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How to test for glucose intolerance

8 November, 2012

How to test for glucose intolerance

Gestational diabetes represents a serious health risk to both mothers and babies, so early recognition is crucial. Noreen Dunnachie describes the correct way to administer the oral glucose intolerance test.
Midwives magazine: Issue 6 :: 2012

Gestational diabetes represents a serious health risk to both mothers and babies, so early recognition is crucial. Noreen Dunnachie describes the correct way to administer the oral glucose intolerance test.

The definition and, therefore, recognition and treatment of gestational diabetes have changed somewhat over the years (McCance et al, 2010). Current literature describes the condition as varying degrees of glucose/carbohydrate intolerance that is first diagnosed during pregnancy, usually resolving after birth. In addition, it is now noted that glucose intolerance in pregnancy may represent undiagnosed type 2 diabetes or, rarely, type 1 diabetes (Bothamley and Boyle, 2009). Since untreated gestational diabetes mellitus (GDM) can increase the risks of fetal macrosomia, birth trauma to mother and baby, the need for induction of labour or CS, perinatal death, transient neonatal morbidity and even childhood obesity and diabetes (NICE, 2008), appropriate diagnosis and timely treatment are paramount.

Although controversy remains around the most appropriate techniques for screening and diagnosing GDM, the benefit of using the oral glucose tolerance test (OGTT) is acknowledged by the Scottish Intercollegiate Guidelines Network (SIGN) (2010), which recognises the significant relationship between maternal glucose levels and pregnancy outcomes. Since the majority of women with GDM are asymptomatic (Robson and Waugh, 2008), it is important to identify and recognise the risk factors as advised by both SIGN (2010) and NICE (2008), which include a booking BMI of greater than 30, previous GDM, previous delivery of a macrosomic baby (4.5kg or more), a family history of diabetes and a family origin with a high prevalence of diabetes. All women should be assessed for the presence of these risk factors at their booking visit and the significance of the OGTT explained.

The test
The test is usually carried out at around 24 to 28 weeks’ gestation, unless there is a history of previous GDM, in which case an OGTT may be performed earlier. Following an overnight fast, a venous blood sample is obtained for the measurement of fasting blood glucose levels and glycated haemoglobin (HbA1c), as well as routine haemoglobin assessment. The woman is then given a drink of a 75g glucose load dissolved in 150ml of water, which she should be encouraged to drink within 10 minutes. A further blood glucose sample should be obtained at a two-hour interval.

As this test is assessing the physiological reaction to a measured dose of glucose, it is imperative that the midwife advises the woman to remain fasted until the test is complete. Although local guidelines may vary, it is generally recommended that the woman also refrains from smoking and rests within the unit until a light diet is offered prior to discharge. In addition, the midwife should take this opportunity to offer advice and generate discussion with the woman regarding exercise and her diet.

Fast prior to test

Drink glucose

Blood test

Monitor blood glucose

The woman should be informed of the timescale involved in receiving her results (normally within 24 to 48 hours), and be given a brief overview of follow-up, should her results be impaired. The adoption of internationally agreed criteria for GDM is recommended by SIGN (2010). Impaired glucose tolerance results suggestive of GDM are a fasting blood glucose level greater or equal to 5.1mmol/l, or a two-hour result greater than or equal to 8.5mmol/l. Consequently, SIGN (2010) advises that a fasting result greater than 7.0mmol/l, or a two-hour result greater than 11.0mmol/l, is indicative of pre-existing type 1 or type 2 diabetes and, therefore, should be managed within a multidisciplinary clinic.

When considering the results of the HbA1c level, it is noted that although this measurement generally reflects average blood glucose levels over the previous two to three months, NICE (2008) does not advocate using this test routinely in the second and third trimesters of pregnancy. This is due to difficulties with the interpretation of results and is further highlighted by Nielsen et al (2004), who found that pregnant women have slightly lower HbA1c concentrations than non-pregnant women. This is of clinical importance when considering an appropriate reference range in pregnancy.

Following an impaired OGTT, the woman will be required to monitor her blood glucose levels at home and record them in a diary, so adequate training in how to use a blood glucose meter must be given. Although there is no established standard as to how frequently a woman should check her blood glucose level, the ultimate goal is to maintain a level within acceptable glycaemic targets (Conway, 2012).

It is recognised that for the majority of women with GDM, optimal glycaemic control can be achieved through dietary and lifestyle changes alone. Therefore, appropriate information and education needs to be offered and tailored to meet the needs of women on an individual basis (Bothamley and Boyle, 2009; Conway, 2012). In addition, women with GDM should be reassessed in the postnatal period (generally around six weeks postpartum) in order to clarify the diagnosis and exclude existing diabetes (SIGN, 2010). This opportunity is paramount, as many women with GDM will go on to develop type 2 diabetes in later life. Early identification and adoption of therapeutic measures can delay and even prevent onset (Conway, 2012).

Noreen Dunnachie
Midwife, Ayrshire Maternity Unit, University Hospital Crosshouse

Illustrations by Ben Hassler


Bothamley J, Boyle M. (2009) Medical conditions affecting pregnancy and childbirth. Radcliffe Publishing: Oxford.

Conway DL. (2012) Gestational Diabetes Mellitus. In: Queenan JT, Spong CY, Lockwood CJ. (Eds.). Queenan’s management of high-risk pregnancy: an evidence-based approach – sixth edition. Wiley-Blackwell: Oxford.

McCance DR, Maresh M, Sacks DA. (2010) A practical manual of diabetes in pregnancy. Wiley-Blackwell: Oxford.

NICE. (2008) Diabetes in pregnancy: management of diabetes and its complications from pre-conception to the postnatal period. NICE: London.

Nielsen LR, Ekbom P, Damm P, Glümer C, Frandsen MM, Jensen DM, Mathiesen ER. (2004) HbA1c levels are significantly lower in early and late pregnancy: In: McCance DR, Maresh M, Sacks DA. (Eds.). (2010) A practical manual of diabetes in pregnancy. Wiley-Blackwell: Oxford.

Robson SE, Waugh JJS. (2008) Medical disorders in pregnancy: a manual for midwives. Blackwell Publishing: Oxford.

Scottish Intercollegiate Guidelines Network. (2010) Guideline 116: Management of Diabetes. Scottish Intercollegiate Guidelines Network: Edinburgh. See: http://www.sign.ac.uk/pdf/sign116.pdf (accessed 15 October 2012).

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