Short answer: A fasting blood glucose test measures the amount of glucose (sugar) in your blood after at least eight hours without food or drinks other than water. It shows how your body handles sugar at rest, and it is one of the tests used to screen for and diagnose diabetes, usually together with HbA1c.
At a glance
| Measures | Glucose in the blood after at least eight hours without food |
|---|---|
| Units | mg/dL (US) or mmol/L (Europe); 1 mmol/L = 18 mg/dL |
| LOINC | 1558-6 (fasting glucose) |
| Ordered for | Screening and follow-up of diabetes, usually with HbA1c |
| Moves with | Food, stress, illness, some medicines, the time of the sample |
| Keep | Value, unit, fasting or not, and the date |
What the test measures
Glucose is the main fuel of the body's cells, and insulin keeps its level in the blood within a narrow range. After a night without food, the level reflects how well this regulation works without the effect of a recent meal.
The report may say "glucose, serum", "plasma glucose" or "fasting glucose". Glucose measured in whole blood (for example with a finger-prick meter) reads somewhat lower than in plasma, so the two are not directly comparable.
When it is ordered
Common reasons, which your doctor adapts to you:
- as part of a routine check-up or a screening for diabetes;
- when symptoms such as thirst, frequent urination or weight loss need checking;
- to follow diabetes or prediabetes over time.
How the result is written on the report
| Result on the report | LOINC code | Usual unit | Also seen as |
|---|---|---|---|
| Glucose, fasting | 1558-6 | mg/dL | mmol/L (×0.0555) |
| Glucose, serum or plasma | 2345-7 | mg/dL | mmol/L |
| Glucose, whole blood | 2339-0 | mg/dL | mmol/L |
To convert, mmol/L = mg/dL × 0.0555 (or divide by 18): 90 mg/dL ≈ 5.0 mmol/L, 126 mg/dL ≈ 7.0 mmol/L. The US uses mg/dL; the UK, Canada and many European countries use mmol/L.
Reading your report
Prepare as the lab tells you: usually 8–12 hours without food, water allowed, and your usual medicines unless your doctor says otherwise. Illness, stress, some medicines and a late meal can raise the value. Guidelines use repeated tests and other measures (HbA1c, a glucose tolerance test) before a diagnosis, so a single result is a starting point for a conversation with your doctor, not a conclusion.
Keeping the results in your record
Scan or photograph the report in Anpheros Daily: each value is read with its unit and the laboratory's reference range, you check it, and it is stored with its LOINC code as an HL7 FHIR Observation, a format used by many hospitals and national health systems. Results from different laboratories line up on one chart over time, and the lab unit converter helps when two reports use different units. Our guide on how to read lab results explains reference ranges and flags in general; the lab results guide for a new doctor shows how to share them.
What this page does not do
It explains what the test is and how the result is written. It does not interpret your result, diagnose or recommend treatment: the meaning of a value depends on your history, your other results and how the sample was taken, and that is your doctor's job.
Related pages
Sources
Frequently asked questions
How long do I need to fast before a glucose test?
Usually at least 8 hours, often overnight. Water is allowed. Follow the instructions of your lab or doctor.
How do I convert glucose from mg/dL to mmol/L?
Multiply by 0.0555 (or divide by 18). For example, 100 mg/dL is about 5.6 mmol/L.
Is my glucose meter reading the same as the lab result?
Not exactly. Meters usually test whole blood from a finger and have a tolerance; the lab tests plasma. Use the lab for diagnosis and the meter for day-to-day tracking, as your doctor advises.
Can I drink coffee before a fasting glucose test?
Usually no: only water. Black coffee can affect some results. Ask your lab if in doubt.
Try it in Anpheros Daily
Photograph your lab report and Anpheros Daily lines up each value with its reference range, so you can compare reports from different laboratories.
How it works in the app →
Anpheros builds a medical record that the patient controls and can share with any clinic, on an HL7 FHIR R4 platform, based in the EU. These guides help you understand and organise your health; they do not replace your doctor.
About Anpheros →
AnpherosThe more you log, the clearer your record
Your health history in one place, under your control.
Free · Works in your browser · No card
