In syphilis infection, penicillin is the drug of choice. A history of penicillin allergy? Do skin testing.
Two minor determinants for penicillin allergy account for 10-20% of positive skin tests. These are not commercially available. The major determinant is available as Pre-Pen (costs $300),
However, the clinical relevance of these 2 minor determinants is unclear. Skin tests may be positive, but clinical reactions occur at a much lower frequency.
However, even without minor determinant, negative predictive value of testing with Pre-Pen & PCN G is very good.
10-20% of PCN-allergic patients may not react to Pre-Pen (major determinant) and PCN G (minor determinants substitute). Should you test with minor determinant? Yeah, if you have it! (minor determinants not commercially available as of 2011).
In PCN skin testing, intradermals should be done in duplicate. Intradermal testing for penicillin seems very safe. Reaction rate was 0.12% in a Mayo clinic study of 1710 patients undergoing PCN skin testing. It is safe for outpatient use.
Cross reactivity of penicillin to cephalosporins
97% of patients with negative PCN skin test will also tolerate other beta-lactam antibiotic.
Cross reactivity of penicillin to cephalosporins is low, but the reported frequency varies “all over the place”. Probably less than 1% risk of cross-reactivity. If PCN skin test is negative, there is virtually no risk. Compare to PCN: 3% of patients with negative penicillin skin test will still have reaction to penicillin.
Use clinical history to decide whether to pursue full dose treatment or graded dose challenge after a negative PCN skin test.
If PCN skin test is positive, treatment can be pursued after drug desensitization. Can be done orally or IV.
Pencillin desensitization does not need to be repeated for weekly doses, IF the half life of the penicillin used is long enough.
It can be difficult to differentiate anaphylaxis from "red-man syndrome" after administration of vancomycin.
“Red man syndrome” is volume dependent. If reaction happens within minutes, more likely to be vancomycin anaphylaxis. "Red-man syndrome" is generally related to infusion rate, and is primarily due to non-specific histamine release, not allergy.
Antihistamines and slow infusion rate should prevent “red man syndrome” with vancomycin.
Drug challenge and desensitization
Almost a quarter of the allergists in the AAAAI meeting room do neither drug challenges nor desensitization!
Most drug desensitizations occur in the inpatient setting - ICU preferred due to 1:1 nursing, rather than danger of procedure.
Image source: Wikipedia, GNU Free Documentation License.