Fasting Blood Sugar
Also known as: Fasting sugar test, FBS, BSF, sugar test, khali pet sugar test, nihar munh sugar
The amount of sugar (glucose) in your blood after not eating for at least 8 hours.
Why it's ordered
Screening for and diagnosing diabetes and prediabetes.
Preparation
Fast for 8–12 hours. Plain water is allowed. Take your usual medicines unless your doctor says otherwise. If you take diabetes medicines or insulin, ask your doctor how to take them on the morning of the test, because fasting with these medicines can cause low sugar.
Typical adult values
| Normal | below 100 mg/dL (5.6 mmol/L) |
|---|---|
| Prediabetes | 100–125 mg/dL (5.6–6.9 mmol/L) |
| Diabetes | 126 mg/dL (7.0 mmol/L) or higher, confirmed on a repeat test |
Ranges vary between labs and methods. Always use the range printed on your report.
If it's high
Suggests prediabetes or diabetes. Illness, stress and steroid medicines can also raise sugar temporarily.
If it's low
Below 70 mg/dL is low (hypoglycaemia), most often with diabetes medicines. Shakiness, sweating or confusion need prompt sugar intake.
Good to know
Fasting longer than 14 hours, or eating anything (including tea with sugar), makes the result unreliable.
Common questions
How long should I fast before a fasting sugar test?
Fast for 8–12 hours. Plain water is allowed.
Can I have tea with sugar before the test?
No. Eating anything, including tea with sugar, makes the result unreliable.
Should I take my medicines before the test?
Take your usual medicines unless your doctor says otherwise. If you take diabetes medicines or insulin, ask your doctor first, because fasting with them can cause low sugar.
Is a fasting sugar of 110 high?
Yes, 110 mg/dL falls in the prediabetes range of 100–125 mg/dL. Always use the range printed on your report.