Predictors and clinical implications of a false negative glucose challenge test in pregnancy

A common approach to screening for glucose intolerance in pregnant women is the use of a 50g glucose challenge test (GCT) in the late second trimester, followed by a diagnostic oral glucose tolerance test (OGTT) when the GCT is positive (1-hour post-challenge blood glucose ≥ 7.8 mmol/L). As women with a negative GCT do not undergo the diagnostic OGTT, it is possible that they could have undiagnosed gestational diabetes (GDM) or gestational impaired glucose tolerance (GIGT). Thus, we sought to characterize predictors of a false negative GCT and its clinical implications.

Two hundred two women with a negative GCT screening test underwent subsequent OGTT, which enabled their stratification into two groups: (1) those with normal glucose tolerance on OGTT (n = 166) and (2) those with either GDM (n = 8) or GIGT (n = 28).

The 36 women found to have GDM or GIGT at the time of OGTT had higher GCT glucose values than those with normal glucose tolerance (mean 6.6 mmol/L vs. 5.9 mmol/L, P < 0.001), as well as poorer insulin sensitivity (P = 0.004) and pancreatic beta-cell function (P < 0.001). On logistic regression analysis, the only significant independent predictor of GDM/GIGT was the GCT glucose value (OR 2.19; 95% CI 1.40 to 3.41, P < 0.001). However, there was no clear GCT glucose threshold for identifying GDM/GIGT, consistent with its modest area under the receiver operating characteristic curve (0.71). Importantly, there were also no differences between the two groups in median length of gestation, median birth weight, prevalence of macrosomia, or proportion of low Apgar scores.

False negative GCTs cannot be readily predicted by risk factors. However, their clinical implications at delivery may be benign.