How many hours do I need to fast before a blood test?
Fasting glucose needs 8 to 12 hours, water allowed. Cholesterol mostly no longer requires it, though triglycerides read truer fasted. HbA1c, thyroid, kidney and liver tests need no fasting at all, so the honest answer depends on which test you are having.
When in doubt, ask whoever ordered it, or book early morning and skip breakfast to cover everything.
Skip the pre-test theater: crash-behaving for three days paints a false picture. The useful result reflects your actual life, and water before the draw makes veins easier, not results worse.
See a doctor promptly if
- Do not fast without medical advice if you take insulin or a sulfonylurea for diabetes. The risk of a hypo outweighs the tidier number
- Feeling shaky, sweaty, confused or faint while fasting means eat something now and rebook the test
- Never skip regular prescription medicines to fast unless the requesting doctor specifically told you to
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Answered by the AskDoc Medical Team · Medically reviewed under our Medical Director, MBBS · How we review
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