Fasting Blood Sugar Test

A Fasting Blood Sugar (FBS) test measures glucose in a blood sample collected after the fasting period advised for the test. It may be used as part of diabetes assessment or monitoring. Follow the preparation instructions from your healthcare professional or laboratory.

A fasting plasma glucose test measures blood glucose after a defined fasting period. It is commonly used as one of the laboratory methods for identifying diabetes or prediabetes, and it can also be used alongside HbA1c or an oral glucose tolerance test.

The fasting period matters because food and caloric drinks raise blood glucose. The American Diabetes Association describes fasting as at least eight hours with no food or drink except water. A single abnormal result may need confirmation unless the clinical picture is clear, so the number should be interpreted in context rather than used for self-diagnosis.

Fasting glucose is particularly useful when it is collected under a clearly defined fast. Water is usually allowed, but calories from food, milk, sugar, juice or sweetened drinks can invalidate the fasting condition. The clinician may also ask about medicines taken that morning. If a fasting value is borderline, HbA1c can provide complementary information because it reflects a longer period, while repeat fasting glucose can show whether the result persists. People with symptoms of very high or very low glucose should seek clinical assessment rather than waiting for routine website interpretation.

Fasting blood sugar may be ordered when evaluating blood-glucose control or possible diabetes, or as part of ongoing monitoring. The requested fasting period and medicine instructions should be confirmed for the individual patient.

Fasting plasma glucose is interpreted using diagnostic cutoffs rather than a simple 'normal range'. In nonpregnant adults, values below 100 mg/dL are generally in the normal category, 100–125 mg/dL in the prediabetes range, and 126 mg/dL or higher can meet a diabetes threshold. Except in appropriate symptomatic clinical circumstances, an abnormal diagnostic result is usually confirmed with repeat testing or another accepted diabetes test.

The result represents glucose at one point in time. HbA1c provides a longer-term view, while illness, stress, medicines and the quality of the fast can influence fasting glucose.

One fasting glucose result does not describe day-to-day glucose control and should not be used to diagnose or exclude diabetes without the appropriate clinical process. Pregnancy uses different diagnostic criteria. If the requested test is part of a combined panel, preparation instructions may differ, so the laboratory or clinician's fasting instructions should be followed.

Fast for at least eight hours unless the ordering clinician or laboratory gives different instructions. Water is usually allowed. Tell the booking team if the patient has diabetes, is pregnant, is taking glucose-lowering medicine, or has been advised not to fast.