Assay of prick test inoculum volume. II. Average values and individual variability

The amount of reagent introduced into the skin by a prick test is critical in studies on the reproducibility and standardization of the method.

The purpose of the present study was to measure the average volume and the individual variability of the inoculum performed with a prick test standardized technique.

Two hundred forty prick tests--16 inoculations per subject--were performed on the volar sides of arms of 15 healthy volunteers. The tests were performed by a skilled tester whose technical performance complied with the criteria of good reproducibility described in international guidelines. A 1-mm tip standard device and a 50% glycerosaline solution labeled with Tc99m were used for the test. The inoculum size was calculated using a direct assay method based on the gamma camera.

The average volume of the prick test inoculum was equal to 0.016 microliters, with a remarkable dispersion of the values around the mean (median, 15906: range, 418 to 82253 picoliters). Further, we observed great variability from one subject to another and great variability in the same subject from one skin site to another. A statistical analysis of the data shows that this variability depends on the individual characteristics of the subjects examined. A skilled tester using a standardized technique is not responsible for significant variability.

Even when performed by a skilled operator and with standardized techniques, the prick test shows great limits of reproducibility, at least as far as the size of the inoculum volume is concerned. The variability of the inoculum depends, in a statistically significant way, on the subject's individual characteristics and therefore can be reduced only within certain limits by the standardization and perfectibility of the technique.