Evaluation of GHb Assays in France by National Quality Control Surveys

OBJECTIVE To evaluate the state of the art concerning GHb assays through analysis of a large-scale quality control survey and to compare the results with those of previous surveys.

RESEARCH DESIGN AND METHODS A lyophilized hemolyzate prepared from human erythrocytes containing a physiological HbA_1c level (5.5%) was sent to 3,500 French laboratories in February of 1995 and assayed as a patient's sample under routine conditions. Distribution of values was analyzed from the reported results for each method. The results were compared with the assigned value (acceptable range: ± 20%) and with the upper value of the reference range currently used.

RESULTS Results were obtained from 2,674 laboratories, among which 39% used cationexchange chromatography methods, 37.5% affinity chromatography, 16% immunological methods, and 7.5% electrophoresis. The number of laboratories using immunological methods increased from 100 to 400 between 1993 and 1995. The overall interlaboratory coefficient of variation (CV) was 20.2%, with within-method CVs ranging between 3.2 and 29.5%. Method-to-method accuracy varied dramatically, with mean HbA_1c values ranging from 4.4 to 8.2%. Results from 75% of the laboratories were comprised in the acceptable range; 88% of them reported a value within the normal range of the method used.

CONCLUSIONS The interlaboratory variability of results illustrates the difficulties encountered by diabetologists in the follow-up of diabetic patients using results obtained from different laboratories. It demonstrates the usefulness of the internationally developed standardization process of GHb measurements and points out the need for laboratories to fulfill good practices.