Gestational diabetes mellitus screening and diagnosis criteria before and during the COVID-19 pandemic: a retrospective pre-post study

o the Editor: Meloncelli and colleagues1 have provided clear evidence that a fasting venous plasma glucose (FVPG) assessment may be used to decrease the number of unnecessary oral glucose tolerance tests (OGTTs) for low risk women. This would be a very welcome step forward. However, it does not directly address the problem of the discrepant results and false positives when only the OGTT is used for the diagnosis. How should we interpret a woman with a screening FVPG value of 4.8 mmol/L, but whose FVPG value is 5.1 mmol/L on a subsequent OGTT when the one- and two-hour results are normal? Given the test–retest (un)reliability of FVPG, such discrepant results will be common. We can quantify this using results from a recent meta-analysis,2 which estimated that the average coefficient of variation of FVPG was 5.7%. This implies that 95% of FVPG results (using 1.96 times the coefficient of variation) would be in a ± 11.4% range. For example, if a woman's true FVPG value was 5.0 mmol/L, then 95% of her results (noting that 0.114 × 5.0 = 0.57) would be 5.0 ± 0.57, that is, 4.43–5.57 mmol/L, which seems unreliable given the thresholds. The further from the threshold, the less likely a false positive becomes; the Box shows our calculation of the proportion of false positive FVPG values for different true average values.