23 November 2023

Fasting blood glucose a rival in GDM screening

Pregnancy Women

Fasting blood glucose tests can effectively replace oral glucose tolerance tests in low-risk pregnant women, according to new Queensland research.

Ruling out GDM in women with fasting blood glucose levels of less than 4.7mmol/L did not increase adverse perinatal outcomes for mothers or children, according to study in the MJA on women during the first year of covid.

“That runs counter to the current guidelines in Australia, but it would save an awful number of women from having a glucose tolerance test,” said study author and Brisbane endocrinologist Associate Professor Michael d’Emden.

Professor d’Emden and colleagues compared perinatal outcomes between 2019 and 2020, when covid fears prompted pregnant women to screen with a fasting blood glucose test at 24 to 28 weeks rather than attend a clinic for the hours long OGTT.

Fasting blood glucose scores below 4.7mmol/L were considered to exclude GDM and scores above 5.0mmol/L triggered treatment. Women with scores within that window required a further oral glucose tolerance test.

“What we showed is that if you had a fasting blood glucose level of less than 4.7mmol/L, your chances of having an abnormal glucose tolerance test was less than 5%,” Professor d’Emden said.

“Then 80% of people who are having just a simple fasting glucose test would either be excluded from having gestational diabetes, or the diagnosis was confirmed on the basis of their fasting glucose level being 5.1mmol/L or above.”

The proportion of GDM diagnoses rose slightly from 13.6% to 14%, they found.

Yet adverse perinatal outcomes – such as gestational hypertension, pre-term delivery, large or small for gestational age, hypoglycaemia and respiratory distress – were similar between those excluded from GDM in 2019 and those excluded after a fasting blood glucose test in 2020.

Caesarean deliveries did climb by an estimated 3.9 percentage points in 2020, equalling an extra 6.8 surgeries per 1000 births. But this rise in caesareans during covid was an international trend, the authors noted. 

Professor d’Emden said there was growing concern about the 2011 changes to the diagnostic criteria, which lowered the threshold for a GDM diagnosis.

“We think that there’s been a substantial increase in the number of people who are diagnosed with the condition,” he said. “Yet there isn’t necessarily any evidence that a lot of these people benefit from the diagnosis in terms of reducing their risk of pregnancy-associated or neonatal complications of gestational diabetes.

“In fact, many of these people who are diagnosed are actually at very little risk.”

Despite these findings, RANZCOG said there would be no change in their position on GDM testing and diagnosis, and the Australasian Diabetes in Pregnancy Society said more evidence would be needed to change their screening guidelines.

“RANZCOG’s statement on GDM (C-Gyn 7) is aligned to the GDM testing and diagnosis recommended by The Australasian Diabetes in Pregnancy Society (ADIPS),” said a RANZCOG spokesperson.

“The College recommends a screening regimen is a 75gram two-hour Pregnancy Oral Glucose Tolerance Test (POGTT) [at 26 to 28 weeks of gestation].”

Professor David Simmons, endocrinologist and president of the Australasian Diabetes in Pregnancy Society, told Rheumatology Republic more evidence was needed to change the group’s guidelines.

Professor Simmons said ADIPS would meet in February or March next year to discuss incorporating new evidence from randomised controlled trials into the current diagnostic criteria.

The oral glucose tolerance test was “certainly not a comfortable test”, the Distinguished Professor of Medicine at Western Sydney University said.

“But the value that you get from it is very high. In spite of its variation and a whole range of other criticisms, it’s a good way to identify those at risk and into the long term as well,” said Professor Simmons.

Professor Simmons said the MJA study was not a randomised controlled trial and it used data gathered during covid when lots of factors changed, such as exercise habits, diet and sleep, so it was unknown how that influenced the proportion with GDM.

The focus on this Queensland population did not include culturally and linguistically diverse groups either, he said.

“What we know from other studies is that those who are of Asian descent are much more likely to have a high one-hour or two-hour glucose result,” said Professor Simmons.

“Only doing a fasting glucose would mean that some of the groups who are more disadvantaged would be essentially excluded and we would be increasing disparities in health outcomes.”

Bond University evidence-based medicine researcher Professor Paul Glasziou said the data was useful, and using a fasting blood glucose as the initial test would save many women from having the full oral glucose tolerance test.

“However, there was no reduction in the percent of women diagnosed with GDM,” he said.

“The interpretation is important as women can currently be given a diagnosis of GDM based on a single slightly elevated [fasting venous plasma glucose] on the OGTT.

“So a question is, how to interpret the results for women who are between 4.7mmol/L but less than 5.1mmol/L on the screening [fasting blood glucose], but just over 5.1mmol/L on the OGTT – that is discrepant results. All glucose tests have a test-retest variation which is important to consider.”

MJA 2023, online 6 November