New call to review gestational diabetes screening

The diagnosis of gestational diabetes mellitus (GDM) has been a controversial area for decades, with guidelines based largely on consensus agreement.
 
Diagnostic criteria also vary depending on location, which can be confusing for clinicians and patients alike.
 
What experts do agree on, however, is that untreated GDM poses risks to both the mother and baby, which is why screening is undertaken during pregnancy.
 
In Australia, GDM affects around 10% of pregnancies but this number could be lower, according to the authors of a Perspective published in the Medical Journal of Australia.
 
The authors warn that our current one-step screening approach, when used in conjunction with criteria set out by the International Association of the Diabetes and Pregnancy Study Group (IADPSG), is overdiagnosing GDM by 25%.
 
This means one in four pregnant women are undergoing unnecessary interventions which can lead to harm.
 
Speaking to newsGP, lead author Professor Jenny Doust from the University of Queensland said the current screening method, which utilises a 75 g oral glucose tolerance test (OGTT) at 24–28 weeks’ gestation, is outdated.
 
She explains that the current issues can be traced back to a number of large trials that came out between 2005 and 2010, which showed that hyperglycaemia was associated with adverse pregnancy outcomes and that these outcomes improved with GDM treatment.
 
‘At the time, however, there was no clear cut off for GDM, so an arbitrary decision was made by IADPSG to make the threshold 1.75 times the median,’ Professor Doust told newsGP.
 
The Australian Diabetes in Pregnancy Society (ADIPS) subsequently came out in support of the one-step approach using IADPSG criteria over the alternative Carpenter-Coustan two-step approach, despite objection from the RACGP and National Institutes of Health (US).
 
In the article, Professor Doust writes the objection was largely because there was a lack of evidence that the subset of women who are identified by the one-step approach, but who would have been considered normal by the two-step approach, benefit from the increased monitoring and interventions involved with the diagnosis of GDM.
 
While the one-step screening approach has remained in place in Australia over the past decade for pregnant women at population risk of GDM, cut offs for the OGTT still vary, with the RACGP having its own preferred criteria in contrast to that used by ADIPS.
 
Cut-offs aside, a 2021 trial conducted in the US appears to validate prior concerns raised over a one-step screening approach, which is why Professor Doust and her colleagues are calling for a review of GDM screening in Australia.
 
Despite almost twice as many pregnant women being diagnosed with GDM using the one-step rather than two-step approach in the trial, no health benefits were seen for either the mother or the infants.
 
The US findings are consistent with another Australian observational study that also showed no benefit in perinatal outcomes after the introduction of the one-step approach.
 
‘The [current] criteria were introduced without evidence of benefit,’ the authors write.
 
They say the one-step approach involves considerable potential for harm, including life disruptions for the women diagnosed, psychosocial burdens, risk of more invasive forms of delivery, potential harms to the infant from restricted diets and the use of insulin, as well as increased costs and poor targeting of resources.
 
Antenatal shared care GP Dr Angela Rassi isn’t surprised by the findings and told newsGP that GDM diagnosis has been a ‘tricky’ area for many years.
 
‘The guidelines and diagnostic criteria differ centre to centre, and these usually differ from the RACGP recommendation,’ she said.
 
‘For those GPs doing shared antenatal care, this means that they need to be across the differences in criteria for each centre in which they provide care.
 
‘This has become even more complicated during the pandemic as some centres have changed their guidelines or approach [including to the two-step] to minimise the time pregnant women are spending in contact with others.’
 
Like Professor Doust, Dr Rassi believes peak bodies should take note of the US findings and hopes that it may provide better clarity around GDM diagnosis.
 
‘This is a fairly robust and interesting study showing non-superiority of the current one-step process versus the two-step process,’ she said.
 
‘In terms of clinical outcomes, this study has shown a one-step process is not better than a two-step process, and in fact, the two-step process may be more tolerable for patients.’
 
When asked what the two-step Carpenter-Coustan process looks like, Professor Doust said it involves a 50 g glucose challenge in the non-fasting state, followed by a 100 g OGTT if the 50 g challenge is positive.
 
‘The threshold is lower with the ADIPS one-step approach than it is for Carpenter-Coustan, but we think that the lower threshold is actually not as important as the fact that women get tested twice,’ she said.
 
‘Part of the problem with the one-step approach is that there is so much biological variability in glucose measures, similar to blood pressure for example, so some women might be a little over on the day they get the 75 g OGTT.
 
‘And then they get labelled as having GDM, but the latest findings show that there’s no evidence this has any benefits and there’s actually evidence that it comes with harms.
 
‘We really need to rethink the criteria’.
 
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antenatal care GDM gestational diabetes pregnancy