Fasting Before Blood Work: Water, Coffee and How Long
Fasting before blood work isn't always needed. When it is, follow the lab: what counts as fasting, whether water and coffee are OK, and what to ask if you take medicines.
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- Blimbi Editorial
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- Blimbi editorial team
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Editorial review against cited sources, not medical review. Educational, not medical advice.
In 30 seconds
Fast before blood work only if the person ordering the test asks you to, and follow their instructions. For a fasting glucose test, fasting means no calories for at least 8 hours, so plain water fits. Coffee, smoking and alcohol can affect results, and a European consensus recommends non-fasting cholesterol tests for routine use.
- Not every blood test needs fasting: A1C doesn't, and European experts recommend non-fasting cholesterol tests for routine use.
- For a fasting glucose test, the American Diabetes Association defines fasting as no calories for at least 8 hours.
- Cutting carbs in the days before a glucose tolerance test can give a falsely high result.
Do you need to fast before blood work?
Only if the person who ordered the test tells you to. Eating changes some results and not others, and labs don’t all use the same definition of fasting, so their instructions come first.
A European lab-medicine working group found that definitions of fasting before blood tests vary widely, and called for standard preparation that sets how long to fast and what is allowed, such as water, coffee, tea or smoking.Source 1
The American Diabetes Association's 2026 standards define fasting for a fasting glucose test as no caloric intake for at least 8 hours. In its protocol for gestational diabetes, the ADA asks for the oral glucose tolerance test in the morning after an overnight fast of at least 8 hours.Source 2
The same standards note that the A1C test does not require fasting.Source 2
A joint European consensus of atherosclerosis and laboratory-medicine societies recommends non-fasting cholesterol (lipid) tests for routine use, while a fasting test may be considered when triglycerides are very high.Source 3
| Test | Fasting? | What the guidance says |
|---|---|---|
| Fasting glucose | Yes | No calories for at least 8 hours |
| Oral glucose tolerance test | Yes | Follow your lab's schedule; don't cut carbohydrates in the 3 days before |
| A1C | No | Fasting isn't required |
| Cholesterol (lipid profile) | Often not | Non-fasting tests recommended for routine use; your clinician may still ask you to fast |
| Other tests | Ask | Follow the lab's instructions |
Can you drink water or coffee before blood work?
Plain water has no calories, so it fits the definition of fasting for a glucose test. Coffee is a question for your lab: a lab-medicine working group lists it among the things that can affect results, along with smoking and alcohol.
The working group lists eating habits, what a meal contains, how long the fast lasts, time of day, smoking, coffee and alcohol among the factors that change the body's response after eating, and calls for controlling them before blood tests.Source 1
What if you already do intermittent fasting?
Tell the person ordering the test how you eat, and don’t stretch your fast beyond what the lab asks. Before an oral glucose tolerance test in particular, a low-carb eating pattern can skew the result.
The American Diabetes Association notes that restricting carbohydrate in the days before an oral glucose tolerance test can falsely raise the result, and advises at least 150 g of carbohydrate a day for the 3 days before.Source 2
Mixed evidence
Professional guidance, not trials
These points come from diabetes standards, a European cholesterol consensus and a lab-medicine working group. They cover test preparation, not reading results, and your lab may set different rules.Source 2Source 3Source 1
Who should ask before fasting for a test?
Frequently asked questions
Talk to a qualified health professional before fasting if you are pregnant or breastfeeding, under 18, living with or recovering from an eating disorder, have diabetes, take insulin or a medicine affected by meal timing, are an older adult, or are acutely ill. Do not change the dose or timing of any medicine without your clinician. See Blimbi’s safety guide.
Frequently asked questions
How long should you fast before blood work?
As long as your lab or clinician says. For a fasting glucose test, the American Diabetes Association defines fasting as no calories for at least 8 hours; other tests may need a different time or none at all.Source 2
Can you drink water before blood work?
Plain water has no calories, so it fits the definition of fasting for a glucose test. Check your lab's instructions, because rules vary.Source 2Source 1
Can you drink coffee before blood work?
Ask your lab first. A lab-medicine working group lists coffee among the factors that can affect results, along with smoking and alcohol. If you're unsure, stick to water.Source 1
Do you need to fast for a cholesterol test?
Often not. A European consensus recommends non-fasting cholesterol tests for routine use, but your clinician may still ask you to fast, for example when triglycerides are very high.Source 3
Sources
Standardization of collection requirements for fasting samplesA.M. Simundic et al. Elsevier BV, May 1, 2014.DOI: 10.1016/j.cca.2013.11.008 · PMID: 24269503Clinical guideline or consensus
2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes-2026American Diabetes Association Professional Practice Committee for Diabetes. American Diabetes Association, January 1, 2026.PMID: 41358893Clinical guideline or consensus
Fasting is not routinely required for determination of a lipid profile: clinical and laboratory implications including flagging at desirable concentration cut-points—a joint consensus statement from the European Atherosclerosis Society and European Federation of Clinical Chemistry and Laboratory MedicineBørge G. Nordestgaard et al. Oxford University Press (OUP), July 1, 2016.DOI: 10.1093/eurheartj/ehw152Clinical guideline or consensus
Fasting Safely with DiabetesMartin M. Grajower. National Institute of Diabetes and Digestive and Kidney Diseases, August 26, 2020.Official guidance
Low blood sugar (hypoglycaemia)NHS. NHS, August 3, 2023.Official guidance