Selective versus universal screening for gestational diabetes mellitus: an evaluation of predictive risk factors

Abstract

In 1998, both the American Diabetes Association (ADA) and the WHO Consultation on diabetes published recommendations on diagnosis and classification of diabetes mellitus that included comments on GDM.^4,5 The Australasian Diabetes in Pregnancy Society (ADIPS) has also published GDM guidelines.⁶

Both the ADA and ADIPS recommendations acknowledge that there are variable levels of risk for GDM and, consequently, that selective rather than universal screening can be considered. Selective screening both reduces costs and, for women deemed not to need screening, eliminates the minor physical inconvenience of the procedure and any anxiety raised by the possibility of suffering diabetes. Both ADA and ADIPS list risk factors for GDM (Box 1). The WHO Consultation's delineation of risk factors for GDM was less clear.⁵

We tested the hypothesis that selective screening for GDM is a practicable alternative to universal screening. We also investigated the effect of using the different age criteria of ADIPS and ADA as a basis for selective screening.

Study population

Data retrieval

Information on risk factors for these women was obtained from medical records containing details of pregnancy management and delivery, endocrinologist's and dietitian's notes and laboratory records (by R X D). If racial heritage was unclear, patients were telephoned at home to obtain more details. Body mass index (BMI) was available for only 290 of the 313 women (93%), but other data were available for over 99%.

Control group: For the 5719 women without GDM, information on age was also obtained from the hospital medical records. However, it was impracticable to investigate racial heritage as closely for this group as for the case group. Therefore, if country of birth was recorded as being in Europe, Asia or Central and South America, it was used for risk categorisation (45.5% of women). All other women were allocated to risk groups in the same proportions as found in the case group. While this biases the outcome in favour of the null hypothesis, it is more accurate than making no such allocation at all.

BMI and family history were not available for the 5719 women without GDM. Therefore, the BMI comparison used BMI data obtained from 303 consecutive non-diabetic women presenting for a glucose challenge test at about 28 weeks' gestation as part of a 1995 study at Sunshine Hospital.⁸ As the patient catchment area was unchanged between 1995 and 1998, this group should represent an unbiased sample of women who presented between 1995 and 1998.

For the family history comparison, a recent estimate of prevalence of diabetes mellitus in Australia⁹ was used for the non-GDM patients. Background risk was corrected for the bias caused by the tendency of patients with diabetes to visit their doctors twice as often as non-diabetic patients.¹⁰ It was also doubled to give a worst-case estimate, as each parent might pass on heritable risk independently.

Statistical analyses and ethics approval

Ethics approval for this study was not required by the Victorian Health Services Act 1988 and was not sought. Data were permanently de-identified after analysis.

Results

Risk-factor comparison

To determine the value of racial heritage as a predictor of GDM in isolation from family risk, we determined the OR for high-risk racial heritage among women with no family history of diabetes mellitus. This OR was not statistically different from the earlier OR for high-risk racial heritage that included women with a family history.

Furthermore, birth in Australia, New Zealand or North America (of non-Indigenous background) does not necessarily equate with low heritable GDM risk. Of the 313 women with GDM, 94 were born in these countries, but 19 of these had high-risk racial heritage.

Finally, we also assessed whether glycosuria in pregnancy or previous GDM had any extra value as predictors of GDM. All women with these risk factors qualified for screening on other grounds.

Effect of selective screening

Selective screening on the basis of at least one risk factor, using the ADIPS age criterion (≥30 years), would have missed 12 women with GDM (95% CI, 6-19; 4%). Using the ADA age criterion (≥25 years), selective screening would have missed only two women with GDM (95% CI, 0-5; 0.6%). Furthermore, χ² tests showed that the proportions of women with GDM who had risk factors other than age did not differ significantly between age groups (≥30 years, ≥25 years and all ages); all P values exceeded 0.67.

Among the women without GDM, 83% were aged 25 years or over and 48% were aged 30 years or over. A selective screening policy could therefore have saved testing up to 17%-52% of women without GDM, depending on the age threshold used and the presence of risk factors other than age.

However, our data also show that it is important to examine risk factors other than age even when the lower age threshold is used, as the other factors underlying susceptibility to GDM operate irrespective of age. Family history and heredity are immutable, and obesity may also be partly under genetic control. These observations are also consonant with the theory that pregnancy unmasks diabetes mellitus prematurely.¹²

We also found that previous GDM and glycosuria in pregnancy added nothing to the above four criteria for screening. However, this does not mean that GDM in a previous pregnancy should be ignored. It is often regarded as a criterion for a full OGTT, without a prior glucose challenge test, earlier than 28 weeks' gestation; the wisdom of this practice is not doubted.

Not only are the ADA criteria for screening diagnostically safer than the ADIPS criteria, they are also more precise in their definitions. Australian women would benefit if ADIPS recommendations were brought into line with ADA recommendations.

Our conclusions differ from those of Moses and colleagues, who found no benefit from selective screening in their study in the Illawarra region of New South Wales.^13,14 Indeed, Moses has championed universal screening.¹⁵ However, the Illawarra protocol for GDM screening varied from contemporary practice, as it did not measure fasting glucose level or stringently control the time between the glucose load and blood sampling, making comparison difficult. Its outcomes have been questioned.^16,17

In North America, a recent, albeit small, retrospective study of GDM screening in Michigan specifically assessed the ADA selective screening recommendations and concluded that they can be used as they miss "few" (4%) women with GDM.¹⁸

A larger study was reported by the Toronto Trihospital Investigators.¹⁹ They proposed a scheme that used among its criteria those later published by ADA to differentiate risk levels for GDM, sparing 35% of pregnant women the need for a glucose challenge test. This is at least twice as efficient as using the ADA criteria in our population, which potentially spared up to 17% of women a glucose challenge test (depending on the presence of risk factors other than age). However, an accompanying editorial concluded that the Trihospital criteria were "so hard to discern" that universal screening would continue as the only practicable alternative.²⁰ For busy clinicians, simple systems are essential.

Our study differed from the Toronto study in that only women with positive results on a glucose challenge test proceeded to an OGTT, while, in Toronto, all women had a full OGTT. We will have missed the small number of women who would have had positive results on an OGTT despite their negative results on a challenge test -- perhaps 3%, based on the Toronto data. Short of performing a full OGTT on all pregnant women, which is impracticable, this group will always escape detection.

The Toronto ORs for risk factors among those with GDM generally accord with ours (1.6 for age ≥35 years [95% CI, 1.1-2.5], 3.2 for BMI ≥25.1 kg/m² [95% CI, 2.1-4.8], 4.8 for Asian race [95% CI, 3.0-7.6]), but our results differ in two ways. Toronto race groupings, apart from "Asian", are difficult to interpret and, by using "white" and "black", ignore the extreme variation among "white" Europeans. Secondly, family history of diabetes mellitus in Toronto did not correlate significantly with higher risk of GDM, whereas our findings strongly support the inclusion of family history among criteria for a glucose challenge test.

Our study, along with the Michigan and Toronto studies, indicates that a selective approach to GDM screening in pregnancy is justifiable. Our simplified algorithm for selective screening is shown in Box 5. In practice, the proportion of women spared a glucose challenge test by selective screening will vary between populations. Consequently, whether selective screening is locally practicable will be a decision for individual groups of obstetricians, endocrinologists and pathologists with local knowledge. It is clear that consideration of a patient's age, rigorous questioning about racial and family history and accurate measurement of height and weight can reduce the need for screening among suitable populations. Selective screening can reduce costs and maternal anxiety, with negligible loss in diagnostic power.

Nevertheless, GDM poses still further challenges. Australia needs a better-directed, more organised, totally inclusive approach to follow-up of women who have had GDM. The third who will go on to develop diabetes mellitus need to be tracked, monitored, and managed prospectively into a healthier future.

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Authors' details

Reprints will not be available from the authors.
Correspondence: Dr R X Davey, Western Hospital, Gordon Street, Footscray, VIC 3011.
richard.daveyATwh.org.au

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