Fasting Blood Glucose Test Explained: Preparation and Results

A fasting plasma glucose (FPG) test measures the amount of glucose in a blood sample after a person has not eaten for a specified period. Clinicians may use it to screen for or help diagnose prediabetes and diabetes. It is a snapshot, unlike the A1C test, which estimates average glucose over a longer period.

Follow the lab's preparation instructions

For an FPG test, a clinician commonly asks for at least eight hours without food; sips of water are generally allowed. Do not assume that every glucose test requires fasting. An oral glucose tolerance test, random glucose test, pregnancy screening test, and A1C each have different procedures. Confirm instructions with the ordering clinician or laboratory, especially if fasting could affect medicines or another health condition. Do not change medication timing without medical advice.

A number needs context

NIDDK explains that fasting glucose results are used in clinical evaluation, alongside symptoms and other findings. This guide does not provide a personal diagnostic threshold. A diagnosis often requires repeat or confirmatory testing unless a clinician determines the situation is clear. Pregnancy uses different testing approaches and thresholds. A single result is not a personalized diagnosis or a reason to change treatment on your own.

Ask what happens next

Ask which test was ordered, whether you should fast, when and how results will be shared, and whether another test is needed. Tell the clinician about medicines, acute illness, pregnancy, and any difficulty fasting. Results can differ between FPG and A1C because the tests measure different aspects of glucose.

If a result is outside the expected range, a healthcare professional can explain how it relates to symptoms and health history.

What the journal evidence adds

The 2023 Diabetes Care guideline describes laboratory glucose measurement as one component of screening, diagnosis, and monitoring. The method and sampling circumstances matter; a venous laboratory result is not interchangeable with every home-device reading.

Before fasting, confirm the instructions for the exact test, including what to do about medicines and what to do if you feel unwell. If a result seems inconsistent with other measurements, ask the clinician how they will reconcile the information. Keep the test name, date, and units when sharing the report. Confirmation, diagnosis, and treatment goals depend on your circumstances. This article does not provide a cutoff chart or ask readers to repeat prolonged fasting or adjust glucose-lowering treatment to produce a different number.

Sources and further reading

• Diabetes Tests & Diagnosis - National Institute of Diabetes and Digestive and Kidney Diseases. NIH patient guidance.
• Fasting for a Blood Test - MedlinePlus, U.S. National Library of Medicine. NIH patient guidance.
• Guidelines and Recommendations for Laboratory Analysis in the Diagnosis and Management of Diabetes Mellitus - Diabetes care (2023). clinical guideline.

Health disclaimer

This article is for general education only. Its information and suggestions are not medical advice, diagnosis, or treatment and may not be right for everyone. Ask a qualified health professional for advice that fits your health, medications, and needs before changing your diet, exercise, supplement use, or care.