Fasting Blood Glucose

Also known as: FBG, Fasting blood sugar, FBS

Fasting blood glucose is the simplest, most widely used test for diabetes screening. Measured after at least 8 hours without food, it reflects your baseline blood sugar control before any meal stimulus.

Specimen

Venous blood (fluoride oxalate tube) or fingerstick capillary blood

Turnaround

1–2 hours (venous); Immediate (point-of-care)

Fasting

Required

Reference Ranges

Measurement	Reference Range	Unit	Note
Normal	70 – 99	mg/dL
Pre-diabetes	100 – 125	mg/dL
Diabetes	≥ 126	mg/dL	Requires confirmation on a separate day
Hypoglycaemia	≤ 70	mg/dL	Symptomatic below 55 mg/dL

Reference ranges are typical adult values. Your laboratory may use slightly different intervals — always use your lab's reported reference.

What This Test Measures

Glucose is the primary fuel for every cell in the body. After fasting, blood glucose reflects the liver's baseline glucose output (gluconeogenesis and glycogenolysis) balanced against insulin's suppressive effect. In diabetes, either insulin is absent (type 1) or cells resist its action (type 2), causing glucose to accumulate.

When Is This Test Ordered?

Diabetes and pre-diabetes screening in adults over 35
Annual monitoring in patients with risk factors (obesity, family history, gestational diabetes)
Diagnosis when combined with symptoms of hyperglycaemia
Monitoring hypoglycaemia in insulin-treated patients
Gestational diabetes screening in pregnancy

When Results Are High

126 mg/dL or above on two separate fasting tests confirms diabetes. 100–125 mg/dL is pre-diabetes — a window for lifestyle reversal. Studies show 5–10% weight loss and 150 minutes/week of exercise reduce progression to diabetes by 58% (DPP study).

When Results Are Low

Below 70 mg/dL is hypoglycaemia. Below 55 mg/dL causes symptoms: sweating, shakiness, confusion. A fasting glucose below 60 mg/dL in a non-diabetic warrants investigation for insulinoma or factitious hypoglycaemia.

Fasting glucose vs. HbA1c: which should you track?

Fasting glucose captures a moment in time; HbA1c integrates the past 90 days. They are complementary. Someone with normal fasting glucose but frequent post-meal spikes may have a high A1c. Someone with reactive hypoglycaemia may have a low fasting glucose but impaired glucose tolerance.

For most adults, both tests together give the fullest picture. HbA1c does not require fasting, making it more practical for opportunistic screening.

Factors That Can Affect Your Result

Any food or caloric drink within 8 hours invalidates the test
Stress hormones (cortisol, adrenaline) raise glucose — retest when well
Corticosteroids substantially raise fasting glucose
Sample must be processed within 30 minutes or glucose will fall in the tube (use fluoride tube)
Capillary (fingerstick) values run 10–15% lower than venous values in the fasting state

Tracking This Test Over Time

Fasting glucose can fluctuate by 5–15 mg/dL between tests in the same person. The trend across multiple results — paired with HbA1c — is more informative than any single measurement. A creeping fasting glucose from 88 to 102 mg/dL over three years is clinically significant.

Track your FBG in Looms

Related Tests

HbA1c

HbA1c reflects your average blood glucose over the past 2–3 months by measuring how much glucose has permanently attached to haemoglobin. It is the gold-standard test for diagnosing and managing diabetes.

Medical disclaimer

This page is for educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted in the context of your symptoms, history, and other tests. Always discuss your results with your healthcare provider. Content reviewed by the Looms medical team, 2026-06-01.

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