Investigations

1st investigations to order

oral glucose tolerance test

Test
Result
Test

Performed at 24 to 28 weeks gestation in women with risk factors for GDM (or earlier in those with a previous history of GDM).

The UK National Institute for Health and Care Excellence (NICE) recommends a 75-g 2-hour oral glucose tolerance test (OGTT) at 24 to 28 weeks for any woman with one of more of the following risk factors:[4]

  • BMI >30 kg/m²

  • A previous baby weighing ≥4.5 kg

  • A first-degree relative with diabetes

  • Ethnic origin associated with high prevalence of diabetes. NICE no longer specifies which ethnic origins this includes, leaving this to be decided at a local level or by individual clinicians.

For any woman with a history of GDM in a prior pregnancy, NICE recommends a 75-g 2-hour OGTT (or self-monitoring of blood glucose) as soon as possible after the booking appointment.[4]

Consider an OGTT to exclude GDM if a pregnant woman has glycosuria of 2+ or above on one occasion or glycosuria of 1+ on two or more occasions.[4]

Globally there is variation in protocols for screening and diagnosis of GDM so check your local guidance.

Result

fasting plasma glucose >5.6 mmol/L (100 mg/dL) or 2-hour plasma glucose >7.8 mmol/L (140 mg/dL) indicates GDM according to NICE recommendations.[4] The World Health Organization diagnostic criteria for GDM are any one or more of the following: fasting plasma glucose 5.1 to 6.9 mmol/L (92-125 mg/dL), 1-hour plasma glucose ≥10.0 mmol/L (180 mg/dL) following a 75-g oral glucose load, 2-hour plasma glucose 8.5 to 11.0 mmol/L (153-199 mg/dL) following a 75-g oral glucose load[2]

Investigations to consider

random blood (plasma) glucose

Test
Result
Test

In the setting of symptoms suggestive of hyperglycaemia (e.g., polyuria, polydipsia), a random glucose test may be appropriate.​​ Testing may be confirmed on the following day if glucose elevations are equivocal.

Result

≥11.1 mmol/L (≥200 mg/dL) in the presence of diabetes symptoms diagnostic for overt (rather than gestational) diabetes[1][2]

HbA1c

Test
Result
Test

Recommended as a routine test, performed shortly after any diagnosis of GDM, to identify women who may have overt pre-existing type 2 diabetes (rather than GDM).[4]

Result

≥48 mmol/mol (6.5%) is diagnostic of type 2 diabetes[4][53]

fasting plasma glucose

Test
Result
Test

In the setting of symptoms suggestive of hyperglycaemia (e.g., polyuria, polydipsia), a fasting glucose test (FPG) may be appropriate.​​ Testing may be confirmed on the following day if the result is equivocal.

Result

>7.0 mmol/L (126 mg/dL) is diagnostic of overt (rather than gestational) diabetes.[2] The UK National Institute for Health and Care Excellence recommends diagnosing GDM if FPG >5.6 mmol/L (100 mg/dL);[4] according to the World Health Organization, FPG of 5.1 to 6.9 mmol/L (92-125 mg/dL) is diagnostic of GDM.[2]

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