EVALUATING PROFICIENCY OF SKIN TESTING

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To be useful clinically, diagnostic tests require evaluation of their accuracy, precision, and sensitivity. Once these test properties have been specified, then personnel can be evaluated for proficiency by determining whether their test results meet these specifications. This study addressed whether an FDA proficiency method for ID testing could be modified for use in practice. The modified method employs two doses of FDA licensed histamine base (H): 1.8 mg/ml and 0.1 mg/ml. The relative potency (RP) of these two H concentrations is 18 (1.8/0.1). The RP by ID test was determined as follows: The endpoint dilution and the next 3 serial 3 fold dilutions of each H were determined in each subject after injection of 0.05 ml. The endpoint was the first dilution from the concentrate (Dilution #0) in which an erythema response was absent or equal to the wheal. Acceptable titrations had graded erythema responses with dilutions bracketing 50 mm sum of erythema (SE). The Dilution# producing a SE between 45-55mm or by interpolating between the 2 dilutions bracketing 50mm is the estimated D50. The difference between the two estimated D50s of each H is the RP. The mean difference in estimated D50 in six subjects is 2.5 (2 to 3). This is the estimate of test accuracy. The standard deviation of this mean is less then 0.6. This is an estimate of test precision. The mean estimated D50 of 1.8H is 6 (5.5-7). The mean estimated D50 of 0.1 H is 4 (3-4.5). These latter two means are test sensitivity specifications. The proposed method provides an approach to assess proficiency of ID testing in practice.