Screening

UK recommendations

In the UK, the National Institute for Health and Care Excellence (NICE) recommends offering diagnostic testing with a 75-g 2-hour oral glucose tolerance test (OGTT) at 24 to 28 weeks’ gestation to high-risk women with any one or more of the following risk factors for GDM:[4]

  • BMI >30 kg/m²

  • A previous baby weighing ≥4.5 kg

  • Family history of diabetes mellitus (first-degree relative with diabetes mellitus)

  • Family origin in an area with high prevalence of diabetes mellitus. NICE no longer specifies which family origins this includes, leaving this to be decided at a local level or by individual clinicians.

Women who have had GDM in a previous pregnancy should be offered early self-monitoring of blood glucose or a 2-hour 75-g OGTT as soon as possible after booking (whether in the first or second trimester), and a further 2-hour 75-g OGTT at 24 to 28 weeks if the results of the first OGTT are normal.[4]

GDM should be diagnosed if a pregnant woman has either:

  • A fasting plasma glucose (FPG) >5.6 mmol/L (100 mg/dL), OR

  • A 2-hour plasma glucose >7.8 mmol/L (140 mg/dL).

NICE does not recommend assessing the risk of developing GDM with FPG, random blood glucose, haemoglobin A1c (HbA1c), glucose challenge test, or urinalysis for glycosuria.[4]

It is important to take steps to encourage all pregnant women with risk factors to attend for an OGTT test. A prospective case-control study, performed before the current NICE guidelines were in place, found that without FPG screening, women ‘at risk’ of GDM (as per the NICE criteria above) experienced 47% greater risk of late stillbirth. For those who were screened, this excess was essentially eliminated. Similarly, without GDM diagnosis, women with raised FPG experienced a fourfold greater risk of late stillbirth. For those who were diagnosed, this excess was no longer apparent.[54]

Other guidelines

The International Association of Diabetes and Pregnancy Study Groups recommends a one-step screening test, while the US National Institutes of Health and the American College of Obstetricians and Gynecologists recommend a two-step test.[1][49][50]​ The American Diabetes Association and the US Preventive Services Task Force recognise that there are data to support both approaches.[3][51]

One-step method:[1]​​

  • Perform a 75-g OGTT, with plasma glucose measurement after fasting and at 1 and 2 hours, at 24 to 28 weeks of gestation in women not previously diagnosed with overt diabetes.

  • The OGTT should be performed in the morning after an overnight fast of at least 8 hours.

  • The diagnosis of GDM is made when any one of the following plasma glucose values are exceeded:

    • Fasting ≥5.1 mmol/L (≥92 mg/dL)

    • 1 hour ≥10.0 mmol/L (≥180 mg/dL)

    • 2 hours ≥8.5 mmol/L (≥153 mg/dL).

Two-step method:

  • 1-hour 50-g glucose load test, which does not have to be fasting.[3] Glucose thresholds of ≥7.2 mmol/L (≥130 mg/dL), 7.5 mmol/L (135 mg/dL), or 7.8 mmol/L (140 mg/dL) are considered abnormal.[3] Lower thresholds are more sensitive than the 7.8 mmol/L (140 mg/dL) threshold (88%-99% for 7.2 mmol/L [130 mg/dL] vs. 70%-88% for 7.8 mmol/L [140 mg/dL]) but less specific and more prone to false-positives.[55] There are no randomised controlled trials of differing thresholds; therefore, it is reasonable for institutions to consider the trade-offs specific to the population served when determining a cut-off.​[49] 

  • If glucose levels are greater than the chosen cut-off value (≥7.2 mmol/L [≥130 mg/dL ], 7.5 mmol/L [135 mg/dL ], or 7.8 mmol/L [140 mg/dL]), then a 3-hour 100-g OGTT should be performed (when the patient is fasting).[3] Two or more plasma glucose levels at or above the following thresholds establish diagnosis.[3]

    • Fasting ≥5.3 mmol/L (≥95 mg/dL )

    • 1 hour ≥10.0 mmol/L (≥180 mg/dL )

    • 2 hours ≥8.6 mmol/L (≥155 mg/dL)

    • 3 hours ≥7.8 mmol/L (≥140 mg/dL ).

    The following plasma glucose test results indicate GDM when obtained in pregnant women after the first trimester:

    • Fasting glucose: ≥7.0 mmol/L (≥126 mg/dL); in the absence of unequivocal hyperglycaemia, this must be confirmed on repeat testing.[3]​ This is usually used outside of pregnancy but may be useful in patients with signs or symptoms of hyperglycaemia. 

    • Random glucose level: ≥11.1 mmol/L (≥200 mg/dL) in patients with classic symptoms of hyperglycaemia or hyperglycaemic crisis.[3]​ This is usually used outside of pregnancy but may be useful in patients with signs or symptoms of hyperglycaemia. 

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