Use and interpretation of HbA_1c testing in general practice. Implications for quality of care

Abstract

We evaluate the interpretation and use of HbA_1c results in general practice in relation to diabetes follow-up and case-finding. As part of an external quality assurance scheme for laboratory analyses, two case histories were mailed to all Norwegian GPs with equipment for HbA_1c analysis in their office laboratory (n=566) and to a random sample of GPs without such instruments (n=419). Patient A represented a monitoring situation and the GPs were asked to state changes in HbA_1c signifying any improvement or deterioration of metabolic control. The initial HbA_1c value stemmed from analysis of quality control material in the instrument group and was a preset value for the other group. In patient B, we focused on the use of HbA_1c and other laboratory tests in diabetes case-finding. In the monitoring situation, 22% of GPs in the instrument group misclassified changes in HbA_1c values, since these were less than the analytical uncertainty. Further, when interpreting HbA_1c results, 64-93% of GPs (i) assumed that analytical quality was better than it really was, (ii) did not appreciate biological variation of HbA_1c, or (iii) acted on small differences to be on the safe side. In case-finding, HbA_1c was deemed important by 29% of GPs; doctors choosing not to perform a glucose tolerance test relied more on HbA_1c. GPs have to be aware of analytical quality and biological variation when interpreting HbA_1c results. The present limitations of HbA_1c in the diagnosis of diabetes are not properly understood.