Abstract
Objectives HbA 1c has been recently recommended as the primary diagnostic test for diabetes. This study evaluated the positive predictive value (PPV) and negative predictive value (NPV) of HbA 1c against the oral glucose tolerance test (OGTT) in three locations. Design and methods Three years of data with concurrent OGTT and HbA 1c tests were extracted from Laboratory Information Systems (LIS) and receiver operator (ROC) curves and positive and negative predictive values calculated comparing the OGTT with the HbA 1c values using a 10% prevalence of diabetes. Results The recommended threshold HbA 1c value of 6.5% did not give the optimal combination of NPV (0.93 to 0.92) and PPV (0.40 to 0.61) compared to a threshold HbA 1c value of 7.0% (NPV 0.91 to 0.92, PPV 0.61 to 0.73). Conclusion The optimal HbA 1c value for the diagnosis of diabetes is 7.0% but even at this HbA 1c the PPV is suboptimal and may cause up to 12% of patients without diabetes, as defined by a normal OGTT, to be classified having diabetes mellitus.
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