Allergy Notes: How Accurate Is Penicillin Skin Testing?

The immunogenic components of PCN include:

1. major determinant is benzylpenicilloyl
2. minor determinants are penicillin G (benzylpenicillin), penicilloate, and penilloate

The major determinant, conjugated to a polylysine carrier molecule, has been available as a commercial product (Pre-Pen®) since 2009.

Minor determinants, other than PCN G, are not commercially available. Some allergists synthesize their own minor determinates. AllerQuest, LLC plans to develop a minor determinant mixture.

The predictive value of a negative skin test has been reported to be 97-99%.

Testing with benzylpenicilloyl polylysine (Pre-Pen®) alone identifies up to 90% of patients likely to have IgE-mediated reactions to PCN. The addition of PCN G to the testing regimen raises the predictive value to 97%.

However, some studies have suggested that 3-30% of patients with IgE-mediated allergy to PCN may be missed by routine skin testing.

A positive PCN skin test result should rule out the use of PCN agents and other beta-lactams.

However, a negative PCN skin test does not definitively rule out the risk for an IgE-mediated reaction upon administration of the offending agent.

All patients should receive a graded challenge of the drug in settings with emergency medical support.