Immune System

Drug allergy: evaluation and desensitization principles

Clinical Overview and When to Suspect Drug Allergy

— Type I: IgE-mediated, immediate (minutes–1 h) — urticaria, angioedema, bronchospasm, anaphylaxis (β-lactams, neuromuscular blockers, platinum agents)

— Type II: IgG cytotoxic — drug-induced hemolytic anemia, thrombocytopenia (heparin, methyldopa, quinine)

— Type III: immune complex — serum sickness, vasculitis (cefaclor, sulfonamides, infliximab)

— Type IV: T-cell mediated, delayed (days–weeks) — morbilliform rash, contact dermatitis, SJS/TEN, DRESS, AGEP

— Temporal link: symptoms onset after starting drug (Type I within hours; Type IV typically 4–14 days, longer for DRESS at 2–8 weeks)

— Reproducibility on rechallenge or cross-reactive agent

— Phenotype matches an immunologic pattern (urticaria, mucosal involvement, eosinophilia, hepatitis, nephritis)

— Fever, lymphadenopathy, organ involvement → severe cutaneous adverse reaction (SCAR)

Step 3 management: When a patient reports a drug "allergy," your first task is to characterize the reaction (timing, phenotype, severity, prior tolerance) before accepting the label — this drives whether to avoid, test, premedicate, or desensitize.

Drug allergy = immunologically mediated adverse drug reaction (ADR), distinct from predictable pharmacologic side effects, intolerance, or idiosyncratic toxicity

Represents only ~5–10% of all ADRs, yet drives major prescribing errors when over-labeled (especially penicillin)

Gell and Coombs classification anchors the framework:

When to suspect drug allergy rather than side effect:

High-risk drug classes: β-lactams, sulfonamide antibiotics, NSAIDs, antiepileptics (carbamazepine, lamotrigine, phenytoin), allopurinol, abacavir, vancomycin, contrast media, chemotherapy (taxanes, platinums), monoclonal antibodies

Outpatient Step 3 framing: ~10% of US adults carry a penicillin allergy label, but >95% are not truly allergic on testing — delabeling improves antibiotic stewardship, reduces MRSA/C. difficile, and lowers cost

Presentation Patterns and Key History

— Exact drug, dose, route, indication

— Time from first dose to symptom onset (immediate <1 h vs delayed >6 h)

— Symptom phenotype: cutaneous only vs systemic vs mucosal vs organ-specific

— Duration, treatment given, resolution timeline

— Prior tolerance of the drug or related class (e.g., cephalosporins after "penicillin allergy")

— Concurrent illness (viral exanthem can mimic drug rash, especially EBV + amoxicillin)

— Time since reaction (IgE-mediated penicillin allergy wanes — ~80% lose sensitivity by 10 years)

— Simple morbilliform exanthem 4–14 days in

— Fever + facial edema + lymphadenopathy + eosinophilia + hepatitis → DRESS

— Mucosal erosions, target lesions, skin pain, Nikolsky sign → SJS/TEN

— Sterile pustules on erythema, neutrophilia → AGEP

— Arthralgia + rash + fever 1–3 weeks after drug → serum sickness-like reaction

— GI upset with antibiotics

— Red-person syndrome with rapid vancomycin (direct mast cell, not IgE)

— ACE inhibitor cough or non-allergic angioedema (bradykinin)

— Statin myalgias, metformin diarrhea

Board pearl: A patient labeled "penicillin allergic" who reports only GI upset or isolated headache has an intolerance, not allergy — proceed with the drug; no testing required. Reserve formal evaluation for objective hypersensitivity phenotypes.

History is the single most important diagnostic tool — most drug allergy decisions are made at the bedside without testing

Structured allergy interview elements:

Immediate hypersensitivity clues: urticaria, flushing, pruritus, wheeze, hypotension, GI cramping within 1–6 hours; suggests IgE or mast cell activation

Delayed/T-cell reaction clues:

Reactions that are NOT allergy but commonly mislabeled:

Family history matters for HLA-linked SCARs: HLA-B57:01 (abacavir), HLA-B15:02 (carbamazepine in Asians), HLA-B*58:01 (allopurinol)

Physical Exam Findings and Hemodynamic Assessment

— Acute skin/mucosal involvement + respiratory compromise OR hypotension

— Two or more organ systems after likely allergen (skin, respiratory, GI, cardiovascular)

— Hypotension after known allergen exposure

— Tachycardia + hypotension + warm flushed skin = distributive anaphylactic shock

— Stridor or hoarse voice = laryngeal edema, airway threat

— Urticaria (raised, blanching wheals, migratory, <24 h per lesion) → IgE or mast cell

— Angioedema (non-pitting deep dermal/mucosal swelling, lips, tongue, periorbital)

— Morbilliform/maculopapular eruption on trunk spreading centrifugally → delayed T-cell

— Targetoid lesions, dusky centers, skin pain, positive Nikolsky → SJS/TEN — dermatologic emergency

— Sheet-like superficial pustules on erythematous base → AGEP

— Facial edema + diffuse exanthem covering >50% BSA → suggests DRESS

CCS pearl: In suspected anaphylaxis on the CCS, IM epinephrine 0.3–0.5 mg into the anterolateral thigh comes before IV access, antihistamines, or steroids — order it as the first action, then airway, IV fluids, and monitoring.

Exam serves two goals: classify severity (mild vs SCAR vs anaphylaxis) and identify mimics

Anaphylaxis (Type I) bedside criteria — any one of:

Vital signs to obtain immediately: BP, HR, RR, SpO₂, peak flow if available

Skin exam patterns:

Mucosal involvement (oral, ocular, genital erosions) is a red flag — mandates evaluation for SJS/TEN

Lymphadenopathy + hepatosplenomegaly → DRESS workup

SCORTEN (SJS/TEN severity score): age >40, malignancy, HR >120, BSA detached >10%, BUN >28, glucose >252, bicarbonate <20 — predicts mortality

Cardiac exam: biphasic anaphylaxis can recur 1–72 h after initial resolution; observe accordingly

Always document BSA involvement and presence/absence of mucosal lesions — drives ICU vs burn unit triage

Diagnostic Workup — Initial Labs and Acute Testing

— Serum tryptase drawn 15 min–3 h after symptom onset; elevation supports mast cell activation

— Baseline tryptase 24 h later or weeks out; persistently elevated >11.4 ng/mL → evaluate for mastocytosis (consider KIT D816V)

— Total tryptase >20% above baseline + 2 ng/mL supports anaphylaxis

— CBC with differential (eosinophilia >700 or atypical lymphocytes suggests DRESS)

— CMP — transaminitis, creatinine, electrolytes

— Urinalysis — proteinuria, eosinophiluria (AIN)

— LDH, CK, lipase if multi-organ

— ESR, CRP

— Hepatitis panel and HHV-6/EBV/CMV PCR if DRESS suspected (viral reactivation is part of pathophysiology)

— Direct antiglobulin (Coombs) test for hemolytic anemia

— Peripheral smear, reticulocyte count, haptoglobin, LDH, indirect bilirubin

— Heparin-induced thrombocytopenia: 4T score → PF4-heparin ELISA → serotonin release assay confirmation

Key distinction: Tryptase elevation supports mast cell-mediated reaction (IgE Type I or direct activation like vancomycin/opioids) but does not distinguish allergic from non-allergic anaphylaxis — both warrant the same acute management; specific etiology is sorted out later with skin testing.

Most drug allergy diagnoses are clinical; labs confirm severity, identify organ involvement, and exclude mimics

Acute anaphylaxis labs (optional, do not delay treatment):

Delayed/SCAR workup — order all:

Drug-induced cytopenias (Type II):

Type III (serum sickness, vasculitis): C3/C4 (low), CIC, ANA, ANCA, urine for hematuria

Imaging: CXR if respiratory symptoms; skin biopsy if SCAR phenotype unclear (DRESS vs AGEP vs SJS shows distinct histology)

Drug exposure timeline reconstruction is itself diagnostic — chart all medications including OTC, herbals, contrast, recent vaccines

Diagnostic Workup — Advanced and Confirmatory Studies

— Skin prick test (SPT) first; if negative, intradermal test (IDT)

— Validated reagents exist for penicillin (PrePen — benzylpenicilloyl polylysine — plus penicillin G), insulin, chymopapain, neuromuscular blockers, latex

— Negative penicillin SPT + IDT + oral amoxicillin challenge has >99% negative predictive value

— Hold antihistamines 5–7 days before testing

— Do NOT skin test during active reaction or while on β-blockers (epinephrine resistance if anaphylaxis)

— Gold standard when skin testing unavailable or unreliable

— Used when pretest probability of true allergy is low

— Typically 1/10 then full dose, observe 60 min between

— Distinct from desensitization — challenge confirms tolerance; desensitization induces temporary tolerance in known allergic patients

— Specific IgE (ImmunoCAP) for penicillin, amoxicillin, cefaclor, suxamethonium, chlorhexidine — moderate sensitivity, useful when skin testing contraindicated

— Basophil activation test (BAT) — research/specialty settings

— Patch testing for contact dermatitis, AGEP, fixed drug eruption, mild DRESS (read at 48 and 96 h)

— Delayed-read IDT for some T-cell reactions

— Lymphocyte transformation test — specialty centers

— HLA-B*57:01 before abacavir (mandatory)

— HLA-B*15:02 before carbamazepine in patients of Han Chinese, Thai, Malay descent

— HLA-B*58:01 before allopurinol in Korean, Han Chinese, Thai populations

— HLA-A*31:01 for carbamazepine in European/Japanese

Board pearl: Never skin-test or rechallenge a patient with prior SJS/TEN, DRESS, AGEP, drug-induced hemolytic anemia, or serum sickness — these are absolute contraindications to both challenge and desensitization; the drug must be avoided lifelong.

Definitive drug allergy testing is done after the acute reaction resolves — typically 4–6 weeks later for IgE-mediated, longer for delayed

Skin testing for IgE-mediated reactions:

Drug provocation (graded challenge):

In vitro testing:

Delayed reaction testing:

HLA pharmacogenomic screening BEFORE prescribing:

Risk Stratification and Management Logic

— Step 1: Reclassify the reaction (true allergy vs intolerance vs unknown)

— Step 2: Assess severity (SCAR? anaphylaxis? mild rash?)

— Step 3: Determine necessity (is there a safe alternative of equal efficacy?)

— Step 4: Choose pathway — avoidance, alternative, test/delabel, graded challenge, premedication, or desensitization

— Low: GI upset, isolated headache, remote childhood rash without details, family history only → direct oral amoxicillin challenge in clinic

— Moderate: benign cutaneous reaction >10 years ago, no anaphylaxis → skin testing + challenge

— High: anaphylaxis, recent reaction, multiple drug allergies → formal allergist testing or desensitization if drug required

— SCAR history → permanent avoidance

— Penicillin ↔ cephalosporin cross-reactivity is ~2% overall, higher with shared R1 side chains (amoxicillin–cefadroxil, ampicillin–cephalexin)

— Cefazolin has unique side chain → very low cross-reactivity with penicillins

— Aztreonam shares side chain with ceftazidime only

— Carbapenems: cross-reactivity with penicillins <1%

— Sulfonamide antibiotics do not cross-react with non-antibiotic sulfonamides (thiazides, furosemide, celecoxib)

Step 3 management: A pregnant woman with syphilis and reported penicillin allergy must receive penicillin — there is no equivalent alternative for fetal treatment. Admit, confirm allergy history, and perform inpatient penicillin desensitization, then give benzathine penicillin G — this is a classic Step 3 vignette.

Decision pathway for a patient with reported drug allergy needing that drug class:

Pretest probability tiers for penicillin allergy:

Cross-reactivity essentials:

Desensitization indication: patient has confirmed or strongly suspected IgE-mediated allergy AND the drug is the only effective option (e.g., syphilis in pregnancy needing penicillin, neurosyphilis, aspirin for CAD/AERD, chemotherapy)

Pharmacotherapy — Acute Treatment and Premedication

— Epinephrine 0.3–0.5 mg IM (1:1000) in anterolateral thigh, repeat every 5–15 min as needed

— Place supine with legs elevated unless respiratory distress

— High-flow O₂, IV access ×2 large bore, isotonic crystalloid 1–2 L bolus for hypotension

— H1 antihistamine (diphenhydramine 25–50 mg IV or cetirizine 10 mg)

— H2 antihistamine (famotidine 20 mg IV)

— Corticosteroid (methylprednisolone 60–125 mg IV) — does not prevent biphasic reaction reliably but standard adjunct

— Bronchospasm: nebulized albuterol

— Refractory hypotension: epinephrine infusion, vasopressin if needed

— Glucagon 1–5 mg IV for patients on β-blockers with refractory hypotension

— Contrast media protocol (Greenberger): prednisone 50 mg PO at 13, 7, and 1 h pre-procedure + diphenhydramine 50 mg 1 h pre — for prior contrast reaction

— Use low-osmolar or iso-osmolar contrast and a different agent than the one that caused prior reaction

— DRESS: stop offending drug, systemic corticosteroids (prednisone 1 mg/kg/day taper over 8–12 weeks), supportive care; cyclosporine if refractory

— SJS/TEN: burn-unit-level supportive care, fluid/electrolytes, ophthalmology, wound care; cyclosporine or etanercept may reduce mortality; IVIG controversial; corticosteroids debated

— AGEP: stop drug, topical steroids, usually self-limited

Key distinction: Premedication is appropriate for non-IgE (anaphylactoid) reactions like radiocontrast or vancomycin — it does not reliably prevent true IgE-mediated anaphylaxis, which requires desensitization instead.

Anaphylaxis treatment algorithm:

Observation: minimum 4–6 hours for mild reactions, up to 24 h for severe or biphasic-risk reactions

Discharge with two epinephrine auto-injectors, action plan, allergist referral, MedicAlert

Premedication protocols (do NOT prevent true anaphylaxis but reduce non-IgE/anaphylactoid reactions):

SCAR treatment:

Desensitization — Principles and Protocols

— Tolerance is temporary — lost within 24–48 h after the last dose; must restart desensitization for future courses

— Performed in monitored setting (ICU or step-down) with IV access, epinephrine, and physician at bedside

— Hold β-blockers and ACE inhibitors if possible

— Skin testing before desensitization is not required and does not prevent reactions

— Premedicate with H1/H2 antihistamines ± steroid

— Contraindicated in SJS/TEN, DRESS, AGEP, hemolytic anemia, serum sickness, vasculitis — these are non-mast-cell, antibody/T-cell driven

— Penicillin oral desensitization: start ~100 units, double every 15 min over ~4–5 hours to therapeutic dose; preferred for pregnant syphilis patients

— Penicillin IV desensitization: similar dose-doubling over 4–6 h

— Aspirin desensitization for AERD: start 20–40 mg, escalate to 325 mg over 1–2 days; continue daily aspirin lifelong (loss of tolerance if missed >48 h)

— Chemotherapy (platinums, taxanes, monoclonals): 12- or 16-step protocols using three increasing concentrations over 6 h

— TMP-SMX in HIV patients with prior mild rash: 5-day escalating oral protocol

— Syphilis in pregnancy or neurosyphilis (penicillin)

— AERD with CAD or rhinosinusitis needing aspirin

— Only effective chemotherapy in oncology patients

— Cystic fibrosis with multidrug-resistant infections needing β-lactams

— HIV needing TMP-SMX prophylaxis

CCS pearl: Order "transfer to ICU for penicillin desensitization" with continuous monitoring, then the desensitization protocol, followed by benzathine penicillin G 2.4 million units IM — partial credit is lost if you give penicillin without the desensitization step in an allergic patient.

Desensitization = induction of temporary clinical tolerance in an IgE- or non-IgE-sensitized patient by gradually escalating drug exposure, allowing safe administration

Mechanism: subthreshold doses cross-link mast cell IgE without triggering degranulation; internalizes receptors, depletes mediators, and induces antigen-specific unresponsiveness

Key principles:

Standard protocols:

Indications:

Special Populations — Elderly and Renal/Hepatic Impairment

— Polypharmacy increases attributable risk and complicates causality assessment

— Beers Criteria: many high-risk drugs (sulfonylureas, NSAIDs, anticholinergics) cause ADRs mistaken for allergy

— Reduced epinephrine tolerance: use standard 0.3 mg IM but anticipate cardiac ischemia, arrhythmia; have cardiac monitoring

— β-blocker use is common → glucagon must be available for refractory anaphylaxis

— Skin testing reliability decreases with age due to reduced skin reactivity — false negatives possible; favor in vitro IgE or graded challenge

— Higher mortality with SJS/TEN (SCORTEN uses age >40 as risk factor)

— Many drugs (vancomycin, β-lactams, sulfonamides, allopurinol) accumulate and increase ADR risk

— Allopurinol-induced SCAR risk rises with CKD — start at 100 mg/day or less and titrate; screen HLA-B*58:01 in high-risk ethnicities

— Iodinated contrast — risk for both contrast-induced nephropathy and hypersensitivity; use iso-osmolar, hydrate, hold metformin if eGFR <30

— Antihistamines: cetirizine/levocetirizine renally cleared, reduce dose; fexofenadine preferred in severe CKD

— Desensitization protocols generally do not require dose adjustment (full therapeutic target dose still given), but final maintenance dose must be renally adjusted

— DRESS commonly involves liver — hepatitis can be fulminant; transplant evaluation if INR rising

— Avoid additional hepatotoxins during recovery

— Drugs metabolized by CYP3A4 (e.g., many chemotherapy agents needing desensitization) may need dose reduction

— Acetaminophen safe at ≤2 g/day in cirrhosis if needed for premedication

Step 3 management: In an elderly patient on a β-blocker presenting with anaphylaxis and refractory hypotension despite IM epinephrine and fluids, administer glucagon 1–5 mg IV — it bypasses the β-receptor by directly activating adenylate cyclase via a Gs-coupled glucagon receptor.

Elderly patients:

Renal impairment:

Hepatic impairment:

Special Populations — Pregnancy and Pediatrics

— Syphilis in pregnancy with penicillin allergy = mandatory penicillin desensitization; no alternative is fetally adequate (doxycycline contraindicated, erythromycin doesn't cross placenta well, ceftriaxone data limited)

— Anaphylaxis treatment: epinephrine IM is first-line — fetal risk of untreated maternal anaphylaxis far exceeds epinephrine risk

— Left lateral decubitus position after 20 weeks to relieve IVC compression

— Avoid teratogenic premedications when alternatives exist; diphenhydramine and prednisone are acceptable

— GBS prophylaxis in penicillin-allergic women: cefazolin if low-risk allergy; clindamycin only if susceptibility confirmed; vancomycin if SCAR history

— Most childhood "amoxicillin allergy" labels are viral exanthems (especially EBV-associated rash with amoxicillin) — direct oral amoxicillin challenge in outpatient clinic is now first-line for low-risk reactions per AAP/AAAAI 2022 guidance

— No prior skin testing needed for low-risk pediatric penicillin delabeling

— Children with confirmed allergy: prescribe epinephrine auto-injector:

— 0.1 mg dose for 7.5–15 kg

— 0.15 mg for 15–30 kg

— 0.3 mg for >30 kg

— School action plan required

— HLA-B*15:02 screening before carbamazepine in at-risk Asian pediatric populations

Board pearl: A 4-year-old with a remote history of "amoxicillin rash" but no urticaria, angioedema, or anaphylaxis can undergo a direct oral amoxicillin challenge in the pediatrician's office — no skin testing, no allergist referral required. Delabeling is high-yield.

Pregnancy:

Pediatrics:

Breastfeeding: most antihistamines and steroids are compatible; second-generation H1 (loratadine, cetirizine) preferred over diphenhydramine which can reduce milk supply

Vaccines: egg allergy is no longer a contraindication to influenza vaccine (give standard dose with observation); MMR safe in egg allergy; yellow fever still requires precaution

Complications and Adverse Outcomes

— Death from anaphylaxis — mortality 0.7–2% of hospitalized cases; higher with delayed epinephrine

— Biphasic anaphylaxis: 5% of cases, symptoms recur 1–72 h after resolution

— Protracted anaphylaxis: hours of refractory shock

— Airway loss from laryngeal edema

— Stress cardiomyopathy / Kounis syndrome (allergic ACS)

— SJS/TEN mortality: SJS ~10%, SJS/TEN overlap ~30%, TEN >30%; sepsis from skin loss, fluid/electrolyte derangement, ARDS

— Long-term SJS/TEN sequelae: chronic ocular complications (symblepharon, dry eye, blindness — 30–50%), nail dystrophy, vulvovaginal/urethral stenosis, post-inflammatory pigmentation, chronic lung disease

— DRESS: acute hepatitis (most common cause of death), myocarditis, nephritis, pneumonitis, encephalitis; late autoimmune sequelae months later — thyroiditis, type 1 diabetes, autoimmune hemolytic anemia, lupus

— Acute interstitial nephritis (β-lactams, PPIs, NSAIDs) — eosinophiluria, sterile pyuria, AKI

— Drug-induced liver injury (DILI) — RUCAM score for causality

— Drug-induced hemolytic anemia, thrombocytopenia, neutropenia

— Drug-induced lupus (hydralazine, procainamide, isoniazid, anti-TNF) — anti-histone antibodies

— Drug-induced vasculitis (hydralazine, propylthiouracil) — anti-MPO ANCA

— Penicillin allergy label associated with 30% more SSIs, 14% more C. difficile, 30% more MRSA, longer LOS, higher mortality in surgical patients

— Use of vancomycin, fluoroquinolones, clindamycin as substitutes drives resistance and toxicity

— Estimated US cost: billions annually

Key distinction: DRESS has a multi-week onset, eosinophilia, facial edema, lymphadenopathy, organ involvement, and is associated with viral reactivation (HHV-6), with late autoimmune sequelae. SJS/TEN is a mucocutaneous emergency with epidermal detachment and Nikolsky sign. Treatment, prognosis, and follow-up differ markedly.

Acute complications:

SCAR-specific complications:

Drug-induced organ injury:

Healthcare system harms from over-labeling:

When to Escalate Care — ICU, Consults, and Triage

— Mild urticaria, drug stopped, no systemic features → discharge with 3–5 day antihistamines, allergist follow-up

— Moderate reaction (extensive urticaria, mild bronchospasm responding to treatment) → 4–6 h observation

— Anaphylaxis → minimum 6 h observation; 12–24 h if severe, biphasic risk factors, β-blocker use, or asthma history

— Required epinephrine ≥2 doses

— Persistent hemodynamic instability

— Airway involvement

— Suspected SCAR (any SJS/TEN, DRESS, AGEP)

— Drug-induced cytopenias, hepatitis, nephritis with organ dysfunction

— Planned desensitization

— Need for epinephrine infusion or vasopressors

— Intubation or impending airway loss

— TEN with detachment >10% BSA (or burn unit/specialty SCAR center)

— DRESS with multi-organ failure

— Active desensitization in high-risk patient (cardiac, severe asthma)

— Allergy/Immunology — all SCARs, multiple drug allergies, planned desensitization, ambiguous histories, AERD workup

— Dermatology — biopsy of suspected SCAR, severe drug eruptions

— Ophthalmology — urgently in SJS/TEN (prevent symblepharon)

— Pharmacy — alternative regimen selection, desensitization protocol design

— ID — when antibiotic alternatives are limited (CF, MDR infections)

— Burn center transfer for TEN with extensive detachment

— All anaphylaxis patients: allergist within 4 weeks

— All SCAR survivors: allergist + specialty follow-up (derm, ophtho) at 2 weeks, then 3 and 6 months

— Provide written action plan and EHR allergy list update at discharge

CCS pearl: When charting a patient with anaphylaxis, update the allergy list with the specific drug and reaction phenotype (e.g., "amoxicillin — anaphylaxis 2024") — vague labels like "PCN — allergy" propagate harm. Also order outpatient allergist referral before ending the case for credit.

ED disposition:

Inpatient admission triggers:

ICU criteria:

Consultations:

Outpatient referral cadence:

Key Differentials — Same-Category Immunologic Mimics

— Vancomycin infusion reaction ("red person syndrome"): flushing, pruritus, hypotension during rapid infusion; direct MRGPRX2-mediated mast cell degranulation; slow infusion to >60 min, antihistamine premed — not a true allergy, do not relabel

— Opioids (morphine, codeine): direct mast cell histamine release — urticaria/flushing common; fentanyl rarely causes this and is safe alternative

— Radiocontrast media: most reactions non-IgE; use premedication protocol and alternative agent

— NSAIDs causing urticaria/angioedema: COX-1 inhibition shifts arachidonate to leukotrienes (pseudo-allergy); often cross-reactive across all COX-1 inhibitors; selective COX-2 inhibitors (celecoxib) typically tolerated

— ACE inhibitor angioedema — onset can be years into therapy; treat with airway support, icatibant or C1 inhibitor concentrate if severe; antihistamines/epinephrine ineffective; switch to ARB cautiously (lower but not zero risk) or avoid RAAS

— Hereditary angioedema (C1 esterase inhibitor deficiency) — low C4, recurrent without urticaria, family history

— True (Type III): immune complex, low complement, biologic agents (anti-thymocyte globulin, rituximab, infliximab)

— Serum sickness-like: cefaclor, minocycline, normal complement

Key distinction: ACE inhibitor angioedema does not respond to epinephrine, antihistamines, or steroids because it is bradykinin-mediated, not histamine. The drug must be permanently discontinued and the allergy list must specify "ACEi-induced angioedema" — patient should never be rechallenged or switched within class.

Within drug hypersensitivity, several entities mimic IgE-mediated allergy but require different management:

Non-IgE mast cell activation (anaphylactoid):

Aspirin-exacerbated respiratory disease (AERD / Samter triad): asthma + nasal polyps + aspirin/NSAID sensitivity; treated with aspirin desensitization for chronic disease control

Bradykinin-mediated angioedema:

Serum sickness vs serum sickness-like reaction:

Drug fever alone — diagnosis of exclusion; resolves 72 h after stopping drug

Key Differentials — Non-Immunologic and Other Mimics

— Viral exanthems (EBV, CMV, HHV-6, parvovirus, measles, COVID) — particularly EBV + amoxicillin producing florid morbilliform rash that is not a true allergy

— Scarlet fever, toxic shock syndrome — rash + systemic illness during antibiotic therapy

— Rocky Mountain spotted fever, meningococcemia — petechiae, fever

— Acute generalized eruption of new-onset lupus, Still disease (fever + salmon rash + arthritis)

— Cutaneous vasculitis from infection, malignancy

— Mast cell disease / mastocytosis — recurrent anaphylaxis to multiple unrelated triggers, elevated baseline tryptase, KIT D816V mutation

— Niacin flush — vasodilation, predictable, premedicate with aspirin

— Disulfiram-like reactions (metronidazole, cefotetan + alcohol)

— Serotonin syndrome — drug interaction, not allergy

— Codeine nausea, statin myalgia, metformin diarrhea, SSRI sexual side effects — all pharmacologic, not immunologic

Board pearl: A young child develops a diffuse morbilliform rash on day 7 of amoxicillin for "tonsillitis," with massive cervical lymphadenopathy and splenomegaly — this is EBV mononucleosis, not a drug allergy. Check heterophile antibody/EBV serology and do not label penicillin allergic; the child can receive amoxicillin in the future.

Several conditions masquerade as drug allergy and must be excluded before labeling:

Infectious mimics:

Autoimmune/inflammatory mimics:

Carcinoid syndrome — episodic flushing, diarrhea, wheeze; 5-HIAA testing

Pheochromocytoma — paroxysmal hypertension, palpitations, sweating; plasma metanephrines

Vocal cord dysfunction — inspiratory stridor mimicking anaphylactic laryngeal edema; flow-volume loop shows blunted inspiration

Panic attack / hyperventilation — paresthesias, dyspnea, no urticaria/angioedema; normal tryptase

Scombroid poisoning — histamine in spoiled fish (tuna, mahi); flushing/urticaria/GI symptoms minutes after ingestion; not allergy; outbreak pattern

Toxin and pharmacologic effects:

Drug intolerances (most common mislabel):

Secondary Prevention and Long-Term Plan

— Record specific drug name, exact reaction phenotype, date, treatment given, and outcome — not just "allergy"

— Update EHR allergy module across systems; reconcile at every encounter

— Distinguish "allergy," "intolerance," "contraindication," and "side effect" in the chart

— Provide written drug-avoidance list including cross-reactive agents and trade names

— MedicAlert bracelet for anaphylaxis or SCAR history

— Two epinephrine auto-injectors prescribed for anyone with anaphylaxis or risk thereof (food, insect, idiopathic, drug if re-exposure plausible); demonstrate technique; replace yearly

— Written anaphylaxis action plan; school/work copy for children

— Avoid β-blockers and ACE inhibitors when feasible in patients with recurrent anaphylaxis

— Inpatient and outpatient stewardship — pharmacist or allergist-led

— All penicillin allergy labels reviewed at hospital admission; low-risk patients challenged on the spot

— Particularly valuable preoperatively (cefazolin is preferred surgical prophylaxis)

— Genotype HLA-B*57:01 before abacavir (universal)

— HLA-B*15:02 before carbamazepine in Asian descent

— HLA-B*58:01 before allopurinol in high-risk groups

— TPMT/NUDT15 before thiopurines (not allergy but ADR prevention)

— Tolerance lost in 24–48 h — must redo desensitization for each new course

— Document the protocol used and any breakthrough reactions

Step 3 management: At hospital discharge after anaphylaxis, the action checklist is — (1) two epinephrine auto-injectors with technique demonstration, (2) written action plan, (3) allergist referral within 4 weeks, (4) EHR allergy update with specific reaction, (5) MedicAlert recommendation. Missing any of these is the typical wrong-answer trap.

Documentation discipline:

Patient education and self-management:

Delabeling programs:

Pharmacogenomic prevention:

For desensitized patients:

AERD management: post-aspirin desensitization, continue 325 mg aspirin daily indefinitely to maintain tolerance and control sinonasal disease

Follow-Up, Monitoring, and Counseling

— Primary care visit within 1–2 weeks for medication reconciliation and action plan reinforcement

— Allergist within 4 weeks for skin testing, in vitro IgE, and definitive identification

— Annual renewal of epinephrine auto-injectors; review expiration dates

— Reassess β-blocker/ACEi necessity at each visit

— Dermatology — wound healing, pigmentation, scarring

— Ophthalmology — every 3–6 months for first 2 years (chronic dry eye, symblepharon, vision loss in SJS/TEN)

— Pulmonology if pneumonitis or bronchiolitis obliterans

— Renal/hepatic labs every 1–3 months until normalized

— DRESS-specific autoimmune surveillance: TSH, fasting glucose, CBC, ANA at baseline, 3 months, 6 months, 12 months, then annually for 2–5 years — autoimmune thyroid disease and type 1 DM may emerge months later

— Mental health screening — PTSD common after ICU/burn unit course

— During: continuous cardiac, BP, SpO₂; trained staff with epinephrine ready

— Document any breakthrough symptoms and treatment given

— Subsequent doses must be uninterrupted; missed doses >24–48 h forfeit tolerance

— ENT every 3–6 months for polyp surveillance

— Pulmonology for asthma control (ACT score)

— Reinforce daily aspirin adherence

— Always disclose drug allergies to every clinician, dentist, pharmacist

— Read OTC and combination product labels (e.g., acetaminophen + opioid)

— Travel: carry medications, action plan, translated allergy card

— Vaccinations are generally safe; egg allergy is no longer a barrier for influenza

— Family screening for HLA risk alleles when relevant (allopurinol, carbamazepine, abacavir)

Board pearl: A patient 4 months post-DRESS due to phenytoin develops new fatigue, weight gain, and TSH of 38 — this is autoimmune thyroiditis as a late sequela of DRESS, not recurrence. Initiate levothyroxine and continue scheduled autoimmune surveillance.

Post-anaphylaxis follow-up:

SCAR survivor follow-up:

Desensitization monitoring:

AERD follow-up post aspirin desensitization:

Counseling points:

Ethical, Legal, and Patient Safety Considerations

— Disclose that the procedure carries real anaphylaxis risk (typically 1–5% mild breakthrough; severe rare with proper monitoring)

— Explain tolerance is temporary and must be repeated

— Discuss alternatives, including drug avoidance with inferior outcomes

— Document patient's understanding and signed consent; in CCS, "obtain informed consent" is an explicit order

— A pregnant woman refusing penicillin desensitization for syphilis: respect autonomy after thorough counseling on fetal congenital syphilis risk; involve ethics consult, social work, MFM; document the conversation

— Pediatric patients: parental consent + age-appropriate assent

— Report serious or unexpected ADRs to FDA MedWatch

— Mandatory reporting of vaccine-associated reactions to VAERS

— Hospital incident reporting for medication errors that caused or risked harm

— Allergy lists must be reconciled at every transition — admission, transfer, discharge

— Communicate specific reaction details to receiving clinicians, not just the drug name

— After ED visit for anaphylaxis: faxed/electronic summary to PCP within 48 h; ensure follow-up appointment is scheduled before discharge, not merely advised

— Pharmacy reconciliation at discharge to confirm avoidance of prior offending drug and cross-reactive agents

— Penicillin allergy labels disproportionately affect women and lead to inferior antibiotic stewardship outcomes — proactive delabeling is an equity intervention

— Access to allergists is limited; primary care–based oral challenge protocols expand access

— HLA screening cost and availability may create disparities; institutional protocols help

— Failing to ask about allergies before prescribing is a frequent malpractice claim

— Documenting "NKDA" requires actual inquiry

— Re-administering a drug to a labeled patient without delabeling protocol → liability exposure

Step 3 management: A patient is admitted from another hospital with "penicillin allergy — anaphylaxis" listed; before any β-lactam order, you must personally re-interview, reconcile the allergy list, and document the specific reaction — Step 3 commonly tests this transition-of-care reconciliation as the correct next step.

Informed consent for desensitization:

Capacity and surrogate decision-making:

Mandatory reporting and pharmacovigilance:

Transition-of-care safety (a recurring Step 3 focus):

Health systems and equity:

Medicolegal:

High-Yield Associations and Rapid-Fire Facts

— Abacavir → HLA-B*57:01 hypersensitivity (fever, rash, GI, respiratory) — screen before use

— Carbamazepine → HLA-B*15:02 SJS/TEN in Asians

— Allopurinol → HLA-B*58:01 SCAR; risk amplified by CKD

— Sulfonamide antibiotics → SJS/TEN, hemolysis in G6PD

— Phenytoin → DRESS, gingival hyperplasia

— Lamotrigine → SJS/TEN (especially with rapid titration or valproate co-use)

— Vancomycin → DRESS, linear IgA bullous dermatosis, red-person syndrome

— Heparin → HIT (Type II)

— Methyldopa, penicillin (high-dose), cephalosporins → autoimmune hemolytic anemia

— Quinine, sulfonamides → thrombocytopenia

— Hydralazine, procainamide → drug-induced lupus (anti-histone)

— Propylthiouracil, hydralazine → ANCA vasculitis

— Minocycline → drug-induced lupus, hyperpigmentation, DRESS

— NSAIDs → AERD, AIN, pseudo-allergic urticaria

— ACE inhibitors → bradykinin angioedema

— Cefaclor → serum sickness-like reaction in children

— Anti-thymocyte globulin → serum sickness

— Iodinated contrast → anaphylactoid reaction

— Platinum chemotherapies → IgE allergy after several cycles (cycle 7+ for carboplatin)

— Penicillin ↔ cephalosporin: ~2%; shared R1 side chain raises risk

— Penicillin ↔ carbapenem: <1%

— Penicillin ↔ monobactam (aztreonam): essentially none (except ceftazidime-aztreonam shared R1)

— Sulfonamide antibiotics ↮ non-antibiotic sulfonamides (no clinical cross-reactivity)

— NSAIDs in AERD: all COX-1 inhibitors cross-react; celecoxib usually tolerated

Key distinction: Anaphylactoid (direct mast cell, no prior sensitization needed — e.g., vancomycin, contrast, opioids) is prevented with slow infusion + premedication; true IgE anaphylaxis (requires prior sensitization) is prevented with avoidance or desensitization.

Drug → classic reaction associations:

Cross-reactivity quick recall:

Tryptase pearls: elevated baseline → mastocytosis; transient peak supports anaphylaxis

Mnemonic for SCAR red flags: Fever, Facial edema, Mucosal involvement, Skin pain, Targetoid/bullous lesions, Eosinophilia, Organ dysfunction

Board Question Stem Patterns

Board pearl: When the stem says "the only effective drug for this patient's condition" + "known IgE-mediated allergy," the answer is almost always desensitization, not avoidance or premedication.

Stem 1 — Pregnant syphilis: "28-year-old G2P1 at 22 weeks, RPR 1:64, reactive treponemal test, reports prior penicillin-induced urticaria." Next step: admit and perform penicillin desensitization, then benzathine penicillin G 2.4 million U IM. Distractors: doxycycline (teratogen), erythromycin (inadequate fetal levels), ceftriaxone (insufficient evidence).

Stem 2 — Child with amoxicillin rash: 5-year-old with morbilliform rash on day 8 of amoxicillin for otitis, no urticaria, mucosal lesions, or systemic features. Best next step: label as benign exanthem; future direct oral amoxicillin challenge; do not avoid penicillins lifelong.

Stem 3 — Surgical prophylaxis: Patient labeled "penicillin allergic — itching only" needs prosthetic joint surgery. Best prophylaxis: cefazolin (negligible cross-reactivity due to unique side chain). Distractor: vancomycin (only if SCAR or anaphylaxis history).

Stem 4 — Anaphylaxis on a β-blocker: Persistent hypotension despite IM epinephrine ×3 and IV fluids. Next step: glucagon 1–5 mg IV.

Stem 5 — ACE inhibitor angioedema: Tongue swelling, no urticaria, on lisinopril 3 years. Epinephrine and antihistamines ineffective. Best management: secure airway, discontinue ACEi permanently, consider icatibant; do not switch to another ACEi; avoid future class use.

Stem 6 — DRESS: 3 weeks after starting allopurinol for gout, fever 39, facial swelling, diffuse rash, eosinophils 1800, AST 320, Cr 2.1. Diagnosis: DRESS; management: stop allopurinol immediately, prednisone 1 mg/kg, monitor for HHV-6 reactivation, schedule 12-month autoimmune surveillance.

Stem 7 — SJS/TEN: Day 14 of lamotrigine, painful skin, oral and conjunctival erosions, Nikolsky positive, >20% BSA detachment. Action: stop drug, transfer to burn unit, ophthalmology consult, supportive care; do not desensitize ever.

Stem 8 — Vancomycin red person syndrome: Flushing during 30-min vancomycin infusion. Management: slow infusion to >60 min, antihistamine premedication; continue vancomycin; do not label allergy.

Stem 9 — Contrast premedication: Prior moderate contrast reaction needs CT with contrast urgently. Order: Greenberger protocol (prednisone 50 mg at 13, 7, 1 h + diphenhydramine 50 mg 1 h pre) and use iso-osmolar agent.

Stem 10 — AERD: Asthma + nasal polyps + aspirin-induced bronchospasm now needs aspirin for CAD. Plan: aspirin desensitization, then daily 325 mg indefinitely.

One-Line Recap

Drug allergy is an immunologically mediated subset of adverse drug reactions whose management hinges on accurate reaction phenotyping, evidence-based delabeling of low-risk patients, lifelong avoidance after severe cutaneous adverse reactions, and structured desensitization when a sensitized patient absolutely requires the offending drug.

Step 3 management: The single most exam-relevant sentence — when a known penicillin-allergic patient must receive penicillin (syphilis in pregnancy, neurosyphilis, endocarditis with no alternative), the answer is inpatient desensitization, then full-dose therapy, not avoidance, not switching to doxycycline, and not premedication.

Phenotype first, then act: distinguish IgE-mediated (immediate, mast cell, tryptase, treat with epinephrine, candidate for desensitization) from T-cell–mediated SCARs (DRESS, SJS/TEN, AGEP — never rechallenge, never desensitize) from non-immunologic reactions (anaphylactoid, bradykinin angioedema, intolerance) — each pathway has a distinct therapeutic answer.

Delabel aggressively: 95% of "penicillin allergy" labels are inaccurate; low-risk histories warrant direct oral amoxicillin challenge in clinic — delabeling reduces MRSA, C. difficile, surgical site infections, length of stay, and cost while improving stewardship and equity.

Desensitization principles: indicated when the offending drug is uniquely necessary (pregnant syphilis, AERD needing aspirin, only effective chemotherapy); requires monitored setting, premedication, dose-doubling protocol; tolerance is temporary (24–48 h); absolutely contraindicated in SJS/TEN, DRESS, AGEP, hemolytic anemia, and serum sickness.

Step 3 execution checklist for every drug-allergy encounter: treat acute reaction (IM epinephrine first for anaphylaxis), document specific reaction details in EHR, reconcile allergy list at every transition of care, prescribe two epinephrine auto-injectors with action plan when indicated, refer to allergist within 4 weeks, surveil for late autoimmune sequelae after DRESS, and apply HLA pharmacogenomic screening before abacavir, carbamazepine, and allopurinol in appropriate populations.