Recognizing Acyclovir Reactions
Patients on acyclovir, an antiviral for herpes infections, may experience reactions like rash, nausea, or rare severe hypersensitivity. Testing starts with clinical history: ask about prior exposures, symptoms onset (e.g., hives, swelling within hours), and timing relative to dosing. Immediate reactions suggest IgE-mediated allergy; delayed ones point to T-cell issues.
Skin Testing for Hypersensitivity
Intradermal skin testing is primary for confirming allergy:
- Dilute acyclovir (e.g., 0.1-1 mg/mL) injected into forearm skin.
- Observe for wheal/flare in 15-30 minutes (positive if >3 mm larger than control).
- Prick testing first with undiluted drug if available.
Allergists perform this in controlled settings due to anaphylaxis risk (1-5% false positives reported in case studies). Negative tests don't fully rule out allergy.
Lab Tests for Confirmation
- Serum tryptase: Draw during acute reaction; elevated (>2x baseline) indicates mast cell activation.
- Specific IgE: Rarely available for acyclovir; research assays exist but not routine.
- Basophil activation test (BAT): Measures CD63/63 expression on basophils exposed to drug; sensitivity ~70% in drug allergy studies, used in specialized labs.
Patch testing for delayed reactions: apply acyclovir 1-10% in petrolatum, read at 48-96 hours.
Oral Challenge for Low-Risk Cases
If skin/lab tests negative and history mild:
- Supervised graded challenge: start with 1/100th dose, double every 30-60 minutes to full dose.
- Monitor vitals, ready epinephrine.
Avoid in severe reaction history (e.g., anaphylaxis, SJS/TEN).
When to Test and Alternatives
Test only if reaction suspected and acyclovir essential (e.g., HSV encephalitis). Desensitization protocols exist for confirmed allergies: 12-16 step IV infusion over hours, up to 95% success. Alternatives like valacyclovir may cross-react (80-90%); famciclovir less so.
[1]: AAAAI/ACAAI Drug Allergy Practice Parameters
[2]: JACI: Acyclovir Hypersensitivity Testing
[3]: UpToDate: Antiviral Drug Allergies