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Posted On : 09 Mar 2011

Thanks for answering my query upon sensitivity test before injecting drugs. In continuation further queries are 1. How to prepare an extract i.e. Penicillin solution for sensitivity test _? What should be dilution _?
2. When to interpret test and what is a positive reaction which warns us against injecting a drug _?
3. Once found sensitive is it necessary that same person will always be sususceptible therefore not to test any more and not to think of using that particular drug{s} _?
4.what is minimum age to go for sensitivity test _? Does it need to be tested in neonates _?
5. Minor allergic reactions to commonly used drugs in the form of urticaria andmild angiedema are seen. does it mean that particular drug{s} can not be used anymore in rest of life of person _? In setting where multiple drugs are prescribed in OPD how can we blame one particular drug _?
With Thanks and regards

Expert Answer :

Answer Discussion :

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Answer 1

Penicillin skin testing identifies patients at high risk for an allergic IgE-mediated reaction {immediate and accelerated types}` 50-75 percent of patients with posit ive skin tests will have an allergic reaction compared to 3-4 percent in those with negative skin tests. IgE antibodies mediating immediate reactions {including anaphylaxis} are mainly directed to the minor determinant antigens {benzyl penicillin, benzyl penicilloate and benzyl penilloate}, whereas the major deteminant {benzyl penicilloyl} largely identifies accelerated
reactions. Skin testing with only Penicillin G will fail to identify over half of patients who would be positive if both major and minor determinants were used, and 1, 4 of the cases who will have
immediate reactions including anaphylaxis.

Reagents
Benzylpenicilloyl-polylysine {major pre-Pen G x 10-5 M}
Benzylpenicillin {10-2 M or 6000U, mL}
If available
Benzylpenicilacloic acid {10-2 M}
Benzylpenilloic Acid {10-2 M}
Positive control {histamin1e.0 mg, mL}
Negative control {buffered saline solution}

Dilute the antigen 100 s fold for preliminary testing if there has been an immediate generalized reaction within the past year.

Procedure
Epicutaneous {scratch or prick} test with a drop of each reagent: examine for 15 minutes` if there is no significant wheal {4 mm or greater} proceed to intra-dermal test.

Intradermal test: inject 0.02 mL intradermally with a 27-gauge short-bevelled needle` observe for 20 minutes

Interpretation
Conclusions are only possible if the negative control is negative and the positive control is positive.
Positive test: a wheal greater than 4 mm in mean diameter to any penicillin reagent` erythema must be present
Negative test: the wheals on the penicillin reagents are equivalent to the negative control
Inderterminate: all other results

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