An evaluation of the accuracy and precision of relative potency (RP) estimated by parallel-line skin test (PLST) was carried out in 171 subjects by 26 operators in 9 countries and 12 states. Each clinic tested 5 or more subjects with both histamine base (HB) 1.8 and 0.1 mg/ml using a common PLST protocol. Best-fit lines of sum of erythema (E) on log3 dose were calculated in each subject from intradermal titrations of each HB using 4 serial 3-fold dilutions in singlicate producing graded E bracketing 50 mm. RP was calculated within-subject from the horizontal distance between acceptable parallel regression lines (slope equal to or greater than 10 and not different at p=0.01, r equal to or greater than 0.85). The geometric mean RP=14.5 (nominal RP=18). The pooled within-lab standard deviation (SD) were combined as specified in Amer. Soc. for Testing & Materials Std. E-177 as a measure of between-lab precision (reproducibility). The 95% reproducibility limits (RL) of RP when 4 subjects are tested is 41-160% of the predicted RP. The D50 (3-fold dilution for E=50 mm) was calculated for each HB as an estimate of test sensitivity. Mean D50 of HB 1.8 is 4.46 (RL 5.14- 7.39 for 4 tests). Mean D50 of HB 0.1 is 3.98 (RL 2.86-4.76 for 4 tests). Each subject was also puncture (P) tested with each HB using a bifurcated needle. Mean PE 1.8 = 50 mm (RL 27-29 mm for 4 tests). Mean PE 0.1=10 mm (RL 1-59 mm for 4 tests). These data are applicable to establishing the proficiency of operators using this method as well as defining the accuracy, sensitivity, and precision of this method within and between clinics.

National Institute of Health (NIH)
Food and Drug Administration (FDA)
Intramural Research (Z01)
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