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Editorial
Gestational diabetes: universal or selective screening?Gestational diabetes and its outcomes need to be better defined before we can tackle this question
MJA 2001; 174: 113-114
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Gestational diabetes mellitus (GDM) is defined as carbohydrate
intolerance of variable severity with onset or first recognition
during pregnancy.1 The first recorded case was
described in Berlin in 1823,2 but the term gestational
diabetes did not appear in the medical literature until
1961.3 The initial studies in the
1960s developed diagnostic criteria which identified mothers
likely to develop type 2 diabetes later in life. However, as late as the
mid-1990s, GDM was not widely accepted as a risk factor for type 2
diabetes in Australia.
That situation has now changed, and the Federal Government has established a committee to advise on the significance of GDM. This is particularly relevant given that preliminary results from the Australian Diabetes, Obesity and Lifestyle Study indicate that 7.2% of Australians over the age of 25 years have diabetes, with 50% being undiagnosed.4 The individual, societal and healthcare costs of this diabetes epidemic are significant, particularly as many individuals have complications such as retinopathy by the time of diagnosis.5 As intervention in high-risk groups, with advice on diet, exercise and weight control, may delay the onset of type 2 diabetes and its complications,6 any opportunities for such intervention should not be missed. | ||||||||||
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The initial diagnostic criteria for GDM were subsequently used to
identify risks of complications during the index pregnancy, such as
macrosomia and associated birth trauma. Nevertheless, whether
these diagnostic criteria can be used for predicting both later
diabetes and pregnancy outcome is contentious. These issues will be
largely settled by the Hyperglycaemia and Adverse Pregnancy Outcome
(HAPO) study, which is now under way. This study, funded by the United
States National Institutes of Health, will test 25 000 women in 16
field centres around the world. Two centres are in Australia — at the
John Hunter Hospital, Newcastle, and the Mater Mothers' Hospital,
Brisbane. Results are expected to become public in June 2004 and, it is
hoped, will answer current questions about the association between
various levels of glucose intolerance during pregnancy and adverse
outcomes. Evidence-based criteria for GDM to prevent or minimise
adverse pregnancy outcomes can then be developed. The longer-term
outcomes for the baby also need to be remembered, as evidence is
accumulating that raised glucose levels in utero are a risk
factor for later development of obesity and diabetes.7
Given these considerations, and given that GDM is mostly asymptomatic and that it is not practical to perform glucose tolerance tests on all pregnant women, a systematic screening program is a necessary first step for detecting GDM. This raises three obvious questions: whom to screen, how to screen and when to screen? When to screen? There is general agreement on this issue, as abnormal carbohydrate tolerance is likely to be seen from the beginning of the third trimester. Screening should be carried out between 24 and 28 weeks' gestation, or earlier in women from high-risk ethnic groups or with a previous history of GDM. How to screen? This question remains controversial. The most validated method is plasma glucose measurement one hour after a random 50 g glucose load.1 Many other suggested methods have not gained wide acceptance. Whom to screen? This question is also controversial and is addressed by Davey and Hamblin8 in this issue of the Journal. They investigated 6032 Melbourne women who had undergone universal screening for GDM, concluding that selective screening on the basis of risk factors for GDM is practicable. Selecting out women aged under 25 years who had a body mass index less than 27 kg/m2 and no family history of diabetes, and who did not belong to a high-risk ethnic group (eg, Indigenous Australian, Pacific Islander, Asian or Middle Eastern background), would have missed only two of 313 women diagnosed with GDM (0.6%) and could have avoided up to 17% of screening tests. The concept of selective screening is not new. It has been suggested by both the American Diabetes Association and, in circumstances of limited workforce resources, by the Australasian Diabetes in Pregnancy Society.9 Davey and Hamblin point out that their proposed form of selective screening requires rigorous questioning about racial and family history and accurate measurement of weight and height. Whether this would be done in routine clinical practice is questionable. At least their proposal is less onerous than that of Naylor and colleagues, who developed a complicated scheme to exclude women from screening that reduces screening tests by a third, but which most agree is not practical.10 The other important issue is how many women with GDM would be missed by this selective screening. Davey and Hamblin claim only 0.6% of potential cases would be missed. This is at variance with the studies of Moses and colleagues in another Australian population,11 which found that prevalence of GDM in a similar low-risk group of women was 2.8%. Excluding this low-risk group from screening would still leave 80% of women requiring tests, but would miss nearly 10% of all cases of GDM. Both these estimates of the number who would be missed by selective screening are based on current criteria for diagnosis of GDM, which, as indicated earlier, are not based on studies of pregnancy outcome. It is perhaps wise at this time to wait for the development of evidence-based criteria for pregnancy outcome before making further recommendations on screening for GDM. If possible, universal screening should continue for the present. To fail to diagnose this condition denies the opportunity for intervention to improve pregnancy outcome and the long-term health prospects of both the baby and the mother. J Dennis Wilson
©MJA 2001
Readers may print a single copy for personal use. No further reproduction or distribution of the articles should proceed without the permission of the publisher. For permission, contact the Australasian Medical Publishing Company. Journalists are welcome to write news stories based on what they read here, but should acknowledge their source as "an article published on the Internet by The Medical Journal of Australia <http://www.mja.com.au>". <URL: http://www.mja.com.au/> © 2001 Medical Journal of Australia. We appreciate your comments. |