Routine screening for gestational diabetes: IQWiG finds indication of positive effect

Pregnant women who develop marked increased blood sugar levels during pregnancy can reduce the risk of certain birth complications if they receive treatment. This is a prerequisite for offering all pregnant women routine screening for gestational diabetes. However, potential disadvantages of this type of routine screening have not been thoroughly researched. This is the conclusion of the Institute for Quality and Efficiency in Health Care (IQWiG) in its final report published on 3 September 2009.

Even today, most pregnant women in Germany are unsystematically offered a blood sugar test to identify those women whose blood sugar levels rise too much during pregnancy. However, the women often have to pay for this test themselves. In a project commissioned by the Federal Joint Committee (G-BA), IQWiG has investigated whether this screening should be routinely offered free of charge to all pregnant women insured with a statutory health insurance fund.

Although the Institute has found an indication in its final report on assessing benefit that such a test can reduce the risk of complications in birth, certain test requirements must be met, which has not always been the case in Germany. According to Prof. Dr. med. Peter T. Sawicki, the director of IQWiG, "We cannot be certain that the tests as currently carried out in the doctor's surgery yield more benefit than harm."

Gestational diabetes is a question of definition

During pregnancy it is normal that a woman's metabolic rate changes and sugar takes longer to be absorbed by the body. In May 2008 an international study confirmed that rising blood sugar levels increase the risk, for example, of a Caesarean section or birth injuries. However, there is disagreement over when increased blood sugar levels should be treated, as there is no threshold where these risks increase dramatically.

Nevertheless, it should be noted that a diagnosis of gestational diabetes has far-reaching consequences for a pregnant woman. Not only does she have to accept the unpleasant news that something is not right, she also has to adapt her diet and take more physical exercise. In addition, blood sugar levels have to be measured several times a day and, if they do not drop to the prescribed targets within a short time, daily insulin injections have to be administered.

Treatment can have positive effects

In order to evaluate the benefit of a therapy, the Institute analysed a total of 25 studies. This yielded an indication that treatment reduces the risk of certain rare birth complications in pregnant women with a marked metabolic disorder. One such complication is shoulder dystocia. This is the name given to a delay in the birth with the attendant risk of the baby suffering a lack of oxygen. As midwives and doctors then quickly take countermeasures, injuries to mother and child may occur more frequently.

Potential disadvantages of routine screening have not been researched

Even if there is an indication of benefit from treatment, this does not automatically mean that routine screening is also useful for identifying pregnant women with gestational diabetes. Although some professional associations have been recommending this type of screening for many years, potential harms have not yet been sufficiently investigated. IQWiG could not find any studies which directly showed that routine screening was of more benefit than harm.

In view of this uncertainty, the Institute considered a long list of potential disadvantages. However, the potential risks were not assessed as being so serious that they might cancel out the potential benefit. Thus, the Institute has indirectly deduced an indication that routine screening for gestational diabetes leads to a reduction in perinatal complications.

Two-step test strategy

However, this deduction presupposes that routine screening takes place according to the selection and diagnosis of participants in the relevant therapy studies. Professor Sawicki explains, "We only obtained an indication of benefit from treatment in the case of women who were diagnosed in a particular way."

Experts around the world are not agreed on how women with a metabolic disorder should be routinely identified.
To date, three main options have been under discussion: The first proposal is that all pregnant women should undergo an oral glucose tolerance test (oGTT), which measures the metabolic reaction to the rapid absorption of an larger amount of sugar.

For this test, the pregnant woman has to attend her doctor's surgery in the morning, having fasted overnight, and drink a concentrated glucose solution. Three blood samples are subsequently taken within 2 hours. The diagnosis is confirmed if at least one of the blood sugar level measurements is too high.

Some experts consider this test of several hours' duration to be too cumbersome to be introduced for all pregnant women. They propose a pre-selection process. Consequently, the second proposal for a test strategy is that only those women undergo an oGTT who display specific risk factors for gestational diabetes, such as older age, overweight or diabetes in the family.

The third proposal is to introduce a short version of the glucose tolerance test in the pre-selection. As part of a routine appointment, the pregnant woman would drink a smaller amount of glucose solution and have only one blood sample taken after 1 hour. Only those women whose blood sugar levels were then above a pre-determined threshold would be given an appointment for a full tolerance test.

Other screening strategies were directly compared in studies

One result of the report is that those pregnant women for whom the relevant therapy studies have shown a benefit were selected by undergoing a short glucose test as the first step. "As long as good screening studies are lacking, we find it difficult to recommend other strategies", says Sawicki.

Consequently, many tests for gestational diabetes that are already being offered to pregnant women should be viewed critically. "These tests label many pregnant women as being at risk, without it being clear whether they would actually profit from having treatment", explains Sawicki. A harm is particularly likely if a woman with a mild metabolic disorder during pregnancy is recommended to have treatment which is too strong.

In IQWiG's opinion, therefore, a study that directly compares the advantages and disadvantages of different screening strategies for mother and child is overdue. According to Sawicki, "In view of the number of pregnancies per year in Germany (more than 600,000), such a study could be carried out relatively quickly."

Report preparation procedure

IQWiG published the preliminary results in the form of the preliminary report in January 2009 and interested parties were invited to submit comments. When the comments stage ended, the preliminary report was revised and sent as a final report to the contracting agency, the Federal Joint Committee, at the end of June 2009. Documentation of the written comments and minutes of the oral debate are published in a separate document simultaneously with the final report. The report was produced in collaboration with external experts.

The Institute is awaiting further information on the benefit of treatment from a US study, whose results are expected to be published in the near future. IQWiG then intends to supplement the final report.

An overview of the background, methods and further results of the final report is provided in the following Executive summary.

Contact: Tel. +49 (0)221 35685-0, info@iqwig.de