Arriving without fasting when a test requires it — or fasting unnecessarily and delaying care — is an avoidable mistake that affects thousands of blood tests daily. Here's a clear guide to which tests genuinely need fasting and which don't.
Tests that require fasting (8–12 hours)
- Fasting glucose — food raises blood sugar; non-fasting results cannot be compared to fasting reference ranges
- Oral glucose tolerance test (OGTT) — must be done after fasting with baseline glucose established
- Fasting insulin — used alongside fasting glucose to assess insulin resistance
- Lipid panel (full) — triglycerides are significantly affected by recent food; fasting LDL is more accurate
- Fasting cortisol — though less commonly fasting-specific, morning fasting levels are the standard reference
Tests that do not require fasting
- Full blood count (FBC / CBC) — not affected by food
- HbA1c — reflects 3-month average glucose; a single meal doesn't affect it
- Thyroid function (TSH, T3, T4) — not affected by recent food
- Liver function tests — not materially affected by fasting
- Kidney function (creatinine, urea, eGFR) — not food-dependent
- Inflammatory markers (CRP, ESR) — not food-dependent
- Non-fasting cholesterol — if only total cholesterol and HDL are needed, fasting is not required
Practical tips for a fasting test
Water is always allowed — hydration makes veins easier to find and doesn't affect results. Take your morning medications with a small amount of water unless your doctor specifies otherwise. Book a morning slot so the fast takes you through sleep. Avoid intense exercise the day before, which can affect some results including glucose and creatinine.