For a fasting blood test you usually avoid food and drink except plain water for 8 to 12 hours. Water is allowed and helps the blood draw. Which tests need fasting, and for how long, is set by the lab and the doctor who ordered them, and their instruction overrides general advice.
The rules below are general. The instruction that comes with your test, or the lab that will draw the sample, decides the details.
What fasting means
Fasting usually means no food and no drink except plain water for a set number of hours. While you are fasting, do not chew gum, smoke, or exercise, according to MedlinePlus guidance. Take medicines as instructed by your doctor.
How long
Eight to twelve hours is the usual range. Some tests ask for a specific window, for example nine to twelve hours for a lipid profile. An early morning appointment lets most of the fast happen during sleep.
Longer is not better. A fast that runs much longer than instructed can make you uncomfortable and can change some results. Follow the number you were given.
Which tests usually need fasting
| Test or panel | Fasting usually needed | Notes |
|---|---|---|
| Fasting glucose | Yes | Usually 8-12 hours; a random glucose is a different test |
| Lipid profile | Usually | Commonly 9-12 hours; some guidance supports non-fasting testing, and the lab's instruction decides |
| Basic metabolic panel | Yes | Includes glucose, which food affects |
| Liver function tests | Sometimes | Often requested fasting when bundled with a metabolic panel |
| Kidney (renal) panel | Sometimes | Follow the order |
| HbA1c | No | Reflects average glucose over about two to three months |
| Complete blood count | No | Food does not usually change the count |
| Thyroid tests (TSH, free T4) | No | Follow the lab's instruction if one is given |
| Vitamin D and vitamin B12 | No | Follow the lab's instruction if one is given |
| Iron studies | Often | A morning sample is usual; iron varies through the day |
| Cortisol | Sometimes | Usually sampled early morning; cortisol follows a daily rhythm |
| Oral glucose tolerance test | Yes | A fasting sample, then timed samples after a glucose drink |
| Urine routine | No | A first-morning sample is often requested |
Can I drink water?
Yes, unless your instructions say otherwise. Plain water is allowed during a fasted test, and staying hydrated can make the blood draw easier. Avoid lemon-flavoured or flavoured sparkling water, which may contain sugars or sweeteners.
Tea, coffee, and juice
Plain water is the drink that does not break a fast. Coffee, tea, juice, and soft drinks do, even without sugar, unless the lab's instruction says otherwise. Milk and sugar in a drink definitely count as food.
Medicines and supplements
Ask your doctor whether to take your usual medicines on the morning of the test. Do not stop a prescribed medicine unless you are told to. If you take diabetes medicine or insulin, ask how to time it around the fast before you skip a meal.
Tell the phlebotomist and your doctor about vitamins and supplements. Some, including high-dose biotin, can interfere with certain laboratory assays.
What breaks a fast
- Milk, sugar, or honey in tea or coffee.
- Juice, soft drinks, or flavoured water.
- Gum, sweets, or a mint.
- A supplement or a protein shake.
- Smoking or strenuous exercise during the fast.
If you eat by mistake
Tell the person drawing the sample before the test. You may need to reschedule, or the result may be reported with a note that fasting was not confirmed. Do not hide it; a non-fasting sample read as fasting can mislead the interpretation.
Why fasting appears on the report
A report may note whether the sample was fasting, and some reference ranges are fasting-specific. A remark such as fasting not confirmed is information about the sample, not a judgement about you.
General guidance on fasting duration, water, and what to avoid is drawn from MedlinePlus, Fasting for a Blood Test. Per-test requirements come from the order and the laboratory.