Fasting is not needed for every blood test. A table of common tests, what MedlinePlus, the CDC and NIDDK say about each, and the questions to ask your lab. Your lab’s own sheet decides.
Only some blood tests need fasting, which means nothing to eat or drink but plain water, usually for 8 to 12 hours (MedlinePlus, updated 28 August 2025). MedlinePlus lists blood glucose, cholesterol and a basic metabolic panel as usually needing it; a complete blood count and vitamin D need no special preparation, and HbA1c needs no fast (NIDDK, April 2018).
Your lab’s own instruction sheet, and the doctor who ordered the test, decide for you. The hours on that sheet govern, not the hours on this page, which does not tell you whether to fast and does not read any result.
In short:
- Fasting blood sugar and the glucose tolerance test follow an overnight fast (CDC). A basic metabolic panel usually does (MedlinePlus), and for cholesterol you “may need to fast” (MedlinePlus, CDC).
- HbA1c needs no fast (NIDDK). Vitamin D needs no special preparation, and a complete blood count usually none unless other tests share the sample (MedlinePlus).
- Sources can give different hours for the same test, so your lab’s sheet decides.
Last checked: 23 October 2026, on the pages listed under Sources. Pages and lab rules change, so confirm them with your lab.
Which blood tests need fasting?
The table sums up each page named, for the test in general. It is not a promise about your test.
| Test | Fasting? | How long | Where this comes from |
|---|---|---|---|
| Fasting blood sugar («سكر صايم») | Yes, after an overnight fast. | Overnight | CDC, diabetes tests (page dated 15 May 2024) |
| Glucose tolerance test | Yes. Overnight fast and a blood draw, then the liquid, then your blood sugar is checked again. | Overnight | CDC, same page |
| HbA1c («السكر التراكمي») | No. | None | NIDDK, A1C test (last reviewed April 2018) |
| Cholesterol (lipid panel) | Often. You “may need to fast”; MedlinePlus’s fasting page lists it among tests that usually require it. | 9 to 12 hours on one page, 8 to 12 on another | MedlinePlus, cholesterol (15 October 2024); CDC, cholesterol (May 2024) |
| Basic metabolic panel | Usually. It is on MedlinePlus’s list. | 8 to 12 hours is the usual range | MedlinePlus, fasting (28 August 2025) |
| Liver function tests | Sometimes. Some only need a fast when ordered within a comprehensive metabolic panel. | As above | MedlinePlus, fasting |
| Kidney tests (renal panel, creatinine) | Sometimes. For creatinine, a fast is mentioned when it is part of a metabolic panel. | Up to 12 hours | MedlinePlus, fasting; MedlinePlus, creatinine (5 December 2023) |
| Complete blood count («صورة دم كاملة») | Usually no special preparation. | Several hours, only if other tests on the sample need it | MedlinePlus, blood count (15 October 2024) |
| Thyroid: TSH («الغدة الدرقية») | None listed for the TSH itself. | Several hours, only if other tests need it | MedlinePlus, TSH (30 October 2024) |
| Vitamin D | No special preparation. | None | MedlinePlus, vitamin D (4 December 2024) |
| Any test not listed | Ask. MedlinePlus says unlisted tests may need a fast too. | As your lab’s sheet says | MedlinePlus, fasting |
Read on 23 October 2026. The hours printed on your lab’s sheet, and your doctor, govern.
What does fasting before a blood test mean?
MedlinePlus (updated 28 August 2025) says plain water is allowed, but not flavoured or carbonated water, which may contain sugars or sweeteners. Your provider sets the length and gives you the instructions, and the page says not to fast unless your provider tells you to.
On medicines, it says to ask whether you can take your usual ones, and not to stop any unless told to. The NHS page on blood tests (last reviewed 2 November 2023) says the GP, nurse or specialist should tell you if you need to prepare. If you ate or drank by mistake, tell your provider, says MedlinePlus: the test may be rescheduled.
Complete blood count: fasting or not?
A complete blood count usually needs no special preparation, says MedlinePlus (updated 15 October 2024). If other tests are run on the same sample, you may need to fast for several hours. So ask “is anything else on my request fasting?”, not only “is my blood count fasting?”
Thyroid test (TSH): fasting or not?
MedlinePlus (updated 30 October 2024) lists no fast for the TSH test itself, and says you may need to fast if other blood tests are ordered. It also says you may need to stop certain medicines, so tell your provider everything you take, and stop none unless told to. That page covers TSH only: for other thyroid tests, ask.
Does the HbA1c test need fasting?
No. NIDDK’s page on the A1C test (last reviewed April 2018) says you do not have to fast before the blood is drawn, so it can be drawn at any time of day.
A “sugar test” is not always an A1C. The CDC page on diabetes tests (dated 15 May 2024) lists several, including fasting blood sugar and glucose tolerance, which follow a fast. MedlinePlus adds that not all blood glucose tests require fasting. If your request just says “sugar”, ask which one it is.
Vitamin D, kidney and liver tests: fasting or not?
For vitamin D, MedlinePlus (updated 4 December 2024) says you need no special preparation. For liver and kidney tests, its fasting page says you may sometimes be asked to fast. For creatinine within a metabolic panel, its test page (updated 5 December 2023) says up to 12 hours, and that you may be told not to eat meat for 24 hours before.
Do I need to fast for every test? What a 2026 study found
A study in JAMA Internal Medicine, published online on 28 September 2026, covered 9.76 million results from 101,148 adults at one hospital in the Seoul area, South Korea, and compared 121 tests across fasting times. In 27, the biggest difference between fasting-time groups went beyond the authors’ allowable-error benchmark, and in 10 of those it also showed within the same people. They include glucose, triglycerides, GGT and lipase, higher after fasting under 8 hours; the paper also names total bilirubin and vitamin D.
In the same paper, the authors suggest keeping at least 8 hours of fasting when glucose or a precise lipid measurement is the goal, with more flexibility for other routine tests, and write that fasting beyond 12 hours gave no measurable advantage for any test. They favour test-by-test rules, pending confirmation in multicentre studies. Their limits include one hospital, outpatients only, patient-reported fasting times they could not verify, and an older sample (median age 61). It is one retrospective study, not a guideline, and it does not change your lab’s instruction.
What should I ask the lab before the day?
- Which tests are on my request, and does any of them need a fast?
- How many hours, and may I drink plain water?
- If it is a sugar test, which one: fasting blood sugar, HbA1c or a glucose tolerance test?
- Should I take my usual medicines that morning?
- If I eat or drink by mistake, should I tell you and rebook?
- Can I have the instructions in writing? (MedlinePlus says your provider gives you a copy when they ask you to fast.)
When the results arrive, questions to ask your doctor about your lab report picks up where this page stops. Getting ready for a specialist? One page is enough to prepare. Every form asking for your medicines again? Here is why forms repeat.
In Egypt: what do «صايم» and «فاطر» mean at the lab?
In Egypt people often ask «صايم ولا فاطر؟» before a test, and «صايم» is also the word for keeping the Ramadan fast. For a blood test it means only what your lab’s sheet says: nothing to eat or drink but plain water, for the hours it gives (MedlinePlus, updated 28 August 2025).
We looked for a page from Egypt’s Ministry of Health and Population telling patients how long to fast before blood tests. We searched its website and other official Egyptian sites on 23 October 2026 and found none. That does not prove none exists, so this page gives no official Egyptian number of hours: they come from the lab you book with and the doctor who requested the test. Our suggestion, not an official requirement: ask for them in writing when you book.
Where Medigize stands
Medigize is a navigation tool. It is not a laboratory, and it cannot tell you whether to fast. We are building a record that moves with the patient, shared only with their consent. If you would like to follow that work, you can register your interest.
Frequently asked questions
Which blood tests need fasting?
MedlinePlus (updated 28 August 2025) lists blood glucose, cholesterol and the basic metabolic panel as usually needing it, and liver and kidney tests sometimes. A complete blood count and vitamin D need no special preparation; NIDDK says HbA1c needs no fast. Your lab’s sheet decides.
Can I drink water before a fasting blood test?
Plain water is allowed, says MedlinePlus (updated 28 August 2025), but not flavoured or carbonated water, which may contain sugars or sweeteners.
Do I need to fast for a thyroid (TSH) test?
MedlinePlus (updated 30 October 2024) lists no fast for the TSH itself. You may need one if other blood tests are ordered with it.
How many hours should I fast before a cholesterol test?
The sources differ: MedlinePlus (15 October 2024) says 9 to 12 hours and the CDC (May 2024) says 8 to 12. Follow your lab’s sheet, and your doctor’s advice.
Medigize is a non-diagnostic navigation tool. This is general information, not medical advice. In an emergency, call an ambulance or go to the nearest emergency department.
Sources
- MedlinePlus (US National Library of Medicine): fasting for a blood test, updated 28 August 2025, read 23 October 2026
- MedlinePlus: cholesterol levels, updated 15 October 2024, read 23 October 2026
- MedlinePlus: complete blood count, updated 15 October 2024, read 23 October 2026
- MedlinePlus: TSH test, updated 30 October 2024, read 23 October 2026
- MedlinePlus: vitamin D test, updated 4 December 2024, read 23 October 2026
- MedlinePlus: creatinine test, updated 5 December 2023, read 23 October 2026
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK): A1C test, last reviewed April 2018, read 23 October 2026
- US Centers for Disease Control and Prevention (CDC): diabetes tests, page dated 15 May 2024, read 23 October 2026
- CDC: cholesterol testing, page dated May 2024, read 23 October 2026
- NHS: blood tests, last reviewed 2 November 2023, read 23 October 2026
- Shin S and colleagues, “Fasting Duration and Differences Across Routine Laboratory Tests”, JAMA Internal Medicine, published online 28 September 2026, read 23 October 2026
- Egypt’s Ministry of Health and Population: website, searched with other official Egyptian sites on 23 October 2026 for a patient page on fasting before blood tests; none found
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