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Assay of prick test inoculum volume. II. Average values and individual variability
Insights
The prick skin test shows significant variability in reagent volume delivered, even with standardized methods. This variability is primarily due to individual patient characteristics, not the technique itself.
Area of Science:
- Dermatology
- Allergy Testing
- Clinical Pharmacology
Background:
- Reproducibility and standardization of skin prick testing are crucial for accurate allergy diagnosis.
- The volume of reagent introduced during a prick test significantly impacts study outcomes.
Purpose of the Study:
- To quantify the average inoculum volume in standardized prick testing.
- To assess the individual variability of inoculum volume across subjects and skin sites.
Main Methods:
- 15 healthy volunteers underwent 240 standardized prick tests using a 1-mm device and Tc99m-labeled solution.
- Inoculum volume was measured using a gamma camera-based direct assay method.
- Tests were performed by a skilled operator adhering to international reproducibility guidelines.
Main Results:
- The average prick test inoculum volume was 0.016 microliters (16,000 picoliters).
- A wide dispersion of inoculum volumes was observed (range: 418–82,253 picoliters).
- Significant variability existed between subjects and even between different skin sites on the same subject.
Conclusions:
- Standardized prick testing, even by skilled operators, has inherent limitations in reproducibility regarding inoculum volume.
- Subject-specific characteristics are the primary drivers of inoculum volume variability.
- Technique standardization can only partially mitigate this variability.
Introduction:
The amount of reagent introduced into the skin by a prick test is critical in studies on the reproducibility and standardization of the method.
Objective:
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.
Methods:
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.
Results:
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.
Conclusions:
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.
