HbA1c has no general use in the diagnosis of GDM

(Aotea News, July 2012)

Current guidelines and expert opinion concur that Haemoglobin A1c (HbA1c) has no general use in the diagnosis of gestational diabetes mellitus (GDM ).

The 50g glucose load and, if required by the result of the load, the oral glucose tolerance test remain the tests of choice for most women.

For obese women, it is reasonable to proceed directly to an OGTT at 24–28 weeks without prior glucose load.

HbA1c useful at ‘booking visit’ if high risk of undiagnosed diabetes or GDM

However, there is some evidence that HbA1c may be useful at the booking visit (before 16 weeks’ gestation) in women at high risk of undiagnosed diabetes or GDM.

This use has been endorsed by leading experts in gestational diabetes and is included in clinical practice guidelines for midwives. The test is already being used in Auckland for women in this high-risk category.

The Oversight Advisory Group for laboratories in the Wellington region has agreed that ordering HbA1c in this limited context will be added to the list of tests available to midwives, effective immediately.

Who is at high risk of undiagnosed diabetes or GDM?

Women with any of the following factors are at high risk of undiagnosed diabetes or GDM:

  • previous GDM
  • previous macrosomia
  • age >40 years
  • morbid obesity:
    • Indian/Asian: BMI ≥32
    • Polynesian: BMI ≥37
    • Everyone else: BMI ≥35
  • PCOS
  • two first-degree relatives with diabetes
  • glycosuria
  • on antipsychotic medication
  • on prednisone.

A result for HbA1c ≥41mmol/mol at the booking visit in these high-risk women indicates gestational diabetes.

Note that HbA1c should not be requested at later stages of pregnancy for diagnosis in these high-risk women. Instead, an OGTT is indicated at 16 weeks, without prior glucose load, and again at 24–28 weeks if the initial test is normal.