Patient Guide

Fasting Before a Blood Test: Complete Guide

Name: Fasting Before Blood Test: What to Eat, Drink & Avoid
Creator: Suman Konda
Published: 2026-07-05

Getting fasting wrong before a blood test can produce inaccurate results. Here's exactly what to do, and not do, before your test.

Written by Suman Konda, PharmD, Clinical Pharmacist · Editorial policy · Not medical advice

Last updated: 01 August 2026 · How we check our content

Which tests require fasting?

Test	Fasting Required?	Fasting Duration
Fasting Blood Sugar (FBS)	Yes	8–12 hours
HbA1c	No	Any time
Lipid Profile (Cholesterol)	Yes (ideal)	9–12 hours
KFT (Creatinine, Urea)	No	Any time
LFT (Liver Function)	Preferred	8 hours
CBC (Blood Count)	No	Any time
Thyroid (TSH)	No	Any time (morning preferred)
Vitamin D, B12	No	Any time
Ferritin / Iron Studies	Preferred	8 hours (morning)
Uric Acid	Preferred	4–8 hours
CRP / ESR	No	Any time
PSA (Prostate)	No	Any time (avoid ejaculation 48hr before)
Urine Routine	No	First morning sample preferred

Can I drink water while fasting?

YES: plain water is allowed and recommended

Drinking plain water (no sugar, no flavouring) during a fasting period does NOT affect blood sugar, lipid or most other test results. In fact, drinking water before the test helps: it keeps you hydrated, makes veins easier to find, and reduces dizziness after blood is drawn. Drink 2–3 glasses of plain water before your test.

What you CANNOT have during fasting

Tea or coffee, even without sugar. Caffeine and milk affect test results
Fruit juice: high sugar content spikes blood glucose
Any food: biscuits, fruits, anything solid
Chewing gum: stimulates insulin release
Alcohol: affects liver enzymes and triglycerides for 24–48 hours

What about medications?

Take most regular medications with a sip of water

Unless your doctor specifically says to skip your medications before the test, continue taking them with a small sip of water. This especially applies to blood pressure medication, thyroid medication (levothyroxine) and heart medications: missing a dose can be dangerous. However: avoid metformin if you're having a contrast scan same day. Always ask your doctor or lab for specific guidance on your medications.

Best time to get a fasting blood test

Ideal: 7–9 AM after overnight fast, Most people fast naturally overnight while sleeping. A morning appointment means less hunger and discomfort. Stop eating at 10 PM the previous night, drink water in the morning, go to the lab by 8 AM. Eat immediately after the test.

What happens if I accidentally ate?

Tell the lab technician and your doctor immediately. For fasting blood sugar and lipid profile, eating beforehand will give falsely elevated results. Most labs will reschedule rather than draw an inaccurate result. For tests that don't require fasting, eating is not a problem at all.

Special situations

Diabetics: Ask your doctor specifically whether to take insulin or diabetes tablets before a fasting test
Children under 5: Prolonged fasting can cause hypoglycaemia, ask your paediatrician for specific guidance
Pregnant women: Keep the fasting period shorter (6–8 hours maximum) and eat immediately after
Thyroid medication (levothyroxine): TSH is ideally measured before taking the morning dose