How to perform a penicillin G (Penicillin G) sensitivity test on a patient with a potential history of allergies or reactions to penicillin?

How to Perform Penicillin G Sensitivity Testing

Penicillin G sensitivity testing should be performed using sequential skin prick testing followed by intradermal testing with the major determinant (benzylpenicilloyl polylysine/Pre-Pen), penicillin G, and ideally the minor determinants (penicilloate and penilloate), conducted in a monitored setting equipped to manage anaphylaxis. 1

Patient Selection for Testing

Testing is indicated only for patients with a reported history of penicillin allergy who require penicillin therapy—routine testing in patients without a history of penicillin allergy is not recommended due to the extremely low rate of anaphylaxis in the general population. 1, 2

• Only 10-15% of patients who report penicillin allergy are truly allergic, and less than 5% have confirmed allergy with appropriate testing. 1, 3, 4
• The prevalence of positive skin tests decreases over time: 93% test positive 7-12 months after a reaction, but only 22% remain positive 10 years or more after the reaction. 5

Pre-Testing Preparation

Antihistamines must be discontinued before testing to avoid false-negative results: 1, 2

• Chlorpheniramine or terfenadine: 24 hours before testing
• Diphenhydramine or hydroxyzine: 4 days before testing
• Astemizole: 3 weeks before testing

High-risk patients require special precautions: 1, 6

• Patients with a history of penicillin-related anaphylaxis, asthma, or those on beta-blockers should be tested with 100-fold dilutions of full-strength reagents before proceeding to full-strength testing
• All testing must be performed in a facility equipped to treat anaphylaxis with trained personnel and immediate access to epinephrine, oxygen, intravenous steroids, and airway management equipment 1, 2, 7

Required Skin Testing Reagents

The complete battery of reagents includes: 1, 6

Major Determinant:

• Benzylpenicilloyl poly-L-lysine (Pre-Pen): 6 × 10⁻⁵ M

Minor Determinants:

• Penicillin G: 10⁻² M (3.3 mg/mL or 6000 U/mL)
• Benzylpenicilloate: 10⁻² M (3.3 mg/mL)
• Benzylpenilloate (or penicilloyl propylamine): 10⁻² M (3.3 mg/mL)

Controls:

• Positive control: Commercial histamine (1 mg/mL)
• Negative control: Diluent (usually phenol saline)

Important caveat: Testing with only Pre-Pen and penicillin G detects 90-97% of allergic patients, but misses 3-10% who are positive only to minor determinants. 1 Research shows that 64% of hospitalized patients with positive tests react to compounds other than the major determinant, with 23% reacting exclusively to penicilloate and 3% exclusively to penilloate. 8

Step-by-Step Testing Procedure

Step 1: Epicutaneous (Prick) Testing

Always perform prick testing first to minimize risk of systemic reactions: 1, 6, 9

1. Place duplicate drops of each skin-test reagent on the volar surface of the forearm
2. Pierce the underlying epidermis with a 26-gauge needle without drawing blood
3. Wait 15 minutes and measure wheal diameter
4. A positive test is defined as: Average wheal diameter ≥4 mm larger than negative controls 1, 6
5. The histamine control must be positive to ensure results are not falsely negative

Step 2: Intradermal Testing (If Prick Tests Are Negative)

Proceed to intradermal testing only if prick tests are negative: 1, 6

1. Inject 0.02 mL of each reagent intradermally into the volar forearm using a 26/27-gauge needle
2. Measure the initial wheal size immediately after injection
3. Wait 15 minutes and remeasure
4. A positive test is defined as: Wheal diameter >2 mm larger than initial wheal AND >2 mm larger than negative controls 6

Interpretation and Management Based on Results

If Skin Tests Are Negative (Full Battery Available):

Patients can receive conventional penicillin therapy safely. 1, 2 The negative predictive value exceeds 95% and approaches 100% when combined with oral challenge. 2 Research shows that among 222 patients with negative skin tests who were challenged, only 9 (4%) had definite positive reactions. 3

If Skin Tests Are Positive:

Desensitization is required before administering penicillin. 1, 6 Approximately 20% of tested patients will have positive skin tests. 8

If Only Pre-Pen and Penicillin G Are Available (Minor Determinants Unavailable):

This creates a clinical dilemma because 3-10% of allergic patients will be missed: 1

• Positive tests: Desensitization required
• Negative tests: Two management options exist:
  • Conservative approach: Regard the patient as possibly allergic and proceed with desensitization 1
  • Alternative approach: Perform gradual test-dosing with oral penicillin in a monitored setting equipped to treat anaphylaxis 1

Safety Profile and Complications

The systemic reaction rate to penicillin skin testing is low but not zero: 9

• Overall systemic reaction rate: 0.12% for all patients tested
• Systemic reaction rate in skin test-positive patients: 2.3%
• No fatalities have been reported in modern series, though historical fatalities have occurred 9

Follow-up is essential: Patients should be contacted 5 days after any challenge to evaluate for late hypersensitivity reactions, which occur in approximately 1.7% of cases. 2

Critical Pitfalls to Avoid

• Never skip prick testing and go directly to intradermal testing—this significantly increases the risk of systemic reactions 1, 9
• Do not perform testing without full resuscitation equipment immediately available—serious anaphylactic reactions require immediate emergency treatment with epinephrine, oxygen, intravenous steroids, and airway management 7, 10
• Do not rely solely on patient history—only 15-17% of patients with self-reported penicillin allergy have demonstrable immune-mediated hypersensitivity 4
• Do not assume negative skin tests guarantee safety—oral challenge may still be needed to detect delayed reactions, which account for 47% of confirmed allergies 4