P25 Accuracy of serial dilution concentrations for intradermal skin testing: a neglectable problem?

Abstract

Skin tests are important diagnostic tools. To establish reactivity, a single allergen test concentration is considered to be sufficient. To diagnose sensitivity serial dilution or endpoint titration is recommended. Skin testing in drug allergy is complex, since pathogenetic mechanisms are diverse, and for many drugs cut‐off skin test concentrations have not been established. Therefore, after a negative prick/puncture test with the stock solution, intradermal tests with serial (ten‐fold) dilutions are recommended. Surprisingly in most publications dealing with serial skin test dilution, the technique is not described in detail.Aim: Comparison of the variability of test substance concentrations produced with two serial dilution techniques conducted by experienced allergy nurses.Methods: Two allergy nurses produced dilution series. Isosulfane blue was chosen as test substance, since it enabled easy and sensitive detection. Dilution series of isosulfan blue (aqueous solution of 10 mg/ml) from 1:10 to 1:10’000 were produced under sterile conditions by two dilution techniques: 1) Dilution in vials: 0.5 ml isosulfane blue solution was serially transferred with 1 ml tuberculine syringes to vials containing exactly 4.5 ml NaCl 0.9% 2) Dilution in syringes: 0.1 ml isosulfane blue solution with the higher concentration was aspirated into 1 ml tuberculine syringes followed by 0.9 ml NaCl 0.9%. These procedures were compared with a controlled experiment carried out using piston‐stroke pipettes. The concentrations of isosulfan blue in all dilutions were determined by UV/VIS‐spectroscopy and external standard calibration.Results: In almost all samples the concentrations were higher than expected. In vials the concentrations varied from 83 to 360%, in syringes from 83 to 711% of the expected values. The dilution in vials gave consistently better results than dilution in syringes. A considerable interindividual variability was observed between the conducting nurses, however variability over time was smaller.Conclusions: In routinely performed serial dilutions of allergen test preparation considerable deviations from the expected allergen test concentration have been observed. This might be of minor importance for an individual patient, however, in establishing e.g. cut‐off concentrations for non‐irritant drug concentrations or in multicenter studies such variations may result in major errors. Quality management of skin testing should not only include the recording, injection technique and test reactivity evaluation, but also the exact description of the preparation of test dilutions. In clinical routine dilution techniques should be applied, which give more accurate results.