Is HbA1c a good screening test for diabetes mellitus?

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.