ObstetricsTier 2Topic

Oral glucose tolerance test and polycose test

The tests

Polycose (glucose challenge test, GCT) - a non-fasting 50g oral glucose load with a 1-hour venous glucose measurement - a screening, not diagnostic, test in protocols that use it
75g OGTT - fasting, 1- and 2-hour venous glucose measurements - the diagnostic test for GDM in Australia (ADIPS one-step approach)
Australia predominantly uses the one-step 75g OGTT at 24-28 weeks (+/- an early OGTT if high risk), rather than the two-step (50g GCT then confirmatory 100g OGTT) approach more common in North America

Both tests exploit the normal insulin response curve to an oral glucose load - GDM is defined by an exaggerated/delayed rise reflecting relative insulin resistance/insufficiency

75g OGTT thresholds (ADIPS 2025) - fasting >=5.3, 1h >=10.6, 2h >=9.0 mmol/L; any single value is diagnostic
50g GCT (where used): 1-hour venous glucose >=7.8 mmol/L -> proceed to a diagnostic OGTT; not diagnostic alone
Patient must be fasting overnight (8-14h) for the 75g OGTT; the 50g GCT does not require fasting

Maintain a normal diet in the days preceding the test - do not carbohydrate-restrict, which can falsely lower results
Repeat testing at 24-28 weeks even if an early test was normal, in women with risk factors

HbA1c is used to diagnose overt diabetes but is not sensitive enough alone to diagnose GDM, given the more modest glucose elevations involved

A single abnormal OGTT value is sufficient for a GDM diagnosis, and management is initiated regardless of the degree of elevation above threshold

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