What is the proper procedure for performing a penicillin skin test, including controls, reagents, prick and intradermal steps, and post‑test monitoring?

Penicillin Skin Testing Procedure

Penicillin skin testing should be performed using a standardized two-step approach: first apply full-strength reagents via prick/puncture technique, and if negative, proceed to intradermal testing with appropriate controls, using a positive criterion of ≥3 mm wheal greater than negative control with ≥5 mm flare. 1

Pre-Test Requirements

Personnel and Safety Preparedness

• Testing must only be performed by personnel trained and skilled in application and interpretation of penicillin skin testing 1
• Have immediate access to anaphylaxis treatment equipment and medications, though systemic reactions occur in <2% of skin test-positive patients 1

Patient Selection

Penicillin skin testing is indicated for:

• Patients with history of anaphylaxis to penicillin 1
• Recent reactions suspected to be IgE-mediated 1
• Patients requiring penicillin when direct oral challenge is not appropriate 2

Required Reagents

Major Determinant

• Benzylpenicilloyl polylysine (PPL/Pre-Pen): Use commercially available premixed 6 × 10⁻⁵ mol/L solution 1

Minor Determinants

• Penicillin G: 10,000 units/mL (use intravenous solution off-label) 1
• Penicilloate and penilloate: 0.01 mol/L concentration 1
  • These are not commercially available in the US but are under FDA review 1
  • Research shows 23% of positive reactions occur exclusively to penicilloate and 3% exclusively to penilloate in hospitalized patients 3
  • Do NOT use "aged penicillin" as a substitute—it does not spontaneously degrade to form minor determinants 1

Culprit Drug (if applicable)

• Include the specific penicillin that caused the reaction if available in IV form (e.g., piperacillin-tazobactam) at nonirritating concentration 1
• For amoxicillin reactions: test with amoxicillin 20 mg/mL 2

Controls

• Positive control: Histamine 6 mg/mL for prick test; 0.02 mg/mL for intradermal 2
• Negative control: 0.9% saline 2

Testing Procedure

Step 1: Prick/Puncture Testing

1. Apply full-strength reagents to volar forearm or upper arm using appropriate testing device 1, 2
2. Test all reagents: major determinant, minor determinants, culprit drug (if applicable), positive control, and negative control
3. Wait 15-20 minutes and read results
4. Valid test requires: positive control shows wheal/flare AND negative control remains negative 1

Step 2: Intradermal Testing (if prick test negative)

1. Critical technique for reproducibility:
   • Fill syringe with larger volume (0.05-0.07 mL) first 1
   • Expel excess fluid and air bubbles to obtain exactly 0.02 mL 1
   • Inject to produce baseline 3-5 mm bleb 1
2. Administer on upper arm with same reagents as prick test (except use histamine 0.02 mg/mL concentration) 2
3. Wait 15-20 minutes and read results

Interpretation Criteria

Positive Test Definition

A positive test requires BOTH of the following 1:

• Wheal: ≥3 mm greater than negative control (for either prick or intradermal)
• Flare: ≥5 mm or greater

Important caveat: Some centers use >5 mm compared to negative control as the criterion, which results in 0.5% positive rate versus 1.0% with the ≥3 mm criterion—both allow the vast majority of patients to be delabeled 1

Negative Test

• Wheal <3 mm greater than negative control, OR
• Flare <5 mm
• Negative predictive value >95% 1

Post-Test Management

If Skin Test Negative

Following negative penicillin skin test results, perform an elective challenge with the offending penicillin that caused the historical reaction 1. This serves to:

• Reassure patient, family, and future prescribing clinicians 1
• Confirm true tolerance before future prescribing needs

If Skin Test Positive

• Patient has confirmed IgE-mediated penicillin allergy
• Avoid penicillins and consider alternative antibiotics
• Document allergy status clearly

Monitoring Duration

• During testing: Observe for immediate reactions at each step (15-20 minutes per test phase)
• Post-challenge: If oral challenge performed after negative skin test, observe minimum 60 minutes after final dose 2
• Follow-up: Contact patient at 5 days to assess for delayed adverse events 2

Key Performance Notes

Diagnostic Characteristics

• Skin testing has sensitivity 30.7%, specificity 96.8%—superior to serum-specific IgE (sensitivity 19.3%, specificity 97.4%) 1
• Negative predictive value >95% 1

Safety Profile

Penicillin skin testing using proper technique is safe: <2% of skin test-positive patients experience systemic reactions, and very few are anaphylactic 1