To the Editor: Gestational diabetes mellitus (GDM) is glucose intolerance of variable severity with onset or first recognition during the current pregnancy.1 GDM is one of the conditions requiring an entry on the New South Wales Midwives Data Form. Effective healthcare planning is dependent on accurate data collection. To our knowledge, the verity of the midwives data with respect to GDM, or indeed other entities, has not been checked for many years.

A previous article has demonstrated that the accuracy of GDM data collection is poor, with the incidence of GDM being under-reported.2 Recently, an article from Victoria also showed a recorded rate of GDM about half that of the acknowledged incidence.3 We have recently completed a review of compliance with GDM testing in our area and, knowing the true incidence of GDM, this has allowed us to revisit the accuracy of the data being recorded on the Midwives Data Collection Form.

In the city of Wollongong, NSW, with a population of around 280 000 and about 3000 births each year, all deliveries take place at two public hospitals (Wollongong and Shellharbour) and a private hospital (Illawarra Private Hospital). It is the policy of both the Obstetric Department and the Division of General Practice that all pregnant women should be tested for GDM in accord with the ADIPS guidelines.4

All women who delivered at the three hospitals over the 6-month period from January 2002 to June 2002 were identified from the Labour Ward records. A hospital-based delivery is used by 99.3% of women in the area.5 The results of testing for GDM were determined for all of these women.

There were 1655 deliveries at the three hospitals over the 6-month period. Seven women with known type 1 or type 2 diabetes were excluded, leaving 1648 women whose data could be examined. Women were considered to have been tested for GDM (n = 1518) if they had had either a glucose tolerance test (n = 1502) or a glucose challenge test (n = 16). There were 101 women diagnosed with GDM, giving an overall incidence rate of 6.6% (prenatal clinic, 7.1%; shared-care, 6.6%; private patients, 6.3%).

The most recent midwives data indicate an incidence of 5.7% at the public hospitals and 3.1% at the private hospital. It is thus apparent that the official statistics still underestimate the incidence of GDM. A similar degree of error may also be found for other entries, and hence data should be extrapolated with caution.

A redesign of the collection form may help remove some of the errors and omissions. For the question regarding GDM, we feel accuracy could be enhanced if there were separate “Yes” and “No” boxes, rather than a single check box. This might encourage further consideration of the problem. Accuracy could be further enhanced by allowing space for the glucose tolerance test results at 0 and 2 hours — these would also be useful data in their own right.

In reply: Moses et al are correct in noting that gestational diabetes mellitus (GDM) is under-reported to the New South Wales Midwives Data Collection (MDC). The most recent validation study of the MDC was carried out in 1998. We reviewed a random sample of 1680 medical records from NSW public and private hospitals, representing 1.9% of births reported in 1998. The sensitivity and specificity of reporting of GDM to the MDC were 86.7% and 99.6%, respectively.1 In this sample, the incidence rate of GDM was 3.5% according to the MDC, and 4.0% according to the medical record review.

These population rates are lower than the rates reported by Moses et al among women attending hospitals in Wollongong. In addition to incomplete recording of diagnosed GDM on the MDC, the low rate of recording of GDM in medical records in our sample suggests that GDM was also under-ascertained at a population level. This is probably due to variations in the implementation of pregnancy screening for GDM between clinicians and across NSW hospitals.

In February 2003, the Royal Australian and New Zealand College of Obstetricians and Gynaecologists endorsed the Australian Diabetes in Pregnancy Society GDM Management Guidelines.2 The guidelines recommend universal screening for GDM, noting that selective screening may be appropriate because of limited resources or known low GDM incidence.

The suggestions for trying to improve reporting of GDM by redesigning the MDC form are welcome, and we will certainly consider them at the next review. We are also considering using the hospital Inpatient Statistics Collection (ISC), in which discharge diagnoses are classified according to the International Classification of Diseases, as an alternative source of information on maternal morbidity. We are currently reviewing a random sample of 500 medical records of mothers who gave birth in hospitals throughout NSW. The information obtained will be compared with matched ISC records provided to the NSW Department of Health to determine whether the ISC is a more reliable source of information on maternal morbidity than the MDC. In the longer term, I anticipate that the integration of the MDC with computerised medical records in hospitals will also contribute to improved reporting.

Under-reporting of maternal morbidity, including GDM, is an issue for all state and territory perinatal data collections in Australia. The information is used for planning and evaluation of healthcare services, so it is important that we get it right. I would like to thank Moses et al for raising this issue.