Fasting glucose: the oldest diabetes test, and still a noisy one

Fasting plasma glucose is the concentration of glucose in the blood after at least eight hours without calories. In the US it is reported in mg/dL; most other countries use mmol/L (divide mg/dL by 18 to convert). It is on almost every routine chemistry panel, which is why so many people have years of results without ever having asked for it.

The guideline numbers

The World Health Organization uses a higher lower bound for impaired fasting glucose, 110 mg/dL (6.1 mmol/L), so the same result can be prediabetes in one country's guideline and normal in another's. Online claims that fasting glucose should be "under 85" or "in the 70s" are not guideline categories. The large cohort data below found no significant change in risk across a wide band of normal values.

What raises and lowers it

• Raises: insulin resistance and type 2 diabetes, acute illness or stress, poor sleep, steroids and some other drugs, and the early-morning rise in glucose known as the dawn phenomenon.
• Lowers: weight loss, exercise, glucose-lowering drugs, and long fasts.
• Lab artefact: red and white cells keep using glucose in the tube. A sample that sits unspun at room temperature can read falsely low.

What it predicts

The Emerging Risk Factors Collaboration pooled 698,782 people from 102 prospective studies in the Lancet in 2010. Diabetes roughly doubled the risk of coronary heart disease (hazard ratio 2.00) and ischaemic stroke (2.27). In people without diabetes, the relationship with fasting glucose was weak and non-linear: there was no significant association between 3.9 and 5.59 mmol/L (about 70 to 100 mg/dL). Compared with that band, the hazard ratio for coronary heart disease was 1.11 at 5.6 to 6.09 mmol/L and 1.17 at 6.1 to 6.99 mmol/L. Adding fasting glucose to conventional risk factors did not meaningfully improve prediction.

In the ARIC study covered on the HbA1c page, fasting glucose stopped predicting heart disease once HbA1c was taken into account. Both are observational findings. The point is not that fasting glucose is useless; it is the cheapest early signal of diabetes. It is that small movements within the normal range carry little information on their own.

On GLP-1 drugs and weight loss

In STEP 1, the trial's posted registry results show fasting plasma glucose falling by a mean of 9.2 mg/dL on semaglutide 2.4 mg against 0.4 mg/dL on placebo over 68 weeks, in adults without diabetes. In people with type 2 diabetes the falls are much larger, alongside the HbA1c changes. Because GLP-1 drugs raise insulin release only when glucose is high, low blood sugar on a GLP-1 alone is uncommon; the labels flag the risk mainly when combined with insulin or a sulfonylurea. The GLP-1 blood work page covers monitoring.

How to test

• Fast 8 to 12 hours, water only, and draw in the morning.
• Same conditions each time: a normal night's sleep, no hard training the evening before, no illness.
• Repeat before acting. The ADA requires two abnormal results for a diagnosis without clear symptoms, which tells you how much single values vary.
• Finger-stick meters and continuous glucose monitors are useful for patterns but are not the same measurement as a lab plasma glucose; do not mix them in one series.
• Pair it with fasting insulin from the same draw for HOMA-IR.

Tracking it in Peptide AI

In Peptide AI, fasting glucose sits in the metabolic panel. The app reads it from a lab PDF or manual entry, scores the panel from 0 to 100 alongside an overall grade, flags values that warrant prompt physician consultation, and plots each result against your dose timeline. Log the draw date, whether you fasted, and anything unusual in the days before, such as illness, a hard session or a dose step, so the next result is compared like with like. Bloodwork tracking shows the series next to the dose log, weight and side effects, and the panels page lists every marker the app groups. More markers are in the bloodwork guides.