% combination

— Median onset: 5–10 weeks after initiation, but can occur after a single dose or months after discontinuation (delayed irAE)

— New diarrhea (≥4 stools/day over baseline), urgency, tenesmus, hematochezia, mucus in stool

— Crampy abdominal pain, fever, fatigue, weight loss

— Even mild symptoms warrant evaluation — colitis can progress to perforation rapidly

— Combination therapy (anti-CTLA-4 + anti-PD-1) > CTLA-4 monotherapy > PD-1/PD-L1 monotherapy

— Pre-existing IBD, NSAID use, prior irAE, gut microbiome composition

— Concurrent infection (especially C. difficile, CMV)

Definition: Immune-related adverse event (irAE) involving the colon, triggered by immune checkpoint inhibitors (ICIs) that block CTLA-4 (ipilimumab), PD-1 (nivolumab, pembrolizumab), or PD-L1 (atezolizumab, durvalumab, avelumab) in patients receiving treatment for melanoma, NSCLC, RCC, urothelial, HCC, MSI-high tumors, and others.

Incidence and timing:

When to suspect in any ICI-exposed patient:

Risk factors:

Step 3 management: Always view new GI symptoms in an ICI-treated patient as ICI colitis until proven otherwise, but rule out infection in parallel — do not start empiric steroids before stool studies and at least preliminary endoscopic assessment in moderate–severe cases.

Board pearl: ICI colitis predominantly affects the left colon and rectum (unlike typical CMV, which is right-sided), and CTLA-4 agents produce more severe, diffuse colitis with crypt abscesses resembling acute UC, while PD-1 agents more often cause patchy or microscopic colitis. The phrase "checkpoint inhibitor + diarrhea" on a vignette is essentially diagnostic of suspected irAE colitis.

Presentation Patterns and Key History

— Grade 1: <4 stools/day above baseline, asymptomatic otherwise

— Grade 2: 4–6 stools/day above baseline, abdominal pain, mucus or blood

— Grade 3: ≥7 stools/day above baseline, incontinence, severe pain, limiting self-care

— Grade 4: Life-threatening — perforation, peritonitis, hemodynamic instability

— Median 6 weeks after first dose with ipilimumab; later with PD-1 agents (often after dose 2–4)

— Can present weeks to months after the last dose — patients on "treatment break" or post-discontinuation still at risk

— Hyperacute onset (<1 week) is unusual — consider alternative diagnoses

— Dermatitis (rash, pruritus) — most common irAE overall

— Hepatitis (transaminitis), hypophysitis, thyroiditis, pneumonitis

— Concurrent enteritis (small-bowel involvement) — predominant nausea/vomiting, less diarrhea, may be missed

— Exact ICI agent(s), number of cycles, date of last infusion

— Baseline stool pattern (essential for grading)

— Blood, mucus, nocturnal stools, urgency, tenesmus → suggest colonic inflammation

— Recent antibiotics, hospitalization, healthcare exposure → C. difficile risk

— Travel, sick contacts, raw foods → infectious differential

— NSAID use (can worsen mucosal injury)

— Prior IBD, celiac, microscopic colitis → predisposes to ICI colitis flare

— Fever, severe abdominal pain, distention, hematochezia, hypotension, tachycardia

— Inability to tolerate PO, signs of dehydration

Symptom spectrum (CTCAE grading drives management):

Onset clues:

Associated irAEs to ask about (clustering is common):

Critical history elements:

Red flags requiring urgent evaluation:

Key distinction: Diarrhea alone (loose stools without inflammatory features, normal labs, normal exam) may represent non-colitis ICI-induced diarrhea — typically self-limited, managed symptomatically. True colitis requires inflammatory features (blood, mucus, pain, elevated inflammatory markers, or endoscopic findings) and triggers immunosuppressive therapy.

Physical Exam Findings and Hemodynamic Assessment

— Tachycardia, hypotension, orthostasis → volume depletion or sepsis/perforation

— Fever >38.5°C → consider superimposed infection or severe inflammation

— Tachypnea + abdominal pain → possible perforation, peritonitis

— Cachexia, pallor (anemia from GI losses), dry mucous membranes

— Ill-appearing or toxic = grade ≥3 until proven otherwise

— Diffuse or left-sided tenderness most common (sigmoid/descending colon distribution)

— Rebound, guarding, rigidity → perforation — surgical emergency, stop diagnostics, get CT and surgery consult

— Distention with absent bowel sounds → toxic megacolon (rare but lethal)

— Hyperactive bowel sounds in mild cases

— Gross blood, mucus, tenderness on DRE supports colitis

— Perianal disease (fistula, abscess) is not typical of ICI colitis — consider Crohn's flare or alternative

— Maculopapular rash, vitiligo, lichenoid eruptions, oral ulcers

— Jaundice → concurrent ICI hepatitis

— Thyroid exam (irAE thyroiditis often coexists), pulmonary exam (pneumonitis), neuro exam (rare myasthenic/encephalitic irAEs)

— Mucous membranes, skin turgor, JVP, capillary refill

— Urine output (target ≥0.5 mL/kg/hr)

— Orthostatic vitals if ambulatory

Vital signs — assess severity first:

General appearance:

Abdominal exam:

Rectal exam:

Skin and mucous membranes (look for clustered irAEs):

Other systems:

Volume status assessment:

CCS pearl: In a CCS-style case, the immediate orders for a patient with suspected ICI colitis presenting to the ED should include: vital signs q4h, strict I/Os, IV access ×2, NPO if severe, IV fluids (NS or LR), CBC, CMP, lactate, CRP, stool studies, abdominal exam q-shift, and GI + oncology consults. Document baseline performance status — this guides whether infliximab or vedolizumab will be tolerated downstream and whether ICI can ever be safely resumed.

Diagnostic Workup — Initial Labs, Imaging, and Stool Studies

— CBC with differential: anemia (chronic GI loss), leukocytosis (infection vs inflammation), eosinophilia (other irAEs)

— CMP: electrolyte derangements (hypokalemia, hypomagnesemia, non-anion gap metabolic acidosis from diarrhea), AKI, low albumin (protein-losing enteropathy or chronic illness)

— LFTs: screen for concurrent ICI hepatitis (AST/ALT elevation)

— CRP and ESR: elevated in moderate–severe colitis; useful for monitoring response

— TSH: screen for concurrent thyroiditis

— Lactate: if severe pain or hemodynamic instability — evaluate for ischemia/perforation

— C. difficile PCR or GDH/toxin EIA — must rule out before steroids

— Stool culture (Salmonella, Shigella, Campylobacter, E. coli O157)

— Ova and parasites, Giardia antigen, Cryptosporidium (especially immunocompromised)

— Fecal calprotectin or lactoferrin — surrogate marker; correlates with endoscopic activity, useful for monitoring

— Stool fecal leukocytes (less sensitive, often skipped)

— CT abdomen/pelvis with IV contrast in moderate–severe disease or any concern for complication

— Findings: bowel wall thickening, mesenteric stranding, mucosal hyperenhancement, pericolonic fluid

— Pancolitis pattern more typical than segmental

— Identifies perforation (free air), abscess, toxic megacolon, ischemia

— Plain abdominal films if perforation/obstruction suspected and CT delayed

— Avoid barium studies (risk if perforation)

Initial labs (all suspected cases):

Stool studies — mandatory before immunosuppression:

Imaging:

Board pearl: Always send C. difficile testing before starting steroids — superimposed C. diff is common (recent antibiotics, hospitalization, prior steroid exposure), and missing it leads to clinical worsening despite immunosuppression. CMV testing (serum PCR, biopsy IHC) is required before starting infliximab or escalating immunosuppression, especially in steroid-refractory disease.

Diagnostic Workup — Endoscopy and Histology

— Indicated in grade ≥2 diarrhea/colitis or any case with blood/mucus, severe symptoms, or to confirm diagnosis before prolonged immunosuppression

— Flexible sigmoidoscopy is preferred initially — faster, safer, no bowel prep needed in severe cases, and captures most disease (predominantly left-sided)

— Full colonoscopy if sigmoidoscopy is unrevealing but suspicion remains, or to assess extent before biologics

— Take biopsies even from normal-appearing mucosa — microscopic colitis pattern can occur without visible inflammation

— Erythema, loss of vascular pattern, edema, friability, erosions, ulcerations

— Mayo endoscopic subscore used to grade severity (0–3)

— Deep ulcerations (>1 cm, or >2 mm depth) predict steroid-refractory disease → consider early infliximab

— Pseudomembranes → think C. difficile coinfection

— Linear ulcers or "owl's eye" inclusions → CMV

— Acute colitis pattern: neutrophilic infiltrate, cryptitis, crypt abscesses, apoptotic bodies (especially in crypt epithelium — a hallmark)

— Lymphocytic or collagenous pattern: intraepithelial lymphocytes, thickened subepithelial collagen band (especially with PD-1 agents)

— Granulomas are not typical (suggest Crohn's, TB, sarcoid)

— CMV IHC on biopsies mandatory in steroid-refractory cases

— Quantiferon-TB and hepatitis B serologies (HBsAg, anti-HBc, anti-HBs) before infliximab

— Pregnancy test in women of reproductive age before infliximab/vedolizumab

— HIV testing if not recent

Endoscopy — the diagnostic gold standard:

Endoscopic findings:

Histology:

Additional testing in selected patients:

Key distinction: ICI colitis histology can mimic both UC (crypt abscesses, basal plasmacytosis) and acute self-limited colitis, but the presence of prominent apoptotic bodies in crypt epithelium is highly suggestive of ICI etiology and helps distinguish from typical IBD on biopsy.

Risk Stratification and Management Logic by Grade

— Continue ICI with close monitoring

— Supportive: oral hydration, electrolyte repletion, loperamide acceptable only after infection excluded (controversial; avoid if any inflammatory features)

— BRAT diet, avoid lactose, NSAIDs, caffeine

— Reassess in 48–72 hours; escalate workup if no improvement or worsening

— Hold ICI

— Stool studies, labs, consider endoscopy

— Prednisone 1 mg/kg/day PO (or methylprednisolone IV equivalent if not tolerating PO)

— If no improvement in 48–72 hours, escalate to grade 3 management

— Once improved to ≤grade 1, taper steroids over 4–6 weeks minimum

— Permanently discontinue CTLA-4; PD-1/PD-L1 may be considered for resumption after full recovery in select cases

— Hospitalize

— Methylprednisolone 1–2 mg/kg/day IV

— Endoscopy with biopsy within 48 hours

— If no response in 3 days, add infliximab 5–10 mg/kg or vedolizumab 300 mg IV

— Permanently discontinue ICI

— ICU admission, surgical consult immediately

— IV methylprednisolone 2 mg/kg/day + early biologic

— Broad-spectrum antibiotics if concern for perforation/sepsis

— CTLA-4 agent, combination therapy, deep ulcerations on endoscopy, elevated CRP/calprotectin, pancolitis

Grade 1 (mild): <4 stools/day above baseline, no systemic symptoms

Grade 2 (moderate): 4–6 stools/day, abdominal pain, blood/mucus, limiting instrumental ADLs

Grade 3 (severe): ≥7 stools/day, incontinence, severe pain, limiting self-care

Grade 4 (life-threatening): perforation, peritonitis, ileus, shock

Predictors of steroid-refractory disease:

Step 3 management: The 72-hour rule is the most testable management principle — if a patient on appropriate steroids fails to improve within 3 days, escalate to infliximab or vedolizumab immediately rather than continuing to wait. Delayed escalation is associated with worse outcomes including colectomy and death.

Pharmacotherapy — Corticosteroids and Biologics

— Prednisone 1 mg/kg/day PO for grade 2 outpatient management

— Methylprednisolone 1–2 mg/kg/day IV for grade 3–4 or PO intolerance

— Expect improvement within 48–72 hours; if absent, escalate

— Taper slowly over ≥4–6 weeks (rapid taper → relapse in up to 30%)

— E.g., 60 mg → 40 → 30 → 20 → 10 → 5 → off, decreasing every 5–7 days once at grade ≤1

— Add PJP prophylaxis (TMP-SMX) if steroids ≥20 mg prednisone equivalent for ≥4 weeks

— Add calcium + vitamin D, consider bisphosphonate if prolonged course

— PPI for GI prophylaxis if concurrent NSAIDs or high-dose steroids

— Monitor glucose (steroid-induced hyperglycemia)

— 5 mg/kg IV, may repeat at 2 weeks if needed; some protocols use 10 mg/kg

— Most patients respond within 1–2 doses

— Pre-treatment: TB screening (Quantiferon), HBV serologies, CHF assessment (contraindicated in NYHA III–IV)

— Avoid if perforation, abscess, active sepsis

— 300 mg IV at weeks 0, 2, 6

— Preferred over infliximab when: cardiac contraindications to TNF inhibitor, active or recent infection concern, prior infliximab failure (alternative), or to preserve anti-tumor immunity (gut-selective → less systemic immunosuppression)

— Slower onset than infliximab (response over 1–2 weeks)

— Ustekinumab (anti-IL-12/23), tofacitinib (JAK inhibitor), fecal microbiota transplant (investigational), mycophenolate

— IV fluids, electrolyte repletion (K, Mg, phosphate)

— Avoid antimotility agents (loperamide, diphenoxylate) in moderate–severe disease — risk of toxic megacolon

— Nutrition support; consider TPN if prolonged NPO

Corticosteroids (first-line for grade ≥2):

Infliximab (second-line, steroid-refractory):

Vedolizumab (gut-selective α4β7 integrin inhibitor):

Third-line options (refractory to steroids + first biologic):

Adjuncts:

Board pearl: Vedolizumab is increasingly favored as first biologic in many centers because its gut-selective mechanism minimizes systemic immunosuppression and may better preserve anti-tumor ICI efficacy — a critical consideration in active cancer patients.

Procedural and Surgical Management

— Flexible sigmoidoscopy preferred initially (safer, no full prep)

— Identifies deep ulcerations (>1 cm) → predicts steroid failure → consider early infliximab rather than waiting 72 hours

— Biopsy mandatory even if mucosa appears normal (rule out microscopic colitis, CMV)

— Avoid full colonoscopy in severe colitis (perforation risk); defer until improvement

— To assess healing before resuming ICI

— To evaluate steroid-refractory disease for CMV or alternative diagnosis

— Persistent endoscopic activity despite symptom resolution predicts relapse

— Perforation with peritonitis

— Toxic megacolon (colonic diameter >6 cm + systemic toxicity) refractory to medical therapy within 24–48 hours

— Uncontrolled hemorrhage

— Fulminant colitis unresponsive to maximal medical therapy

— Procedure: subtotal colectomy with end ileostomy — definitive but morbid; mortality elevated in cancer patients on chemo/immunotherapy

— Quantiferon-TB (treat latent TB before infliximab)

— HBsAg, anti-HBc, anti-HBs (HBV reactivation risk with TNF inhibitor)

— HIV (if status unknown)

— CMV PCR (serum) + biopsy IHC in refractory cases

— Pregnancy test

— Echocardiogram if CHF history (TNF inhibitors contraindicated in NYHA III–IV)

— Vaccination status review — avoid live vaccines during immunosuppression

— Central venous access if prolonged TPN or vesicant infusions

— DVT prophylaxis with LMWH (hospitalized, hypercoagulable cancer patients) — caution if active hematochezia

Endoscopy is both diagnostic and risk-stratifying:

Repeat endoscopy may be needed:

Surgical indications (rare but critical):

Pre-biologic workup checklist:

Supportive care procedures:

CCS pearl: In a CCS case where a patient on day 4 of IV methylprednisolone for grade 3 ICI colitis remains symptomatic, the correct next orders are: CMV serum PCR, repeat sigmoidoscopy with biopsy + CMV IHC, infliximab 5 mg/kg IV, surgical consult for awareness, and continue methylprednisolone. Do not taper steroids until clinical and endoscopic improvement.

Special Populations — Elderly and Renal/Hepatic Impairment

— Higher baseline frailty, sarcopenia, and polypharmacy complicate management

— Greater risk of steroid toxicity: hyperglycemia, delirium, osteoporotic fractures, infection, hypertension exacerbation

— Use lowest effective dose; consider closer monitoring of glucose, BP, mental status

— Bone protection: calcium 1200 mg/day, vitamin D 800–1000 IU/day, bisphosphonate if T-score ≤−1.5 and prolonged steroids

— Infection risk amplified — lower threshold for PJP prophylaxis, herpes zoster vaccination (recombinant, non-live) before ICI when possible

— Drug interactions: warfarin (steroids increase INR effect), diabetes meds, antihypertensives

— Functional status drives whether ICI can be resumed — consider goals-of-care discussion early

— Diarrhea-induced volume depletion frequently causes prerenal AKI — aggressive but careful IV fluid resuscitation

— Concurrent ICI-induced acute interstitial nephritis (irAE) can coexist — check UA for WBC casts, sterile pyuria, eosinophiluria

— Steroids do not require renal dose adjustment

— Infliximab and vedolizumab: no renal dose adjustment

— Avoid nephrotoxins: NSAIDs, IV contrast if avoidable, aminoglycosides

— Electrolyte management: hypokalemia, hypomagnesemia from diarrhea common

— Screen for concurrent ICI hepatitis — check AST/ALT/bilirubin at baseline and during treatment

— If hepatitis coexists, steroids still first-line; infliximab contraindicated in significant hepatic dysfunction (hepatotoxicity risk) — use vedolizumab or mycophenolate instead

— Reactivation of HBV with TNF inhibitors — antiviral prophylaxis (entecavir or tenofovir) if HBsAg+ or anti-HBc+ before infliximab

— Albumin replacement rarely indicated unless severe protein-losing enteropathy

Elderly patients (≥65 years):

Renal impairment (CKD/AKI):

Hepatic impairment:

Board pearl: When ICI colitis and ICI hepatitis coexist, avoid infliximab (potential hepatotoxicity); use vedolizumab or mycophenolate mofetil instead. This is a high-yield Step 3 distinction often tested in multi-irAE vignettes.

Special Populations — Pregnancy, Pediatrics, and Cancer-Specific Subgroups

— ICIs are pregnancy category D — cross placenta (especially after 2nd trimester), risk of immune-mediated fetal effects, miscarriage

— Pregnancy generally a relative contraindication to ICI; contraception counseling mandatory for reproductive-age patients

— If colitis develops during pregnancy: corticosteroids preferred (prednisone preferred over dexamethasone — less placental transfer)

— Infliximab and vedolizumab are pregnancy-compatible (used in IBD); minimize dosing in 3rd trimester to limit transplacental transfer; avoid live vaccines in infant for 6 months after in utero exposure

— Avoid methotrexate, mycophenolate (teratogenic)

— ICI use in pediatrics increasing (melanoma, Hodgkin lymphoma, MSI-high tumors)

— Management principles similar; weight-based dosing for steroids (1–2 mg/kg/day prednisone) and biologics

— Growth monitoring critical with prolonged steroids

— Increased risk of IBD flare and ICI colitis (up to 40%)

— Not absolute contraindication, but requires gastroenterology co-management

— Optimize IBD control before ICI initiation; consider vedolizumab as baseline therapy

— ICI risks both graft rejection and irAEs; high-risk decision requiring multidisciplinary review

— ICIs can precipitate severe GVHD post-transplant — extreme caution

— ICIs generally safe in well-controlled HIV; maintain ART; monitor CD4

— Higher flare risk; baseline disease activity should be controlled; multidisciplinary decision

Pregnancy:

Pediatrics:

Patients with pre-existing IBD:

Solid organ transplant recipients:

Hematologic malignancy and allogeneic HSCT recipients:

HIV patients:

Autoimmune disease patients:

Key distinction: In a pregnant patient with grade 3 ICI colitis, prednisone + vedolizumab is the preferred regimen — vedolizumab's gut-selective mechanism and pregnancy compatibility make it the biologic of choice over infliximab when both options are clinically equivalent for severity.

Complications and Adverse Outcomes

— Bowel perforation (1–5% of severe cases) — surgical emergency; mortality 20–40% in cancer patients

— Toxic megacolon — colonic diameter >6 cm + systemic toxicity

— Massive GI hemorrhage from deep ulcerations

— Bowel obstruction from stricture (rare, late)

— Severe dehydration, AKI, electrolyte abnormalities (hypokalemia → arrhythmia, hypomagnesemia, metabolic acidosis)

— Protein-losing enteropathy with hypoalbuminemia, edema

— Sepsis from translocation or perforation

— Steroid toxicity: hyperglycemia/new diabetes, hypertension, osteoporosis, avascular necrosis, mood/psychosis, insomnia, weight gain, cushingoid features, adrenal suppression (taper carefully)

— Opportunistic infections: PJP, CMV reactivation, HSV/VZV, candidiasis, aspergillosis, strongyloides hyperinfection (screen endemic areas)

— Infliximab: infusion reactions, TB reactivation, HBV reactivation, demyelination, lupus-like syndrome, hepatotoxicity, CHF exacerbation

— Vedolizumab: generally well-tolerated; rare PML (much lower than natalizumab), nasopharyngitis, headache

— Chronic ICI colitis — persistent symptoms despite treatment in 5–10%

— Microscopic colitis pattern may persist after symptom resolution

— Recurrence with ICI rechallenge: ~30% recurrence rate; higher with CTLA-4

— Permanent ICI discontinuation may compromise cancer control — but emerging data suggest irAEs (including colitis) may correlate with better tumor response

— Steroids and immunosuppression theoretically may reduce ICI efficacy — data mixed; gut-selective vedolizumab favored to preserve anti-tumor immunity

— Overall ICI colitis mortality ~1–5%, higher in delayed-recognition or steroid-refractory cases

Direct complications of colitis:

Treatment-related complications:

Long-term sequelae:

Oncologic implications:

Mortality:

Board pearl: CMV reactivation in steroid-treated ICI colitis is a critical missed diagnosis on board questions — any patient failing to respond to steroids should have serum CMV PCR and biopsy CMV IHC before assuming steroid-refractory disease and adding more immunosuppression.

When to Escalate Care — ICU, Consults, and Inpatient Triage

— Grade 1 colitis with reliable patient, no red flags, ability to follow up in 48–72 hours

— Daily symptom diary, hydration, electrolyte monitoring

— Telephone or telehealth check-ins

— Grade 2 colitis with poor PO intake, dehydration, electrolyte derangements, comorbidities

— Grade 3 colitis (essentially always admit)

— Failure of outpatient steroid trial after 48–72 hours

— Inability to tolerate oral steroids

— Social factors (unreliable follow-up, distance from care)

— Grade 4 colitis (perforation, peritonitis, shock)

— Hemodynamic instability requiring vasopressors

— Severe acidosis, AKI requiring dialysis

— Massive GI hemorrhage

— Toxic megacolon

— Respiratory failure (concurrent ICI pneumonitis)

— Gastroenterology — for all grade ≥2 (endoscopy, biopsy interpretation, biologic initiation)

— Oncology — coordinate ICI discontinuation/resumption, balance anti-tumor and irAE management

— Colorectal surgery — early notification for grade 3–4 (perforation, toxic megacolon awareness)

— Infectious disease — if CMV, opportunistic infections, or refractory to standard immunosuppression

— Endocrinology if concurrent thyroiditis, hypophysitis, adrenal insufficiency

— Nutrition — for prolonged NPO, TPN management

— Direct handoff between oncology and inpatient team; document ICI agent, last dose date, all irAEs

— Update outpatient oncology team immediately upon admission and discharge

Outpatient management appropriate for:

Inpatient admission criteria (ward):

ICU admission criteria:

Mandatory consultations:

Transitions of care:

Step 3 management: A patient on day 3 of IV methylprednisolone for grade 3 ICI colitis with persistent ≥7 stools/day, ongoing abdominal pain, and CRP not improving requires immediate escalation — order infliximab 5 mg/kg IV the same day (after confirming negative TB, HBV, CMV screening), continue steroids, and obtain repeat sigmoidoscopy. Waiting another 48 hours is the wrong answer.

Key Differentials — Same-Category (GI) Causes of Diarrhea

— C. difficile — recent antibiotics, hospitalization, prior steroid use; pseudomembranes on endoscopy; treat with oral vancomycin or fidaxomicin; must rule out before/with starting steroids

— CMV colitis — especially in immunosuppressed patients on prolonged steroids; deep linear ulcers, owl's eye inclusions; treat with ganciclovir

— Bacterial: Salmonella, Shigella, Campylobacter, E. coli O157, Yersinia — typically self-limited; treat severe cases with abx (avoid abx in EHEC due to HUS risk)

— Parasitic: Giardia, Entamoeba, Cryptosporidium

— Viral: norovirus, rotavirus (usually self-limited)

— Pre-existing IBD can flare during ICI; differentiation often requires pre-treatment endoscopy comparison

— Granulomas, transmural inflammation, fistulas favor Crohn's

— Histologic apoptotic bodies favor ICI etiology

— Watery non-bloody diarrhea, normal endoscopy, diagnosis on biopsy

— Can be ICI-induced (especially PD-1 agents) or drug-related (NSAIDs, PPIs, SSRIs)

— Older patients, vascular disease, sudden onset bloody diarrhea, "thumbprinting" on imaging, watershed areas (splenic flexure, rectosigmoid)

— History of pelvic radiation; rectal predominance, telangiectasias

— Focal LLQ pain, fever, leukocytosis; CT diagnostic

— ICI enteritis — small-bowel involvement; nausea, vomiting, weight loss > diarrhea

— ICI gastritis — epigastric pain, nausea

— ICI hepatitis — transaminitis

— ICI pancreatitis — usually asymptomatic enzyme elevation

Infectious colitis:

Inflammatory bowel disease (UC, Crohn's):

Microscopic colitis (lymphocytic, collagenous):

Ischemic colitis:

Radiation colitis:

Diverticulitis:

Other ICI GI irAEs:

Key distinction: Distinguishing C. difficile from ICI colitis is mandatory before steroids — both can cause bloody diarrhea, fever, and pseudomembranes (pseudomembranes can rarely occur in severe ICI colitis too). Always test, don't assume — C. diff PCR/toxin on every suspected ICI colitis case.

Key Differentials — Other-Category Causes of Diarrhea in Cancer Patients

— Irinotecan — early (cholinergic, within 24 hours, treat with atropine) and late (secretory, treat with loperamide)

— 5-FU/capecitabine — mucositis, secretory diarrhea

— Tyrosine kinase inhibitors (sunitinib, sorafenib, regorafenib) — common cause

— EGFR inhibitors (cetuximab, erlotinib)

— Typically not bloody, lacks endoscopic colitis features

— Acute (during/within weeks of pelvic XRT) or chronic (months–years later)

— Strictures, fistulas, malabsorption in chronic form

— Post-allogeneic HSCT; skin + GI + liver involvement

— Endoscopy with "apoptotic body" features overlapping with ICI colitis

— Flushing, secretory diarrhea, valvular heart disease; elevated 5-HIAA, chromogranin A

— Steatorrhea after pancreatic surgery, chronic pancreatitis, or pancreatic cancer

— Low fecal elastase; treat with pancreatic enzyme replacement

— Post-cholecystectomy, ileal resection, or Crohn's; responds to cholestyramine

— Common during/after chemo; resolves with dietary modification

— Hyperthyroidism (ICI-induced thyroiditis can cause hyperthyroid phase)

— Adrenal insufficiency (ICI-induced hypophysitis or primary adrenalitis) — diarrhea, fatigue, hypotension, hyponatremia, hyperkalemia

— Diabetic autonomic neuropathy

— Antibiotics, magnesium-containing antacids, metformin, colchicine, SSRIs, PPIs (microscopic colitis), NSAIDs

— Diagnosis of exclusion; consider only after thorough workup

Chemotherapy-induced diarrhea:

Radiation enteritis/colitis:

Graft-versus-host disease (GVHD):

Carcinoid syndrome / neuroendocrine tumors:

Pancreatic insufficiency:

Bile acid diarrhea:

Lactose intolerance and dietary causes:

Endocrine causes:

Medication-related:

Functional/IBS:

Board pearl: In an ICI-treated patient with diarrhea + hypotension + hyponatremia + hyperkalemia + fatigue, think ICI-induced primary adrenal insufficiency or hypophysitis with secondary adrenal insufficiency — check morning cortisol, ACTH, electrolytes and give stress-dose hydrocortisone immediately. This is a high-yield endocrine irAE that mimics or coexists with colitis.

Secondary Prevention, ICI Rechallenge, and Long-Term Plan

— Grade 1–2: May resume same ICI after symptoms resolve and steroids tapered to ≤10 mg prednisone/day

— Grade 3 with PD-1/PD-L1 monotherapy: Consider resumption after full recovery; recurrence rate ~30%

— Grade 3 with CTLA-4 (ipilimumab) or combination therapy: Permanently discontinue CTLA-4 component; PD-1/PD-L1 may be resumed cautiously

— Grade 4: Permanent discontinuation of all ICIs

— Slower steroid taper before resumption

— Consider prophylactic vedolizumab in high-risk patients (under investigation)

— Dietary modifications, avoid NSAIDs

— Baseline stool calprotectin and clinical monitoring

— Oral prednisone taper with explicit weekly dose schedule (e.g., 60→40→30→20→10→5→off over 4–6+ weeks)

— PJP prophylaxis (TMP-SMX SS daily or DS MWF) if on ≥20 mg prednisone equivalent for ≥4 weeks

— Calcium + vitamin D; bisphosphonate if extended course or osteoporosis

— PPI for GI prophylaxis while on high-dose steroids

— Glucose monitoring; adjust diabetes regimen for steroid-induced hyperglycemia

— Antiviral prophylaxis (entecavir or tenofovir) if HBV+ and on infliximab

— Stop NSAIDs

— Recognize early symptoms of recurrence (any ≥2 unformed stools above baseline)

— Contact oncology immediately for new diarrhea, abdominal pain, blood in stool

— Carry wallet card documenting ICI use, irAE history, current immunosuppression

— Avoid live vaccines while immunosuppressed

Decision to resume ICI after colitis:

Multidisciplinary decision required — oncologist, gastroenterologist, patient — balancing cancer control vs irAE recurrence risk

Strategies to reduce recurrence on rechallenge:

Discharge medications after colitis admission:

Patient education:

Step 3 management: A patient discharged after grade 3 ICI colitis on prednisone 40 mg daily should be scheduled for oncology follow-up within 1 week, GI follow-up within 2–4 weeks, with weekly labs (CBC, CMP, CRP), fasting glucose monitoring, and a written, dated steroid taper schedule. Premature taper is the #1 cause of relapse — counsel patients not to skip doses or shorten the taper.

Follow-Up, Monitoring, and Counseling

— Weekly clinical assessment (in-person or telehealth) — symptom diary, stool frequency, abdominal pain

— Labs weekly: CBC, CMP, LFTs, CRP, glucose

— Reassess steroid taper at each visit — slow further if symptoms recur

— Fecal calprotectin every 2–4 weeks as objective marker of inflammation

— Consider before resuming ICI in patients with grade ≥3 colitis to confirm mucosal healing

— Persistent endoscopic inflammation despite symptom resolution predicts relapse

— All ICI-treated patients: clinical assessment, labs (CBC, CMP, LFTs, TSH, glucose), cortisol if symptomatic — before each ICI cycle during treatment

— Monitor for other irAEs (skin, thyroid, pituitary, lung, liver, kidney, joints, neurologic)

— Cancer surveillance per disease-specific oncology guidelines

— Symptom recognition — any GI symptom warrants prompt contact

— Steroid adherence — never abrupt stop; carry medical alert if on chronic steroids

— Vaccination plan — annual influenza (inactivated), pneumococcal, COVID-19, recombinant zoster (Shingrix) — no live vaccines during immunosuppression

— Infection precautions — hand hygiene, food safety, avoid sick contacts during immunosuppression

— Contraception — for women of reproductive age while on ICI and for ≥4 months after

— Mental health — cancer + irAE burden → screen for depression, anxiety, financial toxicity; refer to social work, support groups

— Smoking cessation, nutrition optimization, physical activity — general cancer survivorship

— Ensure access to specialty pharmacy for biologics

— Insurance prior authorization for infliximab/vedolizumab — anticipate delays; have oncology pharmacy involved

Short-term follow-up (first 4–6 weeks post-discharge):

Repeat endoscopy:

Long-term monitoring:

Counseling topics:

Health systems considerations:

CCS pearl: At every follow-up visit, the "every cycle" checklist for ICI patients should include: vitals, weight, full ROS for irAEs, CBC, CMP, LFTs, TSH, glucose, and morning cortisol if any concerning symptoms. Building this systematic review into clinic workflow is the highest-yield prevention strategy for late-presenting or stealth irAEs.

Ethical, Legal, and Patient Safety Considerations

— Patients must be counseled on the full spectrum of irAEs before initiating ICI, including life-threatening colitis, pneumonitis, hepatitis, endocrinopathies, and rare cardiac/neurologic events

— Discuss the trade-off: permanent ICI discontinuation may compromise cancer control if severe irAE occurs

— Document discussion in EHR

— After grade 3 colitis, the decision to resume ICI involves balancing cancer mortality risk against ~30% recurrence risk

— Patient values and goals of care are central — some prioritize cancer control, others quality of life

— Document conversation explicitly

— ICI patients are often managed across oncology, hospital medicine, GI, ED, and primary care

— Medication reconciliation errors (missing the ICI on the med list, missing the steroid taper) are a major safety issue

— Use structured handoff tools; ensure ICI status is flagged in problem list

— Provide patient with wallet card listing ICI agent, irAE history, current immunosuppression, emergency contacts

— Diagnostic anchoring — attributing diarrhea to chemo or "just a virus" delays recognition of life-threatening colitis

— Steroid taper errors — premature taper → relapse; abrupt cessation → adrenal crisis

— Missed C. difficile or CMV coinfection — leads to worsening despite immunosuppression

— Live vaccine administration during immunosuppression — potentially fatal

— Severe irAEs should be reported to FDA MedWatch to inform post-marketing surveillance

— Many institutions track irAE outcomes as a quality metric

— ICI access disparities by insurance, geography, and race — ensure equitable access to monitoring, biologics, and specialty care

— Cost of infliximab/vedolizumab can be prohibitive without insurance navigation

— In refractory colitis with progressive malignancy, palliative care consultation is appropriate to align treatment with goals

Informed consent for ICI therapy:

Shared decision-making for ICI rechallenge:

Transitions of care — high-risk handoff:

Patient safety pitfalls:

Mandatory reporting and quality:

Health equity:

End-of-life considerations:

Step 3 management: When discharging a patient on a 6-week prednisone taper after ICI colitis, the highest-yield safety intervention is a written, dated dose-by-dose taper schedule given to the patient, with explicit instructions to contact oncology before any dose changes, plus scheduled follow-up within 1 week — this single step prevents the majority of taper-related relapses and adrenal crises.

High-Yield Associations and Rapid-Fire Clinical Facts

CTLA-4 (ipilimumab) > PD-1/PD-L1 for colitis frequency and severity; combination therapy highest risk (~13–15% any-grade colitis)

Median onset: 5–10 weeks after ICI initiation; can occur after a single dose or months after stopping

Left-sided > right-sided colon involvement; rectum frequently involved

Hallmark histology: crypt epithelial apoptotic bodies + acute cryptitis/crypt abscesses

PD-1 agents more often produce microscopic (lymphocytic/collagenous) colitis pattern

C. difficile must be excluded before starting steroids — most commonly tested pitfall

CMV reactivation in steroid-refractory disease — test serum PCR + biopsy IHC

72-hour rule: No improvement on appropriate steroids → escalate to infliximab or vedolizumab

Infliximab 5–10 mg/kg IV; vedolizumab 300 mg IV (gut-selective, increasingly preferred to preserve anti-tumor immunity)

Permanent ICI discontinuation: Grade 4 always; Grade 3 CTLA-4 always (PD-1/PD-L1 case-by-case)

Pre-infliximab screening: TB, HBV, HIV, CHF, pregnancy

Steroid taper ≥4–6 weeks to prevent relapse; PJP prophylaxis if ≥20 mg prednisone ≥4 weeks

Concurrent irAEs cluster: dermatitis, hepatitis, thyroiditis, hypophysitis, pneumonitis — always check thyroid + LFTs + cortisol

Steroids do NOT clearly reduce ICI anti-tumor efficacy — but use the lowest effective dose; consider gut-selective vedolizumab in active cancer

Deep ulcers >1 cm on endoscopy → predict steroid failure → consider early infliximab

Avoid loperamide in moderate–severe colitis (toxic megacolon risk)

Avoid NSAIDs — worsen mucosal injury

Fecal calprotectin — useful objective marker for monitoring response and relapse

Pre-existing IBD — relative contraindication, not absolute; flare risk ~40%

Solid organ transplant + ICI = high risk for rejection

Allogeneic HSCT + ICI = severe GVHD risk

Live vaccines contraindicated during immunosuppression

Wallet card for irAE history and current immunosuppression

Board pearl: The single most testable fact: "ICI-treated patient with new diarrhea ≥4 stools/day → hold ICI, send C. diff and stool studies, start methylprednisolone IV after infection workup, escalate to infliximab if no response in 72 hours." Master this sequence and most exam questions on this topic become straightforward.

Board Question Stem Patterns

— 62 y/o with metastatic melanoma on ipilimumab + nivolumab, 6 weeks in, develops 5 loose bloody stools/day with crampy abdominal pain. Stool C. diff negative. What's next?

— Answer: Hold ICI, start prednisone 1 mg/kg/day, refer for sigmoidoscopy

— Same patient now on day 3 of IV methylprednisolone 1 mg/kg with no improvement; CRP still elevated. Next step?

— Answer: Check CMV serum PCR + biopsy CMV IHC; if negative, give infliximab 5 mg/kg IV

— Patient on steroids for ICI colitis develops sudden severe abdominal pain, rebound, free air on CT. Next step?

— Answer: Emergency surgical consult for subtotal colectomy; broad-spectrum antibiotics; stop ICI permanently

— Patient on pembrolizumab with diarrhea, hypotension, fatigue, hyponatremia, hyperkalemia. What test confirms diagnosis?

— Answer: Morning cortisol and ACTH — think ICI hypophysitis or primary adrenal insufficiency; give stress-dose hydrocortisone first

— Before starting infliximab for ICI colitis, which screening test is required?

— Answer: Quantiferon-TB and HBV serologies (and HIV, pregnancy test, CHF assessment)

— Patient recovered from grade 3 ipilimumab-induced colitis. Oncologist asks whether to resume ipilimumab + nivolumab. Best answer?

— Answer: Permanently discontinue ipilimumab (CTLA-4); may resume nivolumab (PD-1) monotherapy cautiously after full recovery

— Patient with ICI colitis also has AST 400, ALT 380. Steroids started but inadequate response. Choose next agent.

— Answer: Vedolizumab (avoid infliximab due to hepatotoxicity)

— Patient on pembrolizumab with watery non-bloody diarrhea, normal colonoscopy. Biopsy?

— Answer: Lymphocytic or collagenous colitis pattern with apoptotic bodies — ICI-induced microscopic colitis; treat with budesonide or systemic steroids

— Patient symptoms returned 2 weeks after rapid prednisone taper. Lesson?

— Answer: Taper ≥4–6 weeks; resume higher dose and taper more slowly

Classic stem 1 — straightforward grade 2:

Stem 2 — steroid-refractory:

Stem 3 — perforation:

Stem 4 — adrenal crisis mimic:

Stem 5 — pre-biologic screening:

Stem 6 — rechallenge decision:

Stem 7 — concurrent hepatitis:

Stem 8 — microscopic colitis variant:

Stem 9 — taper failure:

Key distinction: Stems featuring "no improvement after 3 days of steroids" almost always require infliximab or vedolizumab as the next answer — never "increase steroid dose" or "wait another 48 hours."

One-Line Recap

Immune checkpoint inhibitor colitis is suspected in any ICI-treated patient with new diarrhea (especially bloody), requires exclusion of C. difficile and CMV, is graded by stool frequency and severity, is treated with held ICI plus corticosteroids escalated to infliximab or vedolizumab if no response in 72 hours, with permanent ICI discontinuation for grade 4 (and CTLA-4 for grade 3), and a steroid taper of ≥4–6 weeks to prevent relapse.

Workup essentials: CBC, CMP, LFTs, CRP, lactate, stool studies (C. diff, culture, O&P, calprotectin), CT abdomen/pelvis for moderate–severe disease, and flexible sigmoidoscopy with biopsies (apoptotic bodies are the histologic hallmark).

Treatment algorithm: Grade 1 → continue ICI + supportive care; Grade 2 → hold ICI + prednisone 1 mg/kg/day; Grade 3 → hospitalize + IV methylprednisolone 1–2 mg/kg/day + endoscopy within 48 hours + infliximab/vedolizumab if no response in 72 hours; Grade 4 → ICU, permanent ICI cessation, surgical consult for possible colectomy.

Pre-biologic checklist: Always screen for TB (Quantiferon), HBV, HIV, CHF, pregnancy, and CMV before starting infliximab; choose vedolizumab when concurrent hepatitis, cardiac contraindications, or to preserve anti-tumor immunity.

Safety net: Slow steroid taper, PJP prophylaxis if ≥20 mg prednisone ≥4 weeks, written taper schedule at discharge, oncology follow-up within 1 week, GI follow-up within 2–4 weeks, wallet card documenting ICI exposure and irAE history, no live vaccines during immunosuppression, and counseling to report any recurrent GI symptoms immediately.

Board pearl: The two most testable management decisions in ICI colitis are (1) rule out C. difficile before steroids and (2) escalate to infliximab or vedolizumab if no improvement after 72 hours of steroids — never wait longer, never just increase the steroid dose.