This guide is general information from Alpha Laboratory in Banur. Your doctor knows your history, so if their instructions differ from what's here, follow your doctor's advice. If you're unsure about your own test, call us on 8872524007 before you come.
Tests that need fasting
| Test | Fasting | Notes |
|---|---|---|
| Fasting blood sugar (FBS) | 8 to 10 hours | The name says it all |
| Glucose tolerance test (GTT) | 8 to 12 hours | Plus normal meals in the days before |
| Fasting insulin | 8 to 12 hours | Usually paired with fasting sugar |
| Lipid profile | 9 to 12 hours, if your doctor wants a fasting sample | Triglycerides rise after food |
| Iron studies | Overnight, often advised | Morning sample preferred |
| Some health checkup packages | Overnight | Because they include sugar and lipids |
The fasting blood sugar test is the one people get wrong most often. A cup of tea with sugar at 6 am will push the result up and the test has to be repeated.
The lipid profile is a moving target. Many guidelines now accept a non-fasting lipid test for routine screening, because total and HDL cholesterol barely change after a meal. Triglycerides do rise, though, so plenty of doctors still prefer a fasting sample. Check your slip or ask your doctor which they want.
Tests that usually need no fasting: CBC, ESR, HbA1c, thyroid profile, vitamin D, vitamin B12, LFT and KFT (unless your doctor says otherwise), CRP, dengue and most infection tests. You can see the preparation note for any test in our full test list.
How long to fast
For most fasting tests, 8 to 12 hours is the target. In practice that means a light dinner by about 9 pm and a sample between 7 and 9 am the next morning.
Longer is not better. Going past 14 hours can lower your sugar, raise some fat readings and leave you feeling faint at the counter. If you've missed the morning window, it's often wiser to come the next day than to keep fasting into the afternoon.
Our sample collection starts at 7:00 am, so you can give the sample and still have breakfast at a normal time.
Water, tea and medicines
Water: plain water is allowed, and you should drink it. Being well hydrated makes veins easier to find and the draw quicker.
Not allowed while fasting: tea, coffee (with or without sugar), milk, juice, lassi, chewing gum, mints and paan. Smoking is best avoided too. Tea is the most common slip in Punjab mornings, so it's worth saying twice: tea breaks the fast.
The day before: skip alcohol for at least 24 hours before a lipid profile or liver test, and avoid very heavy exercise, which can raise some muscle and liver enzymes. Eat your normal food. Don't diet specially for the test, since the point is to see your usual state.
Medicines: keep taking your regular medicines unless your doctor has told you to change something. We can't advise you to stop or delay any medicine. A few points worth raising with your doctor:
- Diabetes medicines and insulin. Taking them and then fasting can drop your sugar too low. Ask your doctor how to time the dose on test day, and bring the medicine and some food so you can take them right after the sample.
- Thyroid tablets. Some doctors ask you to give the TSH sample before the morning tablet. Ask what yours prefers.
- Biotin supplements (often sold for hair, skin and nails). High doses can distort thyroid and some hormone results on common lab machines. Tell us you take it, and ask your doctor whether to pause it before the test.
- Iron tablets. These affect iron results if taken that morning. Ask your doctor about timing.
Bring a list of everything you take. It helps the reporting pathologist make sense of an odd result.
Morning-only tests (cortisol, prolactin, testosterone)
Some hormones follow a daily rhythm, so the time of the sample matters more than fasting.
- Cortisol is highest in the early morning and falls through the day. Most doctors want a sample around 8 am. If your doctor has asked for an evening sample instead, tell us when you book.
- Prolactin is best tested in the morning after you've been up for a while. Rest for a few minutes before the draw; stress, heavy exercise and breast stimulation can raise it.
- Testosterone is highest in the morning, so samples are usually taken between about 7 and 10 am. Doctors often repeat it on a second morning before reading too much into one low value.
Women's hormones like FSH and LH are timed to the menstrual cycle, usually day 2 to 5. Your doctor will tell you which day.
Before a GTT or PPBS
These two sugar tests need more planning than most.
PPBS (post-prandial blood sugar): eat your normal breakfast or lunch, note the time you start eating, and give the sample exactly 2 hours later. Don't eat more than usual, and don't skip the meal. Sit quietly in between; a long walk can lower the reading. Many people give a fasting sample first, then eat and come back.
GTT (glucose tolerance test): eat normal meals with your usual rice, roti and dal for three days before. Don't cut carbohydrates, as that can make the result look worse than it is. Fast overnight, give a fasting sample, drink a measured glucose drink (75 g in adults) within 5 minutes, and stay at the lab for the next 2 hours. No food, no smoking, no walking around. Bring something to read.
GTT in pregnancy can follow a different plan, and some doctors in India use a single non-fasting 2-hour test. Follow the exact instructions on your prescription.
For children and elderly patients
Children: most children's tests don't need fasting. When they do, the doctor usually sets a shorter fast. Book the first slot of the morning, bring water and a snack for afterwards, and let us know the child's age so we're ready with the right needle and a calm corner. Infants should never be fasted without the doctor's say-so.
Elderly patients: older people, especially those with diabetes, can feel dizzy or shaky after a long fast. Come early, sit down while you wait, and bring food and medicines for right after. If travelling is hard, book home collection or call us and we'll send a phlebotomist.
Still unsure which blood tests require fasting on your prescription? Send us a photo on WhatsApp or call 8872524007, or use the booking form.
FAQ
Can I drink water before a fasting blood test?
Yes. Plain water is allowed and helps. Avoid anything flavoured, sweetened or with milk.
Does a CBC test need fasting?
No. A CBC can be done at any time of day, with or without food.
How to prepare for blood test day if I'm diabetic?
Ask your doctor how to take your medicines on a fasting morning, come early, and carry your tablets and breakfast. Tell our staff you're diabetic when you arrive.
Is the fasting blood sugar test 8 hours or 12 hours?
At least 8 hours with no food or drink other than water. Between 8 and 12 is ideal; don't go much beyond 14.
Does a thyroid test need fasting?
Usually not. Some doctors prefer an early morning sample, and some ask you to give it before your thyroid tablet. Follow their instruction.
What blood tube colors are for which test?
Purple (EDTA) tubes are for CBC, HbA1c and blood group. Light blue (citrate) is for clotting tests like PT/INR. Grey (fluoride) is for glucose. Red or yellow (serum) tubes carry most biochemistry, hormone, vitamin and infection tests.
Can I brush my teeth before a fasting test?
Yes, brushing is fine. Just don't swallow the toothpaste, and skip mouthwash with sugar or alcohol.
What if I accidentally ate something?
Tell us. For a fasting test, we'll usually suggest coming back the next morning. Non-fasting tests on the same slip can still be done.