How long should you fast before a blood test?
The short answer
Most fasting blood tests need 8 to 12 hours with nothing to eat or drink except plain water. Many tests need no fasting at all, so check the instructions for your specific test.
What to do
- Check whether your test needs fasting and for how many hours.
- If you have several fasting tests, follow the longest fasting time.
- Book an early-morning visit so most of your fast happens while you sleep.
- Drink plain water as usual. Skip food, coffee, tea, juice, gum, mints and smoking.
- Take your medications as usual unless your clinician told you otherwise.
- If you ate by mistake, tell the staff at the lab or reschedule your visit.
Which tests need fasting?
Of our tests, 12 need fasting. The other 71 don't: eat and drink as usual.
| Test | Fast for |
|---|---|
| Basic Metabolic Panel | 8 hours |
| C-Peptide | 8 hours |
| Comprehensive Metabolic Panel | 8 hours |
| Glucose, Fasting | 8 hours |
| Homocysteine | 8 hours |
| Insulin, Fasting | 10 hours |
| Iron and Total Iron Binding Capacity | 8 hours |
| Leptin | 8 hours |
| Lipid Panel (Standard) | 9 hours |
| Lipid Panel, Cardio IQ | 9 hours |
| Lipoprotein Fractionation (NMR) | 9 hours |
| Prolactin | Yes |
If any test in your order needs fasting, fast for the whole order: it is one draw.
Which results it changes
| Marker | Effect | How long |
|---|---|---|
| Triglycerides[1][3] | Rise after a meal; about 26 mg/dL on average after a typical meal | Largest changes 1–6 hours after eating; still raised at 4 hours after a light meal |
| Total and LDL cholesterol[1] | Small drop after a typical meal (about 8 mg/dL on average) | 1–6 hours after eating |
| Apolipoprotein B[1] | Not affected by fasting or eating | No fasting needed |
| Lipoprotein(a)[1] | Not affected by fasting or eating | No fasting needed |
| Iron[3] | Rose after a light meal | Still raised 4 hours after the meal |
| Calcium, sodium, potassium and other CMP results[3] | Shifted after a light meal; some by a clinically meaningful amount | Up to 4 hours after the meal |
Tests that don’t need fasting
Many popular tests do not need fasting, including TSH and other thyroid tests, Hemoglobin A1c, Vitamin D, Ferritin, the CBC, Apolipoprotein B and Lipoprotein(a). If your order mixes fasting and non-fasting tests, fast for the whole visit.
8 hours or 12 hours?
There is no single definition of fasting. A review of lab instructions found required fasting times ranging from 8 to 14 hours, and a European lab-medicine working group has suggested 12 hours with water allowed.
For your visit, use the number on your test in the table above. If you have more than one fasting test, the longest time wins.
What breaks a fast?
Fasting means no food and no drinks other than plain water. Juice, milk, soda, sweetened or flavored drinks and alcohol all break a fast. So do gum and mints, which can contain sugar. Treat them like food until after your draw.
Smoking and coffee count too. Research shows smoking, coffee and alcohol change how the body handles food, and fasting instructions commonly ask people to skip coffee, tea and tobacco on the morning of the draw. Some labs allow black coffee; JustLabs recommends plain water only for fasting tests.
Why a morning appointment helps
Results can shift for hours after a meal. In one study, a light meal changed triglycerides, iron, calcium, sodium, potassium and other routine results, and some were still changed 4 hours later.
The simplest way to fast is overnight. Eat a normal dinner, have nothing but water after it, and book an early-morning draw. You can book a time online through JustLabs or walk in during the center's hours. Bring a photo ID and your lab order; most visits take 10 to 15 minutes, so you can eat soon after. The Iron and TIBC test also recommends a morning appointment.
Medications while fasting
Fasting is about food and drinks, not your prescriptions. Take your medications as usual unless your clinician tells you otherwise, with a little water. Never stop a prescribed medicine to prepare for a blood test; ask your prescriber if you are unsure.
A few tests have their own timing rules. For the Iron and TIBC test, skip iron supplements the day before. If you take thyroid medication, see our page on thyroid medication timing before a TSH test.
What if you forgot to fast?
Don't guess. Tell the staff at the lab what and when you ate, or move your visit to another morning.
For a lipid panel, the test can still be done. After a typical meal, triglycerides rise on average about 26 mg/dL and total and LDL cholesterol fall about 8 mg/dL, so a non-fasting result is close but not identical. For fasting glucose, fasting insulin, C-peptide and iron, eating changes the very thing being measured, so rescheduling is the safer choice.
Do cholesterol tests really need fasting?
Guidelines have moved. A 2016 European consensus recommends non-fasting lipid profiles for routine use, and the 2018 US cholesterol guideline allows non-fasting lipid measurements. HDL cholesterol, apolipoprotein B and lipoprotein(a) are not affected by fasting at all.
JustLabs still asks you to fast for its lipid panels. Fasting gives the most consistent triglyceride and calculated LDL numbers, which helps when you compare results over time. The same consensus suggests fasting may be considered when non-fasting triglycerides are very high. If fasting is hard for you, for example because of diabetes or pregnancy, ask your clinician what is best.
Frequently Asked Questions
More on preparing
Sources
- Nordestgaard BG, Langsted A, Mora S, Kolovou G, Baum H, Bruckert E, Watts GF, Sypniewska G, Wiklund O, Borén J, Chapman MJ, Cobbaert C, Descamps OS, von Eckardstein A, Kamstrup PR, Pulkki K, Kronenberg F, Remaley AT, Rifai N, Ros E, Langlois M, European Atherosclerosis Society (EAS) and the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM) Joint Consensus Initiative. Fasting Is Not Routinely Required for Determination of a Lipid Profile: Clinical and Laboratory Implications Including Flagging at Desirable Concentration Cutpoints-A Joint Consensus Statement from the European Atherosclerosis Society and European Federation of Clinical Chemistry and Laboratory Medicine. Clin Chem. 2016 Jul;62(7):930-46. doi: 10.1373/clinchem.2016.258897. PMID: 27235445. PubMed
- Grundy SM, Stone NJ, Guideline Writing Committee for the 2018 Cholesterol Guidelines. 2018 Cholesterol Clinical Practice Guidelines: Synopsis of the 2018 American Heart Association/American College of Cardiology/Multisociety Cholesterol Guideline. Ann Intern Med. 2019 Jun 4;170(11):779-783. doi: 10.7326/M19-0365. PMID: 31132793. PubMed
- Lima-Oliveira G, Salvagno GL, Lippi G, Gelati M, Montagnana M, Danese E, Picheth G, Guidi GC. Influence of a regular, standardized meal on clinical chemistry analytes. Ann Lab Med. 2012 Jul;32(4):250-6. doi: 10.3343/alm.2012.32.4.250. PMID: 22779065. PubMed
- Grankvist K, Sigthorsson G, Kristensen GB, Pelanti J, Nybo M. Status on fasting definition for blood sampling in the Nordic countries - time for a harmonized definition. Scand J Clin Lab Invest. 2018 Nov-Dec;78(7-8):591-594. doi: 10.1080/00365513.2018.1528503. PMID: 30338721. PubMed
- Simundic AM, Cornes M, Grankvist K, Lippi G, Nybo M. Standardization of collection requirements for fasting samples: for the Working Group on Preanalytical Phase (WG-PA) of the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM). Clin Chim Acta. 2014 May 15;432:33-7. doi: 10.1016/j.cca.2013.11.008. PMID: 24269503. PubMed
Content reviewed for accuracy by the JustLabs editorial team.
Educational information, not medical advice. Don’t stop or change a prescribed medicine without talking to your prescriber. If your lab order gives different instructions, follow your lab order.