Proficiency testing of skin prick testers as part of a quality assurance system

BACKGROUND: Skin prick test is an important diagnostic procedure in clinical allergy but documentation of the quality is often missing. METHODS: We describe a proficiency system to evaluate staff members in relation to the international recommended reproducibility in terms of coefficient of variation (CV < 40 %) and the linearity (coefficient of regression >0.85) based on blinded octuplicate histamine testing using histamine 3, 10, 30 and 100 mg/ml. RESULTS: Fourteen trained allergy nurses participated in the proficiency testing. More than 95 % of the nurses, generated coefficient of variation less than 40 %, and for around 35 % of testers the CV were below 20 % based on wheal area. Regarding the linearity (coefficient of regression), only two nurses produced tests with a value below 0.85. On the contrary, 79 % of testers demonstrated a coefficient of regression >0.95. Depending on the gentleness of the prick procedure, the inter-nurse variability in wheal area varied more than twofold corresponding to a 10-doubling of histamine concentration. This would never have been detected without using a proficiency testing system. CONCLUSION: The described histamine testing provides an objective system for the evaluation of basic skin test quality assessment standards especially for documentation in scientific studies.