Investigations
1st investigations to order
oral glucose tolerance test (OGTT)
Test
Performed at 24-28 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 OGTT at 24-28 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 early 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 gestational diabetes mellitus (GDM) according to NICE recommendations.[4] The World Health Organization (WHO) 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
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
According to the World Health Organization (WHO) and International Association of Diabetes and Pregnancy Study Groups (IADPSG), a random plasma glucose level ≥11.1 mmol/L (≥200 mg/dL) in the presence of diabetes symptoms at any point in pregnancy is diagnostic for pre-existing (rather than gestational) diabetes[1][2]
HbA1c
Test
Recommended as a routine test, performed shortly after any diagnosis of gestational diabetes mellitus (GDM), to identify women who may have overt pre-existing type 2 diabetes (rather than GDM).[4] HbA1c is insufficiently sensitive to substitute for an oral glucose tolerance test as a screening test.[42][43]
fasting plasma glucose
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
Criteria differ between societies: UK National Institute for Health and Care Excellence guidelines recommend diagnosing gestational diabetes mellitus (GDM) if fasting plasma glucose (FPG) is ≥5.6 mmol/L (≥100 mg/dL);[4] according to the World Health Organization (WHO), and International Association of Diabetes and Pregnancy Study Groups (IADPSG), FPG of 5.1 to 6.9 mmol/L (92-125 mg/dL) is diagnostic of GDM, while FPG ≥7.0 mmol/L (≥126 mg/dL) is diagnostic of overt (rather than gestational) diabetes[1][2]
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