Preparation matters because blood biomarkers are meant to help with decision-making, not create false drama. A preventable wobble in triglycerides, glucose, liver enzymes, hydration status, or an assay-sensitive supplement can make a sensible person think something major is wrong when the real issue was timing.
If you are working through the wider category, start with preventative health screening in the UK and biomarker testing UK. This article is narrower. It covers what to do before a blood biomarker test, what usually matters most, and where generic internet advice becomes less useful than the instructions for your exact panel.
1. Start by checking whether fasting is actually required
Not every blood biomarker panel needs fasting. In UK practice, a routine lipid profile is often done with a non-fasting sample. NICE guidance says non-fasting blood samples are suitable for a full lipid profile in most people being assessed for cardiovascular risk. That matters because the internet still talks as if every prevention-focused blood test must be done hungry at 8 a.m. with a thousand-yard stare.
Some tests still do require fasting. Fasting glucose is the obvious example. Some triglyceride-heavy lipid questions, insulin-related work, and certain lab-specific preventative panels may also need an 8-12 hour fast. The practical rule is simple: if the request form or provider says fast, then fast. If it does not, do not invent extra restrictions just to feel virtuous.
British Heart Foundation patient guidance also reflects this more modern approach: cholesterol testing is often possible without fasting, but the laboratory may ask for it depending on the result being checked. That is why “do I need to fast?” should be the first question, not the assumption.
2. Water is usually helpful, not harmful
Plain water is usually allowed even when food is not. NHS and hospital phlebotomy instructions commonly advise people to drink water as normal before attending, including for many fasting blood tests. That is useful for two reasons. First, it helps avoid the slightly absurd situation where a person arrives mildly dehydrated and then wonders why the blood draw was difficult. Second, it may reduce dizziness after the sample.
What counts as breaking the fast is usually anything other than water. Tea, coffee, milk, juice, energy drinks, and often chewing gum are not clever workarounds. If the provider said water only, take that literally.
| Question | Usually fine | Usually avoid unless told otherwise |
|---|---|---|
| During a fasting window | Plain water | Tea, coffee, juice, milk, sports drinks, chewing gum |
| Morning of a non-fasting test | Normal hydration | Anything unusual that is likely to distort your baseline |
| If you feel faint after blood draws | Tell the phlebotomist and hydrate early | Turning up dehydrated and pretending you are fine |
3. Avoid hard exercise if you want a clean baseline
Strenuous exercise can shift biomarkers for a short period. That includes creatine kinase, liver enzymes such as AST and ALT, creatinine, inflammatory markers, glucose handling, and sometimes lipids. In other words, a hard interval session the night before a preventive panel is a good way to make a baseline test less baseline.
This does not mean you must become sedentary. Light walking and ordinary movement are usually fine. The point is to avoid the kind of session that leaves you acutely depleted, inflamed, or glycogen-drained right before the sample. If the purpose of testing is to understand your usual state, then the day before should look fairly usual.
This is especially relevant for people already interested in cellular energy, training load, or recovery. If you are using biomarkers to make sense of fatigue, stress, or performance, you do not want your test dominated by a heroic workout that you would not normally repeat every week.
4. Skip alcohol for 24 hours if you can
Alcohol can acutely affect triglycerides, hydration, glucose regulation, and some liver-related markers. Many NHS trust fasting instructions advise avoiding alcohol for 24 hours before the sample. That advice is not puritanism. It is basic pre-analytical hygiene.
For a preventative test, the best result is not the nicest-looking number. It is the most interpretable number. If you want a useful read on lipids, metabolic markers, or liver enzymes, the easy win is to avoid giving the lab a hangover instead of a baseline.
5. Supplements matter more than most people realise
Supplements are often treated as harmless background scenery, but some of them can alter results directly or interfere with the assay itself. The best-known example is biotin, sometimes sold in hair, skin, and nails formulas. The FDA has warned that biotin can interfere with some lab tests and produce falsely high or falsely low results depending on the platform being used.
That does not mean everyone needs to panic about a multivitamin. It means the lab should know what you take. High-dose biotin is the main one to flag, and some providers ask people to stop it for at least 48 hours before certain tests. Thyroid tests, some hormone assays, vitamin measurements, and other immunoassays may be affected. The right move is disclosure, not improvisation.
The same logic applies to any supplement stack taken for performance, energy, weight loss, or “longevity”. If you are using several products, write them down before your appointment. That makes the result easier to interpret and avoids the bizarre situation where a person forgets the very thing most likely to distort the panel.
6. Medicines are different from supplements
Do not stop prescribed medicines unless your clinician or test provider specifically tells you to. In general, routine medicines should be continued. The major caveat is fasting. If a test requires fasting, medicines that depend on eating, especially insulin or diabetes medication, need proper handling rather than guesswork.
The safest wording is boring but correct: follow the instructions from the requester, and if the plan is unclear, check with your clinician, pharmacist, or provider before the test. That also applies to thyroid medication, iron, and any time-sensitive treatment where the timing of the dose changes the interpretation.
7. What to do the day before
The day before is mostly about not doing anything dramatic. Eat normally unless fasting has been requested. Drink water. Avoid a hard gym session, a heavy night of drinking, and last-minute supplement experiments. Try to sleep sensibly. None of that is glamorous, but it is the difference between a test that reflects your actual physiology and one that reflects one chaotic evening.
- Check the provider email or request form again.
- Confirm whether fasting is required and for how long.
- Lay out any supplement or medicine list you may need to mention.
- Do not schedule your hardest workout of the week for the night before.
- If you are prone to fainting, plan a calm morning and mention it to the phlebotomist.
8. What to do on the morning of the test
On the morning itself, the job is even simpler. If fasting is required, keep it clean: water only, unless your provider says otherwise. If fasting is not required, keep breakfast ordinary. This is not the moment for a “healthy” smoothie the size of a bucket or three coffees because you slept badly.
Wear something practical for the blood draw. Arrive with enough time to sit down if you need a minute. If you have had a difficult blood draw before, say so early. Good preparation is partly biochemical and partly logistical.
Quick preparation checklist
| Prep item | Why it matters | Practical MeScreen view |
|---|---|---|
| Fasting | Can affect glucose, triglycerides, and some panel interpretations | Do it only if your specific test requires it |
| Water | Helps venepuncture and may reduce dizziness | Plain water is usually encouraged |
| Hard exercise | Can temporarily distort CK, liver enzymes, inflammation, glucose, and recovery markers | Avoid vigorous sessions for roughly 24 hours before baseline testing |
| Alcohol | Can shift triglycerides, hydration, and liver-related markers | Best skipped for 24 hours before the sample |
| Biotin and supplements | Some supplements interfere with assays | Disclose everything; biotin deserves special attention |
| Medicines | Stopping them casually can be unsafe and may also distort interpretation | Keep taking prescribed medicines unless the provider tells you otherwise |
Bottom line
Blood test prep is mostly about consistency. Know whether the test is fasting or non-fasting. Drink plain water. Avoid hard exercise and alcohol the day before if you want a cleaner baseline. Tell the lab what medicines and supplements you take, especially biotin. Then let the result tell you something real instead of something avoidable.
If you want a broader view of how biomarker-led prevention fits together, read at-home health tests in the UK, NHS Health Check vs private preventative screening, and the main guide to preventative health screening UK.
Frequently asked questions
Do I always need to fast before a blood biomarker test?
No. Many routine lipid tests can be done non-fasting, but fasting glucose and some lab-specific panels still require fasting. The instructions for your exact panel matter more than generic internet advice.
Can I drink water before a fasting blood test?
Usually yes. Plain water is commonly allowed and may make the blood draw easier. Tea, coffee, milk, and juice usually do not count as “fasting”.
Should I stop supplements before the test?
Do not guess. Tell the lab what you take. High-dose biotin can interfere with some assays, so some providers ask patients to stop it for at least 48 hours.
Should I take my normal medicines?
In general, prescribed medicines should be continued unless your clinician or test provider tells you otherwise. If fasting is involved, ask specifically about diabetes medication, insulin, or other timing-sensitive treatment.
Medically reviewed by Hemal Patel, PhD
Professor of Anesthesiology at UC San Diego School of Medicine, with research interests in mitochondrial biology, caveolin signalling and cellular bioenergetics.
Read Hemal Patel's MeScreen reviewer profile · Verify on UCSD Profiles
References
- NICE. Cardiovascular disease: risk assessment and reduction, including lipid modification. Updated 2023.
- NHS. Blood tests. Patient guidance accessed 2026.
- British Heart Foundation. Cholesterol tests. Patient guidance accessed 2026.
- Oxford University Hospitals NHS Foundation Trust. Fasting blood tests. Patient leaflet accessed 2026.
- Cambridge University Hospitals NHS Foundation Trust. Having a blood test (phlebotomy). Patient information accessed 2026.
- U.S. Food and Drug Administration. Biotin (Vitamin B7): Safety Communication May Interfere with Lab Tests. 2019.
Want a more structured preventative testing plan?
Use the core MeScreen guides to understand which biomarkers matter, when private prevention adds value, and how to interpret results without turning one report into a personality.