Blood glucose test ‘adequate’ for ruling out GDM

Gestational diabetes

By Geir O'Rourke

27 Mar 2024

A plasma glucose test could be adequate for ruling out GDM in low-risk women and reduce the need for unnecessary further testing, Australian research has concluded.

Based on data from 2020, when fasting venous plasma glucose (FVPG) assessment was brought in across most Australian services in response to the COVID-19 pandemic, the finding suggests such a test could be used long-term, the Queensland researchers say.

Their study compared perinatal outcomes for pregnant women assessed for GDM using the standard oral glucose tolerance test (OGTT) procedure in the final six months of 2019, with those screened via FVPG over the same period in 2020.

As with the pre-pandemic protocol, women underwent FVPG assessment at 24-28 weeks gestation, followed by OGTTs for women with FVPG values of 4.7–5.0 mmol/L.

Women with FVPG values below 4.7 mmol/L were empirically classified as not having GDM.

This facilitated a “natural experiment” in which different screening and diagnostic recommendations could be examined, according to the researchers, from the Royal Brisbane and Women’s Hospital and Queensland University.

They found rates of GDM diagnosis were similar across both time periods: 13.6% in 2019 versus 14% in 2020.

Moreover, most perinatal outcomes occurred at roughly the same frequency for women without GDM in 2019 and those for whom it was excluded in 2020 on the basis of FVPG values, they reported in the MJA (link here).

A notable exception was caesarean delivery, for which the estimated probability increase in 2020 was 3.9 percentage points, corresponding to an extra 6.5 caesarean deliveries per 1000 births, the authors said.

Some other outcomes — respiratory distress, neonatal intensive care or special nursery admission, large for gestational age babies — were about one percentage point more likely to occur for women without GDM in 2020 (excluding those diagnosed on the basis of FVPG assessment alone) than for women without GDM in 2019.

“As the results were similar when we compared outcomes for all women without GDM in 2019 and 2020 (except those diagnosed on the basis of FVPG alone), the differences were probably related to the impact of the COVID‐19 pandemic on maternity care during 2020,” they wrote.

“Although Queensland was relatively COVID‐19‐free during 2020, lockdowns and restrictions on movements affected antenatal health care delivery.”

“Reduced physical activity, increased stress and anxiety, and gestational weight gain during the pandemic have been reported and probably influenced outcomes.”

Moreover, increased numbers of caesarean deliveries during the first year of the COVID‐19 pandemic were reported overseas, according to the authors.

They concluded: “The discussions of GDM screening and diagnosis in several countries suggest that an international consensus is unlikely.”

“However, we need to reduce the burden and costs of testing by selecting a method that identifies women who are at lower risk of adverse perinatal outcomes.”

“Using FVPG assessment to identify women at low absolute risk of GDM‐related pregnancy complications as an initial step in GDM screening could benefit a large proportion of pregnant women and save the health system substantial costs.”

Questions raised over borderline cases

In an accompanying editorial (link here), rational care campaigners Professor Paul Glasziou and Professor Jenny Doust agreed the use of FVPG as a screening tool would be a “very welcome step forward”, but raised an issue of “discrepant results and false positives” when only the OGTT was used for the diagnosis.

The key issue involved cases involving patients with FVPG values only slightly above the cut-off at screening but higher on subsequent OGTT.

“Given the test–retest (un)reliability of FVPG, such discrepant results will be common,” they wrote.

“We can quantify this using results from a recent meta-analysis, 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 authors respond

But in a response to Professors Doust and Glasziou, the study authors downplayed the issue (link here).

The problem of discrepant results including false positives (and indeed false negatives) was hardly unique to GDM screening, and applied to virtually any test where a patient’s result was assessed against a study population to determine if treatment was warranted, they noted.

As a result, all test results should be interpreted in their clinical context, such that the same borderline readings could prompt an entirely different response depending on the patient’s risk factors, the authors said.

“There is always normal biological variation from day to day as well as laboratory imprecision,” they wrote.

“Any test that is repeated is likely to give a different result.”

They concluded: “As Australia looks to revise the guidelines for the screening and diagnosis of GDM, consideration should be given to the clinical context and personal risk factors present along with borderline test results.”

“This approach acknowledges the variability and imprecision of screening approaches and ensures health care resources that address the burden of treatment are directed to those who will benefit from care.”

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