Introduction: Oral glucose tolerance testing (OGTT) is used to identify women with gestational diabetes (GDM) with the aim of preventing complications of hyperglycaemia. Only one abnormal glucose reading is required for diagnosis. However, women proceed to 75g oral glucose challenge despite fasting hyperglycaemia (FH). This may result in acute maternal hyperglycaemia, which is of concern as high levels (>12.5-15mmol/l) have the potential to induce fetal hyperglycaemia and acid-base disturbance.
Hypothesis: Women with FH receive unnecessary OGTTs and are at risk of significant acute hyperglycaemia.
Methods: Analysis was performed of 4629 consecutive antenatal 75g OGTTs in pregnant women at a single laboratory. Diagnostic criteria for GDM were fasting glucose ≥5.1mmol/; 1hr ≥10.0mmol/l or 2hr ≥8.5mmol/l. All women completed OGTT irrespective of their fasting glucose level.
Results: 572 (12.4%) of 4629 women were diagnosed with GDM. 312 women (6.7%) had FH (mean 5.5 ±0.62mmol/l, range 5.1–9.9) and therefore were identified as having received an unnecessary challenge. Women with FH reached a 1hr mean level of 9.1mmol/l (highest 19.8mmol/l) and 2hr mean of 7.1mmol/l (highest 23.3mmol/l) after OGTT. These peaks were on average lower than those of GDM women with normal fasting glucose (1hr mean 10.0mmol/l; p<0.0001; 2hr mean 8.3mmol/l; p<0.0001). However, only 6/260 (2.3%) women with normal fasting levels reached glucose >12.5mmol/l, whereas 28/312 (9%) women with FH reached this critical threshold, and 5/312 (1.6%) reached glucose levels >15mmol/l (p=0.003).
Conclusion: Women with FH diagnostic of GDM could avoid an OGTT, as this provides no additional information for their pre-natal care. Cost-savings would be minimal, however iatrogenic hyperglycaemia could be avoided by ensuring fasting level <5.1mmol/l before administering glucose. The logistics of assessing women by awaiting the initial fasting glucose level requires consideration of risk and benefit. Further research is needed to assess the effect of acute hyperglycaemia on fetal wellbeing.