Background: Aboriginal women in the Northern Territory (NT) experience a high diabetes burden. Early pregnancy oral glucose tolerance test (OGTT) screening is recommended but infrequently undertaken. Local guidelines suggest haemoglobin A1c (HbA1c), at threshold 5.7% (39 mmol/mol) as an alternative, although evidence for this is limited. We aimed to assess rates of early (<20 weeks) HbA1c screening and associations of HbA1c with adverse pregnancy outcomes.
Methods: A retrospective study of all pregnancies among Aboriginal women receiving antenatal care in 52 remote clinics across the NT from 2017-2019 was undertaken. Data were extracted from NT Primary Health Care Collection and NT Perinatal Data Collection, supplemented by manual review of individual healthcare records for 902 pregnancies. Outcomes included early gestational diabetes using ADIPS OGTT criteria (early GDM), GDM diagnosed at any gestation, large-for-gestational-age (LGA), neonatal special care nursery admission, caesarean section, preeclampsia and pre-term birth. Associations were assessed using logistic regression.
Results: Of 1184 singleton pregnancies, 75 (6.3%) had pre-existing type 2 diabetes and were excluded. Early pregnancy HbA1c screening was performed in 770 (69%) pregnancies, with 31 (4.0%) having HbA1c ≥5.7% and only one ≥6.5%. Early pregnancy OGTT was undertaken in 182 (24%) of these 770 pregnancies, with 32 (18% of early OGTTs) meeting criteria for GDM, of which 5 (2.3% of early OGTTs) met overt diabetes criteria. Median gestation at time of HbA1c was 7 weeks (IQR 5.1-10.6), compared to 10 weeks (IQR 6.4-14) for early OGTT (p<0.001). HbA1c ≥5.7% had 31% sensitivity and 97% specificity for early GDM, 60% sensitivity and 93% specificity for overt diabetes on early OGTT, and 16% sensitivity and 98% specificity for GDM diagnosed at any gestation. Women with HbA1c ≥5.7% (compared to those with HbA1c <5.7%) had increased risk of LGA (OR 4.1, 95% CI 1.7-10, p=0.002) and neonatal special care admission (OR 2.5, 95% CI: 1.2-5.3, p=0.015), but not caesarean section (OR 1.5, 95% CI: 0.71-3.1, p=0.291), preeclampsia (OR 2.4, 95% CI: 0.68-8.2, p=0.174) or pre-term birth (OR 1.4, 95% CI: 0.56-3.5, p=0.469). There was minimal change in estimates after adjustment for age, smoking, parity and BMI.
Conclusion: Compared to OGTT, uptake of early HbA1c screening was higher and occurred at earlier gestation. Sensitivity of HbA1c against current GDM criteria is poor. Nevertheless, our findings suggest routine HbA1c is worthwhile in this context. Women with HbA1c ≥5.7% carry a higher risk of pregnancy complications and do not require further confirmation with OGTT.