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Abstract

MJA 1997; 167: 14-16  

Introduction

Following the Australasian Diabetes in Pregnancy Society's (ADIPS) 1991 recommendation that all pregnant women should be tested for GDM in every pregnancy,⁵ the NSW Midwives Data Collection (MDC) recorded a gradual increase in the number of women with GDM from 1.5% in 1991 to 2.7% in 1994. In contrast, from surveys in other Australian centres since 1991, the incidence of GDM ranges from 5.5%-8.8%.^4,6-8

The difference between these rates and those recorded by the MDC suggests that many women are still not being tested for GDM in NSW and are at risk of complications which could either be prevented or substantially reduced. The purpose of our study was to estimate the extent of undiagnosed GDM in NSW for the period 1991-1994 by estimating the number of pregnant women who had not been tested for GDM.  

Methods

Incidence of GDM recorded on the MDC was compared with the incidence of GDM found in three different Australian surveys. The sources of these data are shown in Box 1. The three surveys used the 75 g oral glucose tolerance test for the diagnosis of GDM based on the ADIPS criteria.⁵ According to these criteria, GDM is diagnosed if the fasting plasma glucose level is 5.5 mmol/L and/or the two-hour plasma glucose level is 8.0 mmol/L. By combining these data, we obtain an average incidence of GDM, which we refer to as the expected incidence.

At the Mercy Hospital for Women in Melbourne, a 50 g glucose load is used and capillary blood samples taken for plasma glucose measurement; a diagnosis of GDM is made if the glucose level at one hour is 9.0 mmol/L and the glucose level at two hours is 7.0 mmol/L. The incidence of GDM during the period 1991-1994 was 8.8%.⁴ While the glucose load and diagnostic criteria are different, a comparison of the Mercy Hospital and ADIPS criteria found that they diagnosed similar women and in the same proportion.⁹

While the incidence of GDM will vary depending on the ethnic composition of the population being examined, these surveys all included a majority of people of European extraction.  

Validation of the NSW Midwives Data Collection

A 1993 survey carried out in the Wollongong area to determine the incidence of GDM was compared with the recorded MDC incidence of GDM in the area.⁶ The MDC recorded a rate of 5.0%, while the survey found a rate of 7.2%. A 50% addition to the recorded MDC incidence (as indicated by the validation study) gave a figure of 7.5%, which is similar to the 7.2% found in the survey. Therefore, the addition of a 50% correction factor to the recorded MDC incidence of GDM is a reasonable approximation.  

Results

For the whole of NSW, the percentage of women diagnosed with GDM increased from 1.5% in 1991 to 1.9% in 1992 (P < 0.001) and from 1.9% in 1992 to 2.6% in 1993 (P < 0.001). There was no significant change between 1993 and 1994. For the four-year period, the overall recorded MDC incidence of GDM was 2.2%.

In the Sydney metropolitan area, the incidence of GDM ranged from a high of 3.1% in the Central Sydney Area Health Service to a low of 1.5% in the Northern Sydney Area Health Service. The incidence of GDM in the main metropolitan areas of Sydney, Newcastle and Wollongong was 2.4%, which was significantly higher than the country areas (1.6%) (P < 0.001). Some country areas also showed marked changes over the four-year period. For example, the proportion of women diagnosed with GDM increased from 0.8% to 2.7% in the Mid West Area Health Service and declined from 3.4% to 0.8% in the Far West Area Health Service.

By applying the 50% correction factor to the recorded MDC incidence of 2.2%, the estimated incidence of GDM is 3.3%. However, the expected incidence of GDM is 6.6% (Box 1). Therefore, half of the pregnant women in NSW during the period 1991-1994, or 173 534 women, were probably not tested for GDM.  

Discussion

The recorded MDC incidence of GDM for 1991-1994 was lower than the expected incidence. With the application of the 50% correction factor to the recorded MDC incidence (as suggested by the MDC validation study and by data collection in the Wollongong area⁶ ) and a conservative estimate for the expected incidence of GDM, it is possible that half of all the women who delivered in NSW between 1991 and 1994 were not tested for GDM.

Some women may have decided not to be tested for GDM. Others may not have been able to tolerate the glucose solution. In most cases, however, the decision not to test would have undoubtedly been made by the clinician responsible.

Box 2 shows that testing for GDM is not evenly distributed, with a recorded MDC incidence range of 0.7%-3.2%. Of particular concern is that lower than average rates of testing were found in areas where a higher than average rate of GDM could be anticipated. In metropolitan Sydney, the lowest number of recorded cases of GDM were from the Northern Sydney Area Health Service. The MDC has shown that this is also the Area Health Service with the highest proportion of women aged 35 years or over at the time of delivery, a risk factor for GDM. An incidence of GDM of at least 10%⁴ could be anticipated in this age group.

Women with an Aboriginal background have a high incidence of GDM.⁸ The New England and Macquarie Area Health Service, with a high proportion of Aboriginal Australians, had a rate of GDM less than the non-metropolitan average. The Far West Area Health Service also has a high proportion of Aboriginal Australians, and the recorded incidence of GDM there fell over the four-year period from 3.4% to 0.8%.

Observational studies of women with untreated GDM have found a higher perinatal mortality rate than that found in glucose-tolerant women.^2,3 At the Mercy Hospital for Women in Melbourne, Beischer et al. showed that, despite a steady reduction in the overall perinatal mortality rate, women who were not tested for GDM (and therefore not treated) had a significantly higher perinatal mortality rate than women who were tested.⁴ An estimate can be made of the excess perinatal mortality rate for women in NSW who have not been tested for GDM ( Box 3).

Although the recorded incidence of GDM has increased (particularly in the two years after the ADIPS recommendation for universal testing was made), our results confirmed our belief that not all pregnant women in NSW are being tested for GDM. This may have implications for perinatal mortality rates.  

References

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