Simple steps for an accurate report and a comfortable visit to the lab
Published: 15 October 2026 | Updated: 16 October 2026 | By MCT Clinic and Diagnostic Centre, a unit of Millat Charitable Trust
"Khaali pet aana hai?" is one of the most common questions patients ask before a blood test. Some tests are affected by what you have just eaten, so the lab needs a sample taken after a fast. Others need no preparation at all. Getting this right is the easiest way to avoid a misleading report or a second visit.
This guide explains what fasting means, how long it usually lasts, which tests commonly need it and how to prepare. It follows guidance published by MedlinePlus (U.S. National Library of Medicine) and the NHS. The instructions your own doctor gives you always come first.
For a lab test, fasting means not eating or drinking anything except plain water for a set number of hours, often overnight. Tea, coffee, milk, juice, soft drinks, paan and chewing tobacco all count as breaking the fast. So does a "small" biscuit with your morning medicine.
Fasting matters because sugar and fats from a meal enter the blood within hours. A sample taken soon after eating can show raised glucose or triglycerides even in a healthy person, which can lead to a wrong conclusion or an unnecessary repeat test.
There is no single number for every test. The common guidance is:
| Test | Fasting usually needed? | Typical guidance |
|---|---|---|
| Fasting blood sugar (glucose) | Yes | At least 8 hours, water only |
| Oral glucose tolerance test | Yes | 8-hour fast, then a sugary drink and further samples over 2 to 3 hours |
| Cholesterol and lipid profile | Sometimes | 9 to 12 hours when your doctor asks for a fasting sample |
| Triglycerides | Often | As advised by your doctor |
| Random blood sugar | No | Can be done at any time, even after a meal |
| HbA1c | No | Reflects the average of the past 2 to 3 months |
| CBC (complete blood count) | Usually not | Ask if it is combined with a fasting test; see our CBC guide |
If your doctor has ordered several tests together, for example a lipid profile with fasting sugar, follow the longest fasting time in the list. Fasting for much longer than advised is not better; very long fasts can themselves change some results and can make you feel weak or dizzy. You can read more about individual tests in our tests catalogue, and about the HbA1c test in our guide to HbA1c normal range and results.
Yes. Plain water is allowed during a fasting test, and staying hydrated helps. When you are well hydrated, veins are easier to find and the blood draw is usually quicker. Avoid flavoured water, lemon water with sugar, coconut water and anything with calories. Some tests ask for extra water beforehand; if so, the lab will tell you.
Do not stop any medicine unless your doctor tells you to. Tell your doctor about every medicine, vitamin and supplement you take, including over-the-counter products, because some can affect results. If you take medicines or insulin for diabetes, ask your doctor clearly what to do on the morning of a fasting test, since skipping food while taking these medicines can make blood sugar fall too low. Our doctors can advise you during a consultation.
Blood is usually taken from a vein on the inside of the elbow. A strap may be tied around your upper arm, and you will feel a short scratch as the needle goes in. The NHS notes that a blood test usually takes about 10 minutes. Afterwards you will be given cotton wool or a plaster; press gently on the spot for a few minutes. A small bruise is common and fades in a few days.
If you feel nervous about needles or have fainted before, tell the staff before they start. You can lie down for the test, and you are welcome to bring a family member with you. Tell the staff honestly if you ate or drank anything other than water. It is better to repeat the test on another day than to receive a report that does not reflect your real health.
Home sample collection is listed among the services of MCT Clinic and Diagnostic Centre. The same fasting rules apply at home: the sample should be collected at the end of your fasting window, so ask for an early morning visit. To ask about availability, use our contact page. For imaging and specialised tests such as sonography, X-ray and Doppler, see the diagnostics catalogue; some of these, such as certain abdominal scans, have their own preparation instructions.
It depends on the test. A fasting blood glucose test needs at least 8 hours without food or drink other than water. When a cholesterol test needs fasting, it is usually 9 to 12 hours. Always follow the time your doctor gives you.
Yes. Fasting for a blood test means no food or drink except plain water. Water keeps you hydrated and can make it easier to draw blood. Tea, coffee, milk, juice and soft drinks are not allowed during the fast.
Do not stop any medicine on your own. Tell your doctor about all medicines and supplements you take and ask whether to take them before the test. People on diabetes medicines or insulin should ask specifically, because fasting can lower blood sugar.
No. The HbA1c test reflects average blood sugar over about two to three months and is usually done without fasting, unless it is being done together with other tests that need a fast.
Tell the lab staff honestly. Even a small snack can change some results, so the test may need to be done on another day. It is better to reschedule than to get a misleading report.
For information only, not medical advice. This page does not replace advice from your doctor.