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

Abstract

Purpose

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.

Methods

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.

Results

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%.

Conclusions

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.

Keywords:

• Gestational diabetes
• glucose challenge test
• hemoglobin A1C
• HgbA1c
• positive predictive value

Acknowledgements

The authors thank Jasmine L. Hankey for her editorial assistance in preparing this manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors. This was an unfunded study, but the data was provided by LabCorp.