Use and interpretation of HbA1c testing in general practice. Implications for quality of care.

We evaluate the interpretation and use of HbA1c 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 HbA1c 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 HbA1c signifying any improvement or deterioration of metabolic control. The initial HbA1c 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 HbA1c and other laboratory tests in diabetes case-finding. In the monitoring situation, 22% of GPs in the instrument group misclassified changes in HbA1c values, since these were less than the analytical uncertainty. Further, when interpreting HbA1c results, 64-93% of GPs (i) assumed that analytical quality was better than it really was, (ii) did not appreciate biological variation of HbA1c, or (iii) acted on small differences to be on the safe side. In case-finding, HbA1c was deemed important by 29% of GPs; doctors choosing not to perform a glucose tolerance test relied more on HbA1c. GPs have to be aware of analytical quality and biological variation when interpreting HbA1c results. The present limitations of HbA1c in the diagnosis of diabetes are not properly understood.