Patient Guide
Do You Need to Fast Before a Blood Test? A Test-by-Test Guide
Only some blood tests need fasting — most do not. Here is which common tests usually need an empty stomach, how long to fast, what you can drink, and why you should never stop a medicine without asking first.
Quick answer: Most blood tests do not need fasting. Fasting is usually needed only for a few tests — mainly the Fasting Blood Sugar (FBS), the Oral Glucose Tolerance Test (OGTT) and, at St. Luke's, the Lipid Profile. "Fasting" normally means 8–12 hours with nothing but plain water. Tests such as the Full Blood Count, HbA1c, TSH and Vitamin D are usually done without fasting. Always follow the instructions from the doctor who requested your test, and do not stop any medicine unless they tell you to.
People often skip breakfast "just in case" before every blood test. That is understandable, but it is usually unnecessary — and for some people, such as those taking diabetes medicines, an unplanned fast can cause problems of its own. This guide explains which tests usually need fasting, what fasting actually means, and how to prepare so you only have to come once.
Why some blood tests need fasting
When you eat, sugar and fats from the meal pass into your bloodstream for several hours. For most tests this makes no meaningful difference. For a few, it does:
- Blood sugar (glucose) tests — a recent meal raises glucose, so a fasting sugar test needs an empty stomach to show your baseline level.
- Triglycerides (part of the lipid profile) — these fats rise after a meal, especially a fatty one.
The US National Library of Medicine's MedlinePlus service explains that fasting is commonly needed for blood glucose tests and lipid panels, and usually lasts 8 to 12 hours — with the exact time set by your healthcare provider (MedlinePlus: Fasting for a Blood Test).
Which common tests usually need fasting?
The table below reflects the preparation advice published on our own test pages. Your doctor's instructions always come first — if they differ from this table, follow your doctor.
| Test | Usually need to fast? | Typical preparation |
|---|---|---|
| Fasting Blood Sugar (FBS) | Yes | 8–12 hours, water only |
| OGTT (glucose tolerance test) | Yes | 8–12 hours, water only; plan to stay 2–3 hours |
| Lipid Profile | Yes, at St. Luke's | 9–12 hours, water only (see note below) |
| Post Prandial Blood Sugar (PPBS) | No | Eat a normal meal; sample is taken at the time your doctor sets, usually 2 hours later |
| Random Blood Sugar (RBS) | No | No preparation |
| HbA1c | No | Any time of day |
| Full Blood Count (FBC) | No | No preparation |
| TSH (thyroid) | No | No preparation |
| Vitamin D | No | No preparation |
| Liver Function Test (LFT) | Not usually on its own | Fast only if it is booked with an FBS or lipid profile |
| Serum Creatinine / eGFR | No | Avoid a very large meat meal or heavy exercise the evening before |
| Urine Full Report (UFR) | No | First-morning, midstream sample is best |
A note on the lipid profile
Fasting before a cholesterol test has been standard practice for decades, and our lab currently asks patients to fast for the Lipid Profile. You may hear that this is changing: a 2016 joint consensus statement from the European Atherosclerosis Society and the European Federation of Clinical Chemistry and Laboratory Medicine concluded that fasting is not routinely required for a lipid profile in many situations (Nordestgaard et al., European Heart Journal, 2016). Doctors may still request a fasting sample — for example when triglycerides were high before. If your doctor has said "fasting" or "non-fasting", follow that. If you are not sure, a fasting sample is acceptable for both purposes.
What does "fasting" actually mean?
For a fasting blood test, plain water is allowed and encouraged — it keeps you hydrated and makes the blood draw easier. Avoid everything else for the fasting period:
- No food, including biscuits or a "small" snack.
- No tea or coffee — not even plain, without milk or sugar.
- No juice, soft drinks, king coconut water or flavoured water.
- No chewing gum or sweets, and avoid smoking.
- Avoid heavy exercise just before the test.
MedlinePlus also notes that your provider may ask you to avoid alcohol for longer than the fasting period itself (MedlinePlus).
Can fasting be too long?
Yes — longer is not better. Fasting well beyond the time you were given can make you feel faint and does not improve the result. Aim for the window your doctor or the lab gives you.
What about my medicines?
Do not stop or change any medicine because of a blood test unless the doctor who requested it tells you to. Some medicines and supplements can affect certain results, so it helps to tell the lab and your doctor what you take — including vitamins and herbal or Ayurvedic preparations.
If you have diabetes, ask the doctor who requested the test how to manage the fasting period and your diabetes medicines or insulin before you fast. Skipping a meal while taking some diabetes medicines can lower blood sugar too far. If you feel shaky, sweaty, confused or unwell while fasting, treat it and seek help — your safety matters more than the test.
Practical tips for a fasting test
- Book an early-morning slot. Most of the fast then happens while you sleep. Our Ja-Ela lab opens at 07:00 Monday to Saturday and 07:30 on Sunday.
- Check every test on your request form. If only one test needs fasting, you can still do all of them from the same fasting sample — one visit, one needle.
- Drink a glass of water before you leave home.
- Bring something to eat for straight after the sample is taken.
- Tell us if you ate by mistake. It is better to rebook than to get a misleading result. Just let the staff know honestly.
- Consider home collection if travelling while fasting is difficult — for example for elderly parents.
Fasting tests at St. Luke's, Ja-Ela
You can walk in to our lab at No. 67, Old Negombo Road, Ja-Ela without an appointment (Mon–Sat 07:00–19:30, Sun 07:30–12:30). If you would rather not travel on an empty stomach, our home blood collection service covers Ja-Ela, Kandana, Welisara, Ragama, Wattala, Batagama and Thudella, with scheduled home visits in Seeduwa, Katunayake and Negombo. Every test price is listed on our price list.
When you call, read out the tests on your request form and we will tell you whether any of them need fasting.
Frequently asked questions
Can I drink water before a fasting blood test?
Yes. Plain water is allowed and helps the blood draw. Avoid tea, coffee, juice and any flavoured or sweetened drinks.
Can I drink plain tea or black coffee while fasting?
It is best not to. Guidance for fasting tests generally says to avoid coffee and other drinks apart from water during the fasting period.
Do I need to fast for an HbA1c test?
No. HbA1c reflects your average blood sugar over the past two to three months, so a recent meal does not change it meaningfully. It can be done at any time of day.
Do I need to fast for a Full Blood Count?
No. The FBC does not need fasting. If it is booked with a fasting test, you can do both from one fasting sample.
I ate before my fasting test. What should I do?
Tell the staff when you arrive. Depending on the test, we may suggest rebooking so the result is reliable. Non-fasting tests on the same form can usually still be done.
Should I take my blood pressure tablets before the test?
Do not stop or change any medicine unless the doctor who requested the test has told you to. If you are unsure, ask them before the day of the test.
Sources
- MedlinePlus (US National Library of Medicine). Fasting for a Blood Test.
- Nordestgaard BG, et al. Fasting is not routinely required for determination of a lipid profile. European Heart Journal, 2016;37:1944–1958.
- MedlinePlus. How to Understand Your Lab Results.
This article is general health information. It is not medical advice and does not replace the instructions of the doctor who requested your test. If you feel unwell while fasting, eat or drink something and seek medical help.
