Renal & Urinary

Acute interstitial nephritis: drug causes and management

Clinical Overview and When to Suspect Acute Interstitial Nephritis

— Top offenders: PPIs, NSAIDs, beta-lactams (penicillins, cephalosporins), sulfonamides (TMP-SMX), fluoroquinolones, rifampin, allopurinol, 5-ASA, and immune checkpoint inhibitors (ICIs)

— Non-drug causes: infections (Legionella, Leptospira, CMV, EBV, BK), autoimmune (SLE, Sjögren, sarcoidosis, IgG4-related, TINU), and idiopathic

— Unexplained rise in creatinine in a hospitalized or recently discharged patient on a new drug (typically 7–10 days after exposure for first-time sensitization, but 3–5 days with re-exposure, and weeks to months for PPIs/NSAIDs/ICIs)

— AKI with sterile pyuria, WBC casts, mild proteinuria (<1 g/day), and bland sediment otherwise

— Classic triad of fever, rash, eosinophilia is present in <10–15% — its absence does NOT exclude AIN

Acute interstitial nephritis (AIN) is an immune-mediated inflammation of the renal tubulointerstitium causing acute kidney injury (AKI), accounting for 10–15% of unexplained AKI and up to 15–25% of AKI on biopsy in hospitalized patients.

Drug-induced AIN dominates in the modern era (~70–75% of cases), having displaced infection-related AIN as the leading etiology.

When to suspect AIN on Step 3:

Step 3 management: In any AKI workup, perform a meticulous medication reconciliation including OTC NSAIDs, PPIs (often missed), herbal supplements, and recently completed antibiotic courses — the diagnosis is frequently made by temporal pattern recognition, not by lab signature.

Board pearl: PPI-induced AIN is the most commonly missed cause on Step 3 stems — onset is insidious (10–11 weeks), eosinophilia is often absent, and the patient is frequently asymptomatic until creatinine is checked. Always scrutinize the PPI on the med list of any unexplained AKI.

Presentation Patterns and Key History

— Classic (methicillin-era): Fever (~30%), maculopapular rash (~15%), eosinophilia (~20%) — complete triad <10%

— Modern (NSAID/PPI-era): Often oligosymptomatic with only rising creatinine; fever and rash rare with NSAIDs because they blunt the hypersensitivity response

— Beta-lactams, sulfas, rifampin: 7–10 days after first exposure, 3–5 days if re-challenged

— NSAIDs: weeks to months; often with concurrent nephrotic-range proteinuria (minimal change overlay)

— PPIs: weeks to months (median ~10–11 weeks); insidious; eosinophilia uncommon

— Allopurinol: part of DRESS syndrome — fever, rash, lymphadenopathy, hepatitis, AKI, often 2–6 weeks after start

— Checkpoint inhibitors (pembrolizumab, nivolumab, ipilimumab): 3–12 months after initiation; may coincide with other irAEs (colitis, thyroiditis, hepatitis)

— Rifampin: distinctive — abrupt AKI with flank pain, hemolysis, thrombocytopenia on intermittent dosing

— Nonspecific: malaise, nausea, anorexia, flank pain (capsular stretch), low-grade fever, arthralgias

— Polyuria/nocturia from tubular concentrating defect

— Rash distribution (morbilliform), mucosal involvement (rule out SJS/TEN, DRESS)

— All new prescriptions in prior 3 months, OTC NSAIDs (ibuprofen, naproxen), PPIs (omeprazole especially), herbal supplements

— Recent infections, IV contrast, autoimmune symptoms (sicca, arthritis, parotid swelling — Sjögren; uveitis — TINU)

Classic versus modern presentation — Step 3 stems lean modern:

Time course by drug class — high yield:

Symptoms to elicit:

History must capture:

Key distinction: TINU syndrome (Tubulointerstitial Nephritis and Uveitis) — bilateral anterior uveitis with AIN, classically in adolescent girls or middle-aged women; ask about eye pain, redness, photophobia. Ophthalmology must be involved; uveitis may precede, accompany, or follow AKI.

Physical Exam Findings and Volume Assessment

— Morbilliform maculopapular rash on trunk/extremities — classic with beta-lactams, sulfas, allopurinol

— Examine for DRESS features: facial edema, diffuse rash >50% BSA, lymphadenopathy — triggers urgent drug withdrawal and hospitalization

— SJS/TEN red flags: mucosal involvement, Nikolsky sign, skin pain — dermatology emergency

— Conjunctival injection, ciliary flush, photophobia → suspect TINU; slit-lamp confirms anterior uveitis

— Dry eyes (Schirmer test consideration) → Sjögren-associated AIN

— Generalized lymphadenopathy and hepatomegaly → DRESS or sarcoidosis-related AIN

— Parotid enlargement → Sjögren or IgG4-related disease

— AIN patients are typically euvolemic with normal-to-mildly-elevated BP

— No edema, no JVD elevation, no orthostasis (distinguishes from CHF cardiorenal and from prerenal volume depletion)

— Urine output is often preserved (non-oliguric AKI) — a hallmark

— Mild bilateral flank tenderness from interstitial edema/capsular stretch — not the severe unilateral CVAT of pyelonephritis

— Bibasilar crackles + hilar adenopathy on imaging → sarcoid AIN

— Pericardial rub raises concern for uremia if AKI advanced

AIN exam is frequently unremarkable — the absence of dramatic findings is itself a board clue distinguishing AIN from glomerulonephritis (edema, HTN) or ATN (hemodynamic insult).

Skin:

Eyes:

Lymphatic/Constitutional:

Volume status (crucial for Step 3 differential vs prerenal AKI/ATN):

Costovertebral angle:

Pulmonary/Cardiac:

CCS pearl: On a CCS case of unexplained AKI, the physical exam screen should include skin, conjunctivae, lymph nodes, and parotids — these targeted findings differentiate AIN subtypes and trigger the right confirmatory workup (slit-lamp for TINU, ACE level/CT chest for sarcoid, IgG4 for IgG4-RD) without ordering shotgun labs.

Diagnostic Workup — Initial Labs, Urinalysis, and Imaging

— Rising creatinine (often 1.5–3× baseline), elevated BUN with BUN:Cr ratio typically <20 (unlike prerenal)

— Electrolyte derangements from tubular dysfunction: hyperkalemia, non-anion-gap metabolic acidosis (type 1 or type 4 RTA), hypomagnesemia (especially PPI-related), Fanconi-like features in some drug exposures

— Peripheral eosinophilia (>500/μL) — supportive but present in only 20–35%; absent does not rule out

— Eosinophilia is least common with NSAID- and PPI-induced AIN

— Sterile pyuria (WBCs without bacteria) — present in ~80%

— WBC casts — highly suggestive; RBC casts argue against AIN (favor GN)

— Mild proteinuria typically <1 g/day; exception is NSAID-induced AIN with minimal change overlay producing nephrotic-range proteinuria

— Microscopic hematuria possible; gross hematuria rare

— No longer recommended — sensitivity ~30%, specificity ~70%; positive in UTI, atheroembolic disease, prostatitis, GN

— Board pearl: If a Step 3 stem offers "urine eosinophils" as the next best test, do not pick it — it has been formally deprecated. Choose biopsy or empiric drug withdrawal instead.

— FENa often >1% (tubular injury pattern), overlapping with ATN — not discriminatory

— Normal-to-enlarged kidneys with increased cortical echogenicity; rules out obstruction

— Historical adjunct showing diffuse bilateral uptake; rarely used clinically today

Basic metabolic panel:

CBC with differential:

Urinalysis with microscopy — the cornerstone:

Urine eosinophils (Hansel stain):

FENa and FEUrea:

Renal ultrasound:

Gallium-67 scan:

Step 3 management: Order BMP, CBC with diff, UA with microscopy, urine protein-to-creatinine ratio, and renal ultrasound as the initial AKI bundle. Stop or hold the suspected offending agent simultaneously — do not wait for biopsy to act.

Diagnostic Workup — Advanced and Confirmatory Studies

— Histology: Interstitial edema with mononuclear cell infiltrate (T lymphocytes predominant), variable eosinophils, tubulitis, non-caseating granulomas suggest sarcoid, drug (especially anti-TB drugs), or TINU

— Glomeruli and vessels are typically spared — a key distinguishing feature from glomerulonephritis and vasculitis

— AKI without clear etiology and creatinine not improving 5–7 days after stopping suspected offender

— Considering corticosteroid therapy — biopsy confirmation justifies immunosuppression

— Atypical presentation (heavy proteinuria, active sediment, systemic features suggesting GN or vasculitis)

— Multiple potential culprits where targeted withdrawal is unclear

— Clear temporal relationship to a single drug, classic UA, and improvement after withdrawal

— High bleeding risk (anticoagulation, single kidney, uncontrolled HTN) — weigh against diagnostic certainty

— ANA, anti-dsDNA, complement (C3/C4) — SLE-associated

— SSA/SSB — Sjögren

— ACE level, 1,25-OH vitamin D, calcium, CT chest — sarcoidosis

— IgG4 subclass levels — IgG4-related disease

— Hepatitis B/C, HIV serologies

— ANCA, anti-GBM if active urinary sediment to exclude RPGN

— Gallium or FDG-PET occasionally used when biopsy contraindicated

— CT chest if sarcoidosis suspected (bilateral hilar lymphadenopathy)

Renal biopsy is the gold standard for diagnosis of AIN.

When to biopsy on Step 3:

When biopsy may be deferred:

Serologic workup when AIN is confirmed or suspected without obvious drug trigger:

Imaging:

Key distinction: AIN versus ATN on biopsy — AIN shows interstitial inflammation with intact tubular epithelium; ATN shows tubular epithelial necrosis with minimal inflammation. Clinically, AIN has WBC casts; ATN has muddy brown granular casts and a clear ischemic or nephrotoxic insult (sepsis, contrast, aminoglycosides, cisplatin).

Risk Stratification and First-Line Management Logic

— Step 1: Discontinue all suspected offending drugs; if multiple culprits, stop the most likely (PPI > NSAID > recent antibiotic) or stop all if clinically feasible

— Step 2: Supportive care — maintain euvolemia, correct electrolytes (K+, Mg2+, bicarbonate), avoid additional nephrotoxins (contrast, aminoglycosides, ACE inhibitors temporarily)

— Step 3: Monitor creatinine daily inpatient or every 2–3 days outpatient for 5–7 days

— Step 4: If creatinine does not improve within 5–7 days of drug withdrawal → consider renal biopsy and corticosteroids

— Prolonged exposure (>3 weeks) before drug withdrawal

— Older age (>65)

— Pre-existing CKD

— Severe AKI at presentation (Cr >3 mg/dL, need for dialysis)

— Interstitial fibrosis on biopsy (>50% of cortex)

— Delay to corticosteroid initiation >2–3 weeks

— Biopsy-proven AIN with no improvement after drug withdrawal

— Severe AKI requiring dialysis

— ICI-induced AIN (steroids are standard of care here)

— Granulomatous AIN, TINU, sarcoid-related AIN

— Active infection-related AIN without antimicrobial coverage

— Significant fibrosis on biopsy (response unlikely)

Foundational principle: The single most important intervention is identification and prompt withdrawal of the offending agent. Delay in stopping the culprit is the strongest predictor of incomplete renal recovery and progression to CKD.

Stepwise management framework:

Risk factors for poor renal recovery:

Indications for corticosteroid therapy (controversial but commonly used):

Avoid steroids if:

CCS pearl: On a CCS AIN case, discontinue the offending drug in the order sheet immediately, then switch (do not just hold) to an alternative drug class if treatment is needed — e.g., switch omeprazole to famotidine, switch ibuprofen to acetaminophen, switch ceftriaxone to a non-beta-lactam if culture allows. Half-credit if you only hold.

Pharmacotherapy — Corticosteroids and Adjuncts

— Initiation timing: Best outcomes when started within 7–14 days of AKI onset; benefit diminishes substantially after 3 weeks

— Regimen options:

— IV methylprednisolone 250–500 mg daily × 3 days, followed by oral prednisone 1 mg/kg/day (max 60 mg) for 2–4 weeks, then taper over 4–8 weeks (total 8–12 weeks)

— Alternative: Oral prednisone 1 mg/kg/day from the outset for severe but non-dialysis-requiring AIN

— Expected response: Creatinine should decline within 1–2 weeks; if no response by 3 weeks, consider alternative diagnosis or steroid-resistant disease

— Mycophenolate mofetil 1–2 g/day

— Cyclophosphamide (rare, severe granulomatous disease)

— Used in sarcoid AIN, IgG4-RD, and ICI-AIN when steroids fail

— Loop diuretics only if volume-overloaded — do not force diuresis in euvolemic AIN

— Sodium bicarbonate for metabolic acidosis (HCO3 <18) or hyperkalemia bridging

— Phosphate binders, potassium binders (patiromer, SZC) as needed

— Avoid RAAS inhibitors until creatinine stabilizes

— Checkpoint inhibitor AIN: Steroids are first-line; permanent ICI discontinuation for grade 3–4 nephritis; rechallenge possible for grade 2 after recovery

— Rifampin AIN: Permanent avoidance — re-exposure can cause fulminant AKI with hemolysis

— NSAID AIN: Avoid the entire NSAID class lifelong; counsel on OTC ibuprofen, naproxen, ketorolac

Corticosteroids — first-line immunosuppressant when drug withdrawal alone is insufficient.

Steroid-sparing agents for refractory or steroid-dependent AIN (subspecialist territory):

Supportive pharmacotherapy:

Drug-specific considerations:

Board pearl: A Step 3 favorite — when persistent AKI after stopping the drug AND biopsy confirms AIN, the next best step is oral prednisone 1 mg/kg/day, not continued observation. Early steroids = better recovery.

Expanded Pharmacology — Drug-Class Deep Dive

— All PPIs implicated; omeprazole, pantoprazole, lansoprazole most reported

— Mechanism: idiosyncratic T-cell-mediated hypersensitivity

— Insidious onset (weeks-months); eosinophilia often absent

— Recovery often incomplete — increased risk of CKD; substitute H2 blocker (famotidine) as the safer alternative

— Both nonselective (ibuprofen, naproxen, indomethacin) and COX-2 selective

— Dual pathology: AIN + minimal change disease → nephrotic-range proteinuria with AKI is pathognomonic

— Longer latency (months); eosinophilia rare

— Alternative analgesics: acetaminophen, topical agents, gabapentinoids for neuropathic pain

— Beta-lactams (methicillin historically; now penicillin, ampicillin, cephalosporins): classic 7–10 day onset, fever/rash/eosinophilia most likely with this class

— Sulfonamides (TMP-SMX): can cause AIN ± SJS/TEN overlap

— Fluoroquinolones (ciprofloxacin): increasingly recognized; often with rash

— Rifampin: intermittent dosing → abrupt AKI + hemolysis + thrombocytopenia (anti-rifampin antibodies)

— Vancomycin: can cause both AIN and ATN; vancomycin trough monitoring is essential

— Part of DRESS (HLA-B*58:01 association in Han Chinese, Thai, Korean — screen before starting)

— Severe, may require steroids and lifelong avoidance; use febuxostat as alternative (though has its own CV warnings)

— IBD patients on long-term therapy; annual creatinine monitoring is standard of care

— Pembrolizumab, nivolumab, ipilimumab, atezolizumab — incidence 2–5%

— Co-administration with PPI or NSAID amplifies risk — deprescribe PPIs in cancer patients on ICIs

Proton pump inhibitors (PPIs):

NSAIDs:

Antibiotics:

Allopurinol:

5-Aminosalicylates (mesalamine, sulfasalazine):

Immune checkpoint inhibitors:

Step 3 management: When prescribing allopurinol to a patient of Han Chinese, Thai, or Korean ancestry, order HLA-B*58:01 genotyping before starting to prevent SCAR/DRESS/AIN — this is a tested pharmacogenomic checkpoint.

Special Populations — Elderly and Renal/Hepatic Impairment

— Disproportionately affected — polypharmacy (PPIs, NSAIDs, antibiotics, allopurinol commonly stacked) is the dominant risk factor

— Often present with subtle, non-oliguric AKI detected only on routine labs; classic triad even rarer than in younger adults

— Higher risk of incomplete recovery and progression to CKD — baseline nephron loss reduces functional reserve

— Drug withdrawal more complex: pain control without NSAIDs (acetaminophen, careful opioid use), reflux without PPIs (lifestyle, H2 blocker, on-demand antacids)

— Steroid risks magnified: hyperglycemia (often unmasks diabetes), osteoporosis, delirium, infection, GI bleeding — use lowest effective dose, shortest duration, add PPI substitute (here famotidine) plus calcium/vitamin D and bone health monitoring

— AIN superimposed on CKD has worse renal recovery; lower threshold for biopsy and steroid therapy

— Adjust all renally-cleared drugs; avoid additive nephrotoxins

— Closer follow-up post-recovery — many progress to needing nephrology longitudinally

— AIN can still cause incremental damage to residual renal function — preserving urine output matters for fluid management and outcomes

— Steroid metabolism altered; monitor for prolonged effect

— Drug-drug interactions matter — rifampin, allopurinol, mesalamine all have hepatic considerations

— Coexistent hepatorenal physiology complicates assessment of AKI etiology — biopsy threshold is lower

— AIN can mimic acute rejection in kidney transplant; biopsy is essential to distinguish

— Calcineurin inhibitor toxicity is a competing diagnosis

Elderly patients (>65):

Pre-existing CKD:

Patients on dialysis:

Hepatic impairment:

Transplant recipients:

Board pearl: In an elderly patient on chronic PPI + NSAID + recent antibiotic presenting with unexplained AKI, stop ALL three sequentially or simultaneously based on risk-benefit — the PPI is usually the easiest to stop first (substitute famotidine), then NSAID (substitute acetaminophen), then narrow antibiotic if culture-driven. Polypharmacy AIN is a Step 3 staple.

Special Populations — Pregnancy, Pediatrics, and Specific Demographics

— AIN in pregnancy is uncommon but seen with antibiotic exposure (beta-lactams for UTI/pyelonephritis, nitrofurantoin, sulfonamides — though sulfa avoided in third trimester)

— NSAIDs are contraindicated after 20 weeks (oligohydramnios, ductus closure) — so NSAID-AIN in late pregnancy should prompt evaluation of OTC use

— Differential includes preeclampsia, HELLP, acute fatty liver of pregnancy, thrombotic microangiopathy — these dominate the obstetric AKI differential

— Biopsy is feasible but reserved for diagnostic uncertainty with management implications

— Corticosteroids (prednisone) are safe in pregnancy (category C historically; widely used) — use if indicated

— TINU syndrome has peak incidence in adolescent females (median age ~15) — anterior uveitis + AIN, often with positive modified Goldmann-Witmer coefficient; HLA-DRB1*01 association

— Drug-induced AIN in children most commonly from antibiotics (beta-lactams, TMP-SMX), NSAIDs, and antiepileptics (phenytoin, carbamazepine)

— Pediatric AIN generally has better renal recovery than adult AIN

— Ophthalmology referral mandatory in any pediatric AIN — uveitis may be asymptomatic

— Increasing demographic; coordinate care with oncology

— Hold ICI during active AIN; permanent discontinuation for grade 4 or recurrent grade 3

— Common co-factors: PPI use, NSAIDs, prior radiation — minimize all concurrent insults

— Sjögren, sarcoidosis, SLE, IgG4-related disease — AIN may be a presenting manifestation; treat underlying disease

— Trimethoprim-sulfamethoxazole prophylaxis is a recognized AIN trigger — distinguish from rejection by biopsy

Pregnancy:

Pediatrics:

Oncology patients on ICIs:

Patients with autoimmune disease:

Solid organ transplant recipients:

Key distinction: TINU vs sarcoid AIN — both cause uveitis and AIN. TINU has anterior uveitis, younger patients, no granulomas (usually), self-limited course; sarcoid has bilateral anterior or posterior uveitis, hilar adenopathy, non-caseating granulomas on biopsy, hypercalcemia, elevated ACE/1,25-vitamin D, often requires prolonged steroid course.

Complications and Adverse Outcomes

— Severe AKI requiring renal replacement therapy — ~30–40% of biopsy-proven AIN at peak; most recover off dialysis within weeks

— Hyperkalemia — particularly with type 4 RTA pattern; manage with potassium binders (patiromer, SZC), loop diuretics, dietary restriction, IV calcium/insulin-D50 for emergencies

— Metabolic acidosis — both anion-gap (uremic) and non-anion-gap (RTA from tubular dysfunction); bicarbonate supplementation when HCO3 <22

— Volume overload — uncommon in pure AIN; if present, suspect oliguric phase or coexistent CHF

— Uremic complications: pericarditis, encephalopathy, platelet dysfunction — indications for urgent dialysis

— Fanconi syndrome (glucosuria, aminoaciduria, phosphaturia, type 2 RTA) — rare but reported with certain drugs

— Concentrating defect → polyuria, nocturia, nephrogenic-like diabetes insipidus picture

— Hypomagnesemia, hypokalemia — especially with PPI-related disease

— Incomplete recovery in 30–50% of biopsy-proven AIN — persistent CKD with elevated baseline creatinine

— Progression to ESRD in 5–10%, higher with delayed drug withdrawal, fibrosis on biopsy, older age, severe initial AKI

— Recurrence with re-exposure — typically more rapid and severe than initial episode

— Corticosteroid adverse effects: hyperglycemia, hypertension, weight gain, mood changes, osteoporosis, infection, GI bleeding, cataracts, adrenal suppression with prolonged courses

— Opportunistic infections — consider PJP prophylaxis if prednisone ≥20 mg/day for ≥4 weeks

Acute complications during the AIN episode:

Tubular dysfunction syndromes:

Chronic and long-term outcomes:

Treatment-related complications:

CCS pearl: When managing an AIN inpatient case, order daily BMP, daily weights, strict I/Os, and daily creatinine trend; reassess need for dialysis using AEIOU criteria (Acidosis, Electrolytes, Ingestions, Overload, Uremia). Initiate renal diet (low K, low Na, low P) consultation early and vaccinate (pneumococcal, influenza) before starting steroids.

When to Escalate Care — ICU, Nephrology Consult, and Inpatient Triage

— AKI with creatinine ≥2× baseline or absolute Cr >3 mg/dL

— Hyperkalemia >6.0 mEq/L or with ECG changes

— Severe acidosis (HCO3 <15, pH <7.20)

— Volume overload with respiratory compromise

— Uremic symptoms — encephalopathy, pericarditis, bleeding

— Need for urgent biopsy or IV pulse steroids

— Systemic features suggesting DRESS, SJS/TEN, or vasculitis

— Any AKI not improving within 48–72 hours of culprit withdrawal

— Severe AKI requiring potential RRT

— Decision-making on biopsy

— Initiation of immunosuppression

— Suspected systemic disease (sarcoid, SLE, IgG4-RD, vasculitis)

— Emergent dialysis indications (AEIOU): refractory Acidosis, Electrolyte abnormalities (hyperK), toxic Ingestion, volume Overload refractory to diuresis, Uremic complications

— Hemodynamic instability from sepsis (if infection-related AIN)

— DRESS with multiorgan involvement (hepatitis, myocarditis, pneumonitis)

— Ophthalmology — slit-lamp exam in suspected TINU or sarcoid (mandatory in children/adolescents)

— Dermatology — biopsy of rash in DRESS, SJS/TEN; assess severity

— Rheumatology — autoimmune workup

— Oncology — for ICI-induced AIN (decisions on ICI rechallenge)

— Pharmacy — medication reconciliation, identifying culprit and ensuring substitution

— Mild AKI (Cr <2.0) with identified single culprit

— No electrolyte emergencies, stable volume status

— Reliable follow-up within 48–72 hours

— Patient understands and can adhere to drug withdrawal

Inpatient admission criteria for suspected AIN:

Nephrology consultation indications:

ICU triage:

Specialty consults beyond nephrology:

Outpatient management appropriate when:

Step 3 management: For an outpatient discovered to have Cr 2.5 (baseline 1.0), sterile pyuria, and recent PPI initiation 8 weeks ago, the correct disposition is usually stop the PPI (substitute famotidine), admit or arrange next-day nephrology and labs, depending on K+, acid-base, and access to outpatient follow-up. Do not "watch and wait" at home without close monitoring.

Key Differentials — Other Causes of AKI Within the Renal Category

— Most common intrinsic AKI; ischemic (sepsis, hypotension, shock) or nephrotoxic (aminoglycosides, contrast, amphotericin, cisplatin, myoglobin)

— UA: muddy brown granular casts, renal tubular epithelial cells; FENa >2%, urine osm <350

— No fever/rash/eosinophilia; clear precipitating insult

— Volume depletion, CHF, cirrhosis, sepsis (early)

— BUN:Cr >20, FENa <1%, urine osm >500, bland UA

— Responds rapidly to volume resuscitation

— Active sediment with RBC casts, dysmorphic RBCs, heavy proteinuria, hypertension, edema

— Differential: anti-GBM, ANCA vasculitis, lupus nephritis, IgA, post-infectious GN

— Serologies: ANA, ANCA, anti-GBM, C3/C4, ASO, cryoglobulins

— HUS, TTP, malignant HTN, scleroderma renal crisis, drug-induced (calcineurin inhibitors, gemcitabine, quinine)

— MAHA + thrombocytopenia + AKI; schistocytes on smear, elevated LDH, low haptoglobin

— Post-arterial catheterization in elderly atherosclerotic patient

— Livedo reticularis, blue toe syndrome, eosinophilia, low complement, Hollenhorst plaques on fundoscopy

— Mimics AIN with eosinophilia — but vascular procedure history and skin findings distinguish

— Multiple myeloma; AKI + bland UA + low anion gap + hypercalcemia + anemia + bone pain

— UPEP/SPEP, serum free light chains

— Rhabdomyolysis (CK >5000, positive urine heme dip but no RBCs on micro); hemolysis

Acute tubular necrosis (ATN):

Prerenal AKI:

Glomerulonephritis (GN) and RPGN:

Thrombotic microangiopathy (TMA):

Atheroembolic (cholesterol embolic) disease:

Cast nephropathy (myeloma kidney):

Pigment nephropathy:

Acute interstitial nephritis vs ATN — Key distinction: AIN has WBC casts and pyuria with eosinophils; ATN has muddy brown granular casts and tubular epithelial cells. AIN follows drug exposure or infection; ATN follows ischemia or direct tubular toxin. Both are non-oliguric most commonly; biopsy is definitive when uncertain.

Key Differentials — Extra-Renal and Systemic Mimics

— Pyuria with positive urine culture, bacteria, nitrites, leukocyte esterase

— Unilateral CVA tenderness, fever, dysuria; AIN urine is sterile

— Can coexist — pyelonephritis can rarely cause AIN-like infiltrate

— BPH, malignancy, retroperitoneal fibrosis, stones, neurogenic bladder

— Renal ultrasound shows hydronephrosis — must be excluded in every AKI

— Retroperitoneal fibrosis is part of IgG4-related disease, which itself causes AIN — overlap exists

— Advanced cirrhosis with portal hypertension; AKI unresponsive to volume challenge with albumin

— Bland sediment, very low FENa; treat with terlipressin/midodrine-octreotide + albumin

— Decompensated CHF with worsening renal function; volume status assessment is key

— Often improves with diuresis (paradoxically)

— Multifactorial: hypotension, cytokines, microcirculatory dysfunction

— Source control + resuscitation + antibiotics; AIN can complicate antibiotic therapy

— Drug reaction with eosinophilia and systemic symptoms (DRESS): fever, rash >50% BSA, facial edema, lymphadenopathy, eosinophilia, hepatitis ± nephritis (AIN); HHV-6 reactivation common; drugs — allopurinol, anticonvulsants, sulfonamides, vancomycin

— Serum sickness: fever, rash, arthralgias, ± mild renal involvement

— Anaphylaxis: acute, IgE-mediated, not AIN

— ANCA-associated (GPA, MPA, EGPA), polyarteritis nodosa, IgA vasculitis (HSP)

— Multisystem (lung, skin, nerve, GI) + active urinary sediment distinguishes from drug AIN

— Plasma cell dyscrasias; check SPEP/UPEP, free light chains in elderly with unexplained AKI

Urinary tract infection / pyelonephritis:

Obstructive uropathy (post-renal):

Hepatorenal syndrome:

Cardiorenal syndrome:

Sepsis-induced AKI:

Allergic conditions with renal involvement:

Vasculitis:

Light chain cast nephropathy and amyloidosis:

Board pearl: DRESS syndrome is on the differential whenever rash + AKI + eosinophilia + hepatitis present together — the renal lesion in DRESS is typically AIN, and management is immediate culprit drug withdrawal, systemic corticosteroids, monitoring for HHV-6/CMV reactivation, and avoiding the entire drug class lifelong. Allopurinol is the classic DRESS trigger.

Secondary Prevention and Discharge Plan

— Document the culprit drug in the allergy/adverse reaction section of the EMR with the reaction "acute interstitial nephritis" — not just "allergy"

— Educate the patient verbally and in writing; provide MedicAlert information if culprit is a commonly used class (e.g., beta-lactams)

— Avoid cross-reactive drugs: if penicillin-AIN, generally avoid all beta-lactams (some tolerate cephalosporins, but caution); if NSAID-AIN, avoid entire NSAID class including OTC; if sulfa-AIN, avoid sulfonamide antibiotics

— PPI → famotidine (H2 blocker); reassess need for acid suppression annually

— NSAID → acetaminophen, topical NSAIDs (limited absorption), gabapentin/pregabalin for neuropathic pain, physical therapy

— Allopurinol → febuxostat (with CV risk counseling); if neither tolerated, pegloticase for refractory gout

— TMP-SMX → alternative based on indication (nitrofurantoin for cystitis, atovaquone for PJP prophylaxis)

— Mesalamine in IBD → discussion with GI; alternatives include corticosteroids, biologics

— Avoid all nephrotoxins: IV contrast (when possible), aminoglycosides, repeat NSAIDs

— BP control to <130/80 mmHg

— Diabetes control if applicable

— Smoking cessation

— Hydration counseling, especially during illness

— Influenza annually, pneumococcal (PCV20 or PCV15+PPSV23), hepatitis B if progressing to CKD stage 4+

— Reconcile prescription, OTC, and supplement list

— Particular vigilance about patient-initiated NSAID use for headaches, back pain, dysmenorrhea

Definitive lifelong avoidance of the offending drug:

Substitute medications — Step 3 high-yield swaps:

Renoprotective long-term measures:

Vaccination:

Medication review at every visit:

Step 3 management: At discharge, the order set must include: (1) Stop culprit drug permanently; (2) Document allergy in EMR with reaction type; (3) Prescribe substitute medication; (4) Schedule nephrology follow-up in 1–2 weeks; (5) Provide patient education handout; (6) Communicate with primary care via discharge summary noting specific drugs to avoid.

Follow-Up, Monitoring, and Patient Counseling

— Week 1–2 post-discharge: Nephrology visit; BMP, UA, urine protein:creatinine ratio

— Weeks 2–4: Repeat creatinine; assess steroid taper and adverse effects if on therapy

— Months 2–3: Reassess renal recovery — if creatinine has not returned to baseline, document new baseline and stage CKD

— Months 6 and 12: Confirm stability; transition to annual monitoring if recovered

— Lifelong annual creatinine and UA even after full recovery — risk of CKD progression persists

— Glucose weekly initially, then monthly; screen for steroid-induced diabetes (fasting glucose, HbA1c)

— BP at every visit; treat if >130/80

— Weight, edema, mood assessment

— CBC if steroid-sparing agents added

— Bone health: baseline DEXA if expecting >3 months of steroids; calcium 1200 mg/day + vitamin D 800–1000 IU/day; bisphosphonate if T-score ≤−1.5 or fracture risk elevated

— PJP prophylaxis (TMP-SMX or atovaquone if sulfa-allergic) for prednisone ≥20 mg/day for ≥4 weeks

— GI prophylaxis — but caution given PPI association with AIN; use H2 blocker

— Explain the diagnosis: "Your kidneys had an allergic-type reaction to a medication"

— Lifelong avoidance message: even small doses can re-trigger AIN

— Warning signs of recurrence or CKD progression: decreased urine output, edema, fatigue, nausea

— Importance of disclosing AIN history at every healthcare encounter, including dental and urgent care

— Stage CKD by eGFR; albuminuria by urine ACR

— Initiate ACE inhibitor or ARB if albuminuria >30 mg/g and BP allows (after Cr stable)

— SGLT2 inhibitor if eGFR ≥20 and indicated by KDIGO 2024 guidelines for CKD

Outpatient follow-up cadence after AIN episode:

Monitoring parameters on steroid therapy:

Counseling points for patients:

CKD-specific monitoring if incomplete recovery:

Board pearl: A patient who fully recovered from drug-induced AIN still carries elevated lifetime risk of CKD and AKI recurrence — annual creatinine, UA, and medication review are standard of care even years after the index event.

Ethical, Legal, and Patient Safety Considerations

— AIN is frequently a preventable adverse drug event; Joint Commission National Patient Safety Goal 03.06.01 mandates medication reconciliation at every transition of care

— Failure to recognize a recent drug exposure as the AIN culprit during admission or discharge is a documented source of medical error

— Document the offending agent prominently in the problem list, allergies, and discharge summary — this prevents future iatrogenic re-exposure

— Renal biopsy requires consent covering risks: bleeding (1–2% major), hematuria, perinephric hematoma, AV fistula, infection, need for transfusion or embolization; alternatives (empiric treatment, expectant management) must be discussed

— Corticosteroid therapy consent should address infection risk, metabolic effects, mood/psychiatric effects (especially in elderly), and that evidence base is observational, not randomized

— ICI rechallenge in oncology patients with prior AIN requires explicit shared decision-making about cancer benefit vs renal risk

— Patient discharged on a new antibiotic develops AIN 7 days later in the outpatient setting — the discharging clinician retains responsibility to ensure follow-up and explicit instructions on warning signs and when to seek care

— Always send a structured discharge summary to the primary care provider within 48 hours, flagging high-risk medications

— Label adverse drug reactions accurately: "AIN" is not an IgE-mediated allergy and may not contraindicate the same drug class in every situation — but for AIN, lifelong avoidance of the offending agent is standard

— Mislabeling AIN as "rash" leads to re-prescription and recurrence

— Serious adverse drug reactions should be reported to FDA MedWatch — particularly novel agents (ICIs, biologics) where surveillance data are still accruing

— Polypharmacy AIN disproportionately affects elderly, low-income patients with multiple prescribers and limited care coordination — deprescribing and medication review at every visit is an equity intervention

Patient safety — medication reconciliation as a national priority:

Informed consent edge cases:

Transition-of-care risk (Step 3 staple):

Allergy documentation accuracy:

Mandatory reporting:

Health equity:

Step 3 management: When you suspect drug-induced AIN, fulfill three duties simultaneously: (1) clinical (stop drug, treat AKI), (2) documentation (EMR allergy entry with reaction type), and (3) communication (patient education + handoff to PCP). All three must occur before discharge to meet the standard of care.

High-Yield Associations and Rapid-Fire Clinical Facts

Top 5 drug causes (memorize for Step 3): PPIs, NSAIDs, beta-lactams, sulfonamides (TMP-SMX), allopurinol

Triad of fever, rash, eosinophilia — present in <10–15% of cases; absence does NOT exclude AIN

Urine eosinophils — not recommended; sensitivity ~30%, false positives in atheroembolic disease, prostatitis, UTI, GN

WBC casts with sterile pyuria → classic AIN

RBC casts → glomerulonephritis, NOT AIN

Muddy brown casts → ATN, NOT AIN

NSAID-induced AIN → uniquely associated with minimal change disease and nephrotic-range proteinuria

PPI-induced AIN → insidious onset (weeks-months), often without eosinophilia, frequently missed, common Step 3 distractor

Rifampin AIN → intermittent dosing, abrupt onset with hemolysis and thrombocytopenia, antibody-mediated

Allopurinol AIN → part of DRESS; HLA-B*58:01 in Asian populations — screen before prescribing

Checkpoint inhibitor AIN → late onset (3–12 months); concurrent PPI use multiplies risk; permanent ICI hold for grade 3–4

TINU syndrome → adolescent female; uveitis + AIN; ophthalmology consult mandatory

Sarcoid AIN → non-caseating granulomas, hypercalcemia, hilar adenopathy, elevated ACE

IgG4-related AIN → elevated IgG4, often with autoimmune pancreatitis or retroperitoneal fibrosis; storiform fibrosis on biopsy

Gold standard diagnosis → renal biopsy

First-line management → stop the drug; corticosteroids if no improvement in 5–7 days or severe disease

Prednisone dose → 1 mg/kg/day (max 60 mg) × 2–4 weeks, then taper over 8–12 weeks total

Best predictor of recovery → early drug withdrawal; outcome worsens dramatically after 3 weeks

5-ASA-induced AIN → patients with IBD; monitor creatinine annually

CKD risk → 30–50% have incomplete recovery; 5–10% progress to ESRD

Key distinction: Drug-induced AIN (no rash, no eosinophilia, indolent — think PPI/NSAID) vs hypersensitivity AIN (rash, fever, eosinophilia, acute — think beta-lactam/allopurinol) — both AIN, but different stem fingerprints. The treatment principle (stop drug ± steroids) is the same.

Board Question Stem Patterns

Stem 1 — The PPI miss: A 68-year-old woman on omeprazole for 3 months for GERD has Cr rise from 0.9 to 2.1 over 6 weeks. UA: sterile pyuria, WBC casts, trace protein. No fever, no rash, eosinophils normal. Best next step? → Stop omeprazole, substitute famotidine, monitor Cr. (Don't pick urine eosinophils; don't pick steroids first.)

Stem 2 — Classic beta-lactam: A 45-year-old man develops fever, morbilliform rash, and Cr 3.2 (baseline 1.0) 8 days into a course of nafcillin for endocarditis. Eosinophils 12%. Diagnosis? → Drug-induced AIN. Next step? Discontinue nafcillin, switch to vancomycin (with awareness vanco can also cause AIN), consider steroids if no improvement.

Stem 3 — NSAID with nephrotic syndrome: A 60-year-old man on chronic ibuprofen for OA presents with edema, Cr 2.5, urine protein 6 g/day, hypoalbuminemia. UA: WBCs, no RBC casts. Diagnosis? → NSAID-induced AIN with minimal change disease. Biopsy confirms; stop NSAID, prednisone.

Stem 4 — Allopurinol DRESS: Han Chinese man started on allopurinol 4 weeks ago for gout now has fever, diffuse rash >50% BSA, facial edema, LAD, ALT 400, Cr 2.8, eosinophils 18%. Diagnosis? → DRESS with AIN. Stop allopurinol, IV steroids, lifelong avoidance; should have screened HLA-B*58:01.

Stem 5 — TINU: A 14-year-old girl with red, painful eye and photophobia has Cr 1.8, sterile pyuria. Slit-lamp: bilateral anterior uveitis. Diagnosis? → TINU syndrome. Ophthalmology, prednisone, topical steroids for uveitis.

Stem 6 — Checkpoint inhibitor: A 70-year-old man on pembrolizumab × 6 months for melanoma, also on omeprazole, has Cr rise from 1.0 to 2.4. Next step? → Hold pembrolizumab, stop PPI, start prednisone 1 mg/kg/day, nephrology and oncology consult.

Stem 7 — Rifampin re-exposure: TB patient on intermittent rifampin has sudden Cr 3.5, Hgb 8, platelets 80,000. Diagnosis? → Rifampin-induced AIN with hemolysis. Discontinue permanently.

CCS pearl: On CCS AIN cases the highest-yield orders are: stop offending drug → IV fluids cautiously if euvolemic → daily BMP → renal US → nephrology consult → biopsy if not improving in 5–7 days → prednisone 1 mg/kg/day → document allergy → discharge with PCP follow-up in 1 week and nephrology in 2 weeks. Sequence and substitution matter.

One-Line Recap

Acute interstitial nephritis is a drug-driven, immune-mediated AKI in which prompt identification and withdrawal of the offending agent — most commonly PPIs, NSAIDs, beta-lactams, sulfonamides, or allopurinol — is the single most important intervention, with corticosteroids reserved for biopsy-proven disease that fails to improve within 5–7 days.

Diagnostic fingerprint: Sterile pyuria + WBC casts + mild proteinuria + rising creatinine in a patient with a temporally related drug exposure; classic fever/rash/eosinophilia triad is the exception (<15%), not the rule, and is essentially absent with PPI- and NSAID-induced disease.

Treatment hierarchy: (1) Stop the culprit drug (substitute, don't just hold — PPI → famotidine, NSAID → acetaminophen); (2) Supportive care with electrolyte/acid-base management and avoidance of additional nephrotoxins; (3) Renal biopsy if no improvement in 5–7 days or atypical features; (4) Prednisone 1 mg/kg/day × 2–4 weeks with taper over 8–12 weeks for biopsy-proven AIN not recovering with drug withdrawal alone; (5) Lifelong avoidance documented in EMR.

Outcomes reality: 30–50% incomplete renal recovery, 5–10% progression to ESRD; outcome hinges on time to drug withdrawal — every day of continued exposure worsens prognosis, which is why the first move on any unexplained AKI with bland-ish sediment is meticulous medication reconciliation.

Step 3 management essence: Recognize the pattern early, deprescribe boldly, document precisely, follow up reliably — the AIN patient who recovers fully is the one whose clinician noticed the PPI started 10 weeks ago and stopped it before the nephron loss became permanent.