Does serum hemoglobin A1C during early pregnancy predict performance on the 1-hour glucose challenge test?

To determine the negative and positive predictive values of hemoglobin A1c (HgbA1c) levels in predicting abnormal 1-hour glucose challenge test (GCT) results in pregnancy.

This retrospective cohort study consisted of pregnant patients from a private practice obstetrics clinic. The primary exposure was early HgbA1c measurement, and the primary outcome was third trimester 1-hour GCT results above or below 140 mg/dL. We calculated the predictive value of early HgbA1c to identify a 1-hour GCT of ≤140 mg/dL or >140 mg/dL.

Of the 1404 one-hour GCT results, 635 were associated with HgbA1c being measured in a patient on the same day or before the 1-hour GCT. The median interval between the HgbA1c test and the 1-hour GCT was 95 days (IQR 56-122 days). Among women with a hgbA1c less than or equal to 4.9, 5.0 and 5.1, respectively, the probability of their 1 hour GCT being 140 mg/dl or less was 91.3%, 91.0% and 90.1%, respectively (Table 1). Among our study population, 14.5%, 22.8% and 35.0% had an Hgb A1c less than or equal to 4.9, 5.0 and 5.1 respectively. Among women with HgbA1c greater than 6.1, the probability of their 1-hour GCT being greater than 140 mg/dL was 100%. Only 0.3% of our population had HgbA1c greater than 6.1. There was no other cut point for HgbA1c, where the positive predictive value for predicting an abnormal 1-hour GCT was equal to or greater than 90%.

The greatest value of an early HgbA1C test appears to be that very low values (5.1 or less) can predict a normal 1 h GCT with high probability (at least 90%). However, it has limited ability to predict abnormal 1 h GCT. Overall, an early HgbA1C cannot replace the 1 h GCT based on the current body of evidence.