Preparing for a home blood test: fasting, timing and what to do on the day
Most blood tests need no preparation, but a few must be taken after a fast of several hours, usually overnight: fasting blood sugar, and often a lipid profile. Check the instructions for every test you book, book fasting tests for an early-morning collection, drink plain water, and ask your doctor before changing how you take any medicine.
[pending medical review]: this article has not yet been reviewed by a named medical professional.
Key points
- Fasting blood sugar needs at least 8 hours without food; many labs ask for a longer fast before a lipid profile.
- Plain water is usually allowed during a fast; tea, coffee, juice and chewing gum are not.
- Do not skip or delay medicines on your own. Ask your doctor, especially for diabetes and thyroid medicines.
- Book fasting tests for the earliest slot, and eat soon after the sample is taken.
Which blood tests need fasting?
Tests that measure sugar or fats in the blood are the ones most often done fasting, because a recent meal changes the result. Many common tests, such as a complete blood count or HbA1c, do not need fasting at all.
| Test | Fasting usually needed? |
|---|---|
| Fasting blood sugar (FBS) | Yes, at least 8 hours without food |
| Lipid profile (cholesterol, triglycerides) | Often: many labs ask for about 10 to 12 hours, though some doctors order it without fasting |
| Post-meal blood sugar (PPBS) | No fast, and taken a set time after a meal, commonly 2 hours |
| HbA1c | No |
| Complete blood count (CBC) | No |
| Thyroid profile (TSH, T3, T4) | Usually no, but timing around thyroid medicine can matter |
Health check packages often combine fasting and non-fasting tests. If any test in the package needs fasting, prepare as if the whole package does. On Aish Health, each test and package shows whether fasting is needed before you book, so check that line when you choose your lab tests at home.
What does fasting mean for a blood test?
Fasting means no food and no drinks other than plain water for the number of hours your test asks for. Tea and coffee, even without sugar or milk, along with juice, soft drinks, chewing gum and smoking, are best avoided until the sample is taken.
- Plain water is usually fine and makes the vein easier to find. Do not drink large amounts just before the test.
- Finish dinner at a normal time the evening before, then fast overnight.
- Avoid alcohol the day before; many labs ask for 24 hours without it before liver and lipid tests.
- Avoid unusually heavy exercise the day before, which can change some results.
- Fasting for much longer than asked is not better and can itself affect some results.
What time should you book the collection?
Book fasting tests for the earliest slot available, so the fast runs overnight and you are not without food for longer than needed. Some tests must be taken at a particular time, such as a post-meal sugar, or hormone tests like cortisol that change through the day. Book those for the time your doctor asks for.
For a post-meal sugar test, note the time you start eating. The sample is usually taken two hours after that, but follow the timing on your doctor’s form or the test instructions. If you need both a fasting and a post-meal sample, the collection has to be arranged around your breakfast, so mention it when you book.
If your doctor wants results compared over time, for example thyroid or sugar tests every few months, try to have each sample taken at a similar time of day and in the same fasting state, so the numbers are comparable.
Should you take your medicines before a blood test?
Keep taking your regular medicines unless your doctor tells you otherwise, and ask them in advance if you are unsure. Do not stop or delay a medicine on your own because of a blood test.
- Thyroid medicine: some doctors prefer the sample before the morning dose, so ask yours.
- Blood thinners: keep taking them as prescribed and tell the sample collector, as the puncture may need pressing for longer.
- Supplements: tell your doctor about any you take. Biotin, found in many hair, skin and nail supplements, can interfere with some laboratory tests.
On the day of a home blood test
Wear a top with loose sleeves, drink a glass of water, keep your doctor’s test form and the booking code ready, and sit or lie somewhere comfortable with good light. The collection itself usually takes only a few minutes.
- Keep the doctor’s prescription or test request handy so the tests can be matched to it.
- Tell the sample collector if you have fainted during a blood test before, because lying down helps.
- Mention blood thinners, bleeding problems or a side where blood should not be taken, such as the arm on the side of a past breast surgery.
- Have the one-time code from your booking ready; on Aish Health the collection is confirmed with it.
- After the needle is removed, press firmly on the spot for a few minutes to reduce bruising.
- If you were fasting, eat and drink as soon as the sample is taken.
Sample collectors on Aish Health are reviewed and approved by the Aish team before they can take bookings, and once one accepts you can see their name and number. Slots start from 30 minutes ahead where a sample collector is available.
After the test: getting and reading your report
The partner lab processes the sample and uploads the report to your account, where you can download it; each test shows its expected report time before you book. Take the report to the doctor who asked for it, because a result outside the printed range is not always a problem and a result inside it is not always the full picture.
If you had a check-up without a doctor’s request, or you cannot see your doctor soon, a doctor can go through your lab report with you online. For symptoms or a result your doctor has flagged as urgent, book a consultation with a doctor instead of waiting.
A small bruise where the needle went in is common and fades in a few days. See a doctor if the arm becomes very swollen, hot or painful.
Frequently asked questions
Can I drink water before a fasting blood test?
Yes, plain water is usually allowed and helps make the vein easier to find. Avoid tea, coffee, juice and other drinks until the sample is taken, unless your doctor says otherwise.
How many hours of fasting are needed for a blood sugar test?
A fasting blood sugar test needs at least 8 hours without food. Follow the specific instruction shown for your test, as some packages ask for a longer fast.
Do I need to fast for a thyroid test?
Usually not. If you take thyroid medicine, ask your doctor whether the sample should be taken before your morning dose.
Does HbA1c need fasting?
No. HbA1c reflects average blood sugar over the past few months, so a recent meal does not affect it in the way it affects a fasting sugar test.
Can I take my blood pressure tablets before a blood test?
In most cases yes, with a sip of water, but check with your doctor. Do not stop or delay any regular medicine because of a blood test without their advice.
