Trimester-specific reference intervals for IFCC standardised haemoglobin A(1c): new criterion to diagnose gestational diabetes mellitus (GDM)?

We established trimester-specific reference intervals for IFCC standardised HbA(1c) in 311 non-diabetic Caucasian pregnant women (n = 246) and non-pregnant women (n = 65). A selective screening strategy based on risk factors for gestational diabetes was employed. Pregnancy trimester was defined as trimester 1 (T1, n = 40) up to 12 weeks + 6 days, trimester 2 (T2, n = 106) 13 to 27 weeks + 6 days, trimester 3 (T3, n = 100) > 28 weeks to delivery. The normal HbA(1c) reference interval for Caucasian non-pregnant women was 29-37 mmol/mol (DCCT: 4.8-5.5%), T1: 24-36 mmol/mol (DCCT: 4.3-5.4%), T2: 25-35 mmol/mol (DCCT: 4.4-5.4%), and T3: 28-39 mmol/mol (DCCT: 4.7-5.7%). HbA(1c) was significantly decreased in trimesters 1 (P < 0.01) and 2 (P < 0.001) compared to non-pregnant women. Retrospective application of selective screening to Caucasian women of the Atlantic DIP cohort determined that 5,208 met the criteria. 945 of those women (18.1%) were diagnosed with Gestational Diabetes Mellitus (GDM) using the International Association of Diabetes and Pregnancy Study Groups (IADPSG) glucose concentration thresholds. HbA(1c) measurement within 2 weeks of the diagnostic Oral Glucose Tolerance Test (OGTT) was available in 622 of 945 (66%). Applying the decision threshold for T2: HbA(1c) > 35 mmol/mol (DCCT > 5.4%) identified 287 of 622 (46%) of those with GDM. HbA(1c) measurement in T2 (13 to 27 weeks) should be included in the diagnostic armamentarium for GDM. This would reduce the need for diagnostic OGTT in a significant number of women.