Fasting Glucose
Metabolic screening in PCOS
What it is
Fasting blood glucose is the measurement of blood sugar concentration after at least 8 hours without food or caloric drink (typically overnight). It is the simplest standardised test for screening diabetes and prediabetes, and its interpretation is consistent across international guidelines.
It measures blood glucose at metabolic rest — in other words, the body's ability to maintain a low blood sugar without recent food intake. An elevated fasting glucose reflects either insufficient insulin secretion (advanced type 1 or type 2 diabetes) or insulin resistance severe enough to overwhelm the pancreas's compensatory capacity.
Why it matters in PCOS
PCOS is strongly associated with insulin resistance — present in 50 to 70% of people with PCOS regardless of body weight. This insulin resistance substantially increases the risk of developing prediabetes and then type 2 diabetes, sometimes as early as the third or fourth decade.
Fasting glucose is therefore a core component of the metabolic screening recommended in PCOS. The 2023 ESHRE guideline recommends a full metabolic assessment (fasting glucose + OGTT + lipid panel) in all people diagnosed with PCOS, particularly those with overweight, obesity, or a family history of diabetes.
However, fasting glucose has an important limitation in the PCOS context: it is less sensitive than the oral glucose tolerance test (OGTT) for detecting early glucose regulation abnormalities. A normal fasting glucose does not exclude impaired glucose tolerance at the 2-hour OGTT timepoint.
Normal values and diagnostic thresholds
- Normal: < 5.6 mmol/L (< 100 mg/dL)
- Prediabetes (impaired fasting glucose): 5.6–6.9 mmol/L (100–125 mg/dL)
- Diabetes: ≥ 7.0 mmol/L (≥ 126 mg/dL) confirmed on two separate occasions
When is it measured?
Fasting glucose is measured in the initial metabolic workup, after a strict fast of at least 8 hours (ideally 10–12 hours), preferably in the morning. It should be repeated annually in people with PCOS who have metabolic risk factors (overweight, family history of diabetes, borderline prior result). It should be accompanied by an OGTT and a fasting insulin level for a complete assessment of insulin resistance.
Key takeaways
- Insulin resistance is present in 50–70% of people with PCOS
- Normal < 5.6 mmol/L; prediabetes 5.6–6.9 mmol/L; diabetes ≥ 7.0 mmol/L
- Less sensitive than OGTT for detecting early abnormalities in PCOS
- ESHRE 2023: full metabolic panel (fasting glucose + OGTT) in all PCOS workups
- Annual monitoring if metabolic risk factors present