OGTT

What it is

The OGTT (Oral Glucose Tolerance Test) is a blood test that involves drinking a standardised sugar solution containing 75 g of glucose, then measuring blood glucose at fixed intervals. It is the gold-standard test for detecting type 2 diabetes or a pre-diabetic state — and in the context of PCOS, for evaluating glucose tolerance and insulin resistance.

The ESHRE 2023 guidelines recommend the OGTT over a simple fasting glucose test for people with PCOS who have metabolic risk factors (excess weight, family history of diabetes, acanthosis nigricans, elevated HOMA-IR).

How the test works

The test is performed after fasting for at least 8 hours. A first blood sample is taken at time T0 (fasting glucose and insulin). You then drink the glucose solution. Further blood samples are taken at:

• T60 min — glucose and insulin at 1 hour
• T120 min — glucose and insulin at 2 hours

You should remain seated and at rest throughout the test, without eating or smoking. Some protocols include an additional sample at T30 min.

Normal values and glucose interpretation

The diagnostic thresholds for 2-hour (T120) glucose are as follows:

• Normal: < 7.8 mmol/L (< 140 mg/dL)
• Impaired glucose tolerance (pre-diabetes): 7.8 – 11.0 mmol/L (140 – 199 mg/dL)
• Diabetes: ≥ 11.1 mmol/L (≥ 200 mg/dL)

Fasting glucose (T0) is also interpreted: a value > 5.6 mmol/L (100 mg/dL) indicates impaired fasting glucose, and > 7.0 mmol/L (126 mg/dL) is diagnostic of diabetes without needing to wait for T120.

Insulin levels: what the OGTT also reveals

When insulin is measured at all three timepoints, the OGTT can detect compensated insulin resistance — a situation where blood glucose remains normal only because the pancreas is producing excess insulin to compensate.

A peak insulin level at T60 above 150 mU/L is a marker of severe insulin resistance, even when glucose levels remain within normal range. Insulin values are not standardised across laboratories and must always be interpreted in clinical context.

Why fasting glucose alone is not enough

In PCOS, a normal fasting glucose can mask significant glucose intolerance or insulin resistance. The post-load response (at 2 hours) is often more informative. This is why ESHRE 2023 recommends the OGTT rather than fasting glucose alone in PCOS patients with risk factors.

Key takeaways

• 75 g oral glucose, blood draws at T0, T60 and T120 min
• Normal 2-hour glucose: < 7.8 mmol/L
• Pre-diabetes: 7.8 – 11.0 mmol/L at 2h
• Diabetes: ≥ 11.1 mmol/L at 2h
• Simultaneous insulin measurement detects compensated resistance
• Recommended by ESHRE 2023 in the presence of metabolic risk factors