Gestational diabetes mellitus (GDM) screening in morbidly obese pregnant women.

OBJECTIVE

To study the outcomes of two-stage GDM screening of morbidly obese women in our obstetric unit and to evaluate the diagnostic performance of 20-week oral glucose tolerance test (OGTT) values in predicting or excluding late onset GDM.

STUDY DESIGN

A retrospective study in which 190 pregnant women with BMI ≥40 had two-stage screening: a 75g OGTT is performed at 20 weeks and repeated at 28 weeks if the 20-week OGTT was normal. Receiver operating characteristic (ROC) curves for 20-week OGTT values were constructed in order to obtain an optimal cut-off value of fasting and/or 2-h glucose at 20 weeks from which GDM could be predicted or excluded at 28 weeks. Sensitivity, specificity, positive likelihood ratio, and negative likelihood ratio were determined for each of the fasting and 2-h post-load glucose values at 20 weeks.

RESULTS

Forty six (24%) women were diagnosed with GDM. Thirty-two (70%) were diagnosed at 20 weeks and 14 (30%) at 28 weeks. The 2-h cut-off value of ≥6mmol/l at the 20-week OGTT had a negative likelihood ratio of 0.12 to predict GDM at 28 weeks. The low negative likelihood ratio reduces the probability of detecting GDM at 28 weeks from 9% (pre-test probability) to 1% (post-test probability).

CONCLUSION

Nearly 70% of the women were diagnosed with GDM at 20 weeks, which gives an early opportunity to treat maternal hyperglycaemia with consequent health benefits. A 2-h cut-off glucose value of 6mmol/l at 20 weeks OGTT has a low negative likelihood ratio which virtually excludes GDM at 28 weeks. Hence women with a 2h value of <6mmol/l at 20 weeks can avoid a repeat 28 week OGTT test.