How to Prepare for a Blood Test: The 24 Hours Before

1. Do you need to fast?

Only for some markers. Fasting means no food and no drinks except water, usually for 9 to 12 hours. The NHS puts the rule simply:

“For a short time before some tests, you may need to: not eat or drink anything other than water (fast).”

— NHS, Blood tests

Fasting matters most for triglycerides and glucose. Both climb after a meal. It matters less for total and LDL cholesterol, which the NHS now measures either way. Our cholesterol blood test guide covers what a non-fasting lipid result can still tell you.

Thyroid, vitamin D, B12, ferritin and the hormone markers do not need a fast. If you want the detail, the full fasting guide lists every test either way.

2. What time of day should you test?

Timing moves more numbers than food does. Several hormones follow a daily rhythm, so the clock changes the reading.

• Testosterone peaks early and falls through the day. Sample between 7am and 11am.
• Cortisol is highest shortly after waking. Same window, and note the time you sampled.
• TSH runs higher in the early morning and dips in the afternoon. Keep repeat tests at the same time of day.
• Iron also drifts down over the day, so a morning sample reads truer.

One rule covers all four. Take the sample in the morning, and take every future sample at the same hour. Comparing a 7am result with a 4pm one tells you about the clock, not about you.

If you have a menstrual cycle, hormone timing has a second layer. FSH, LH and progesterone move across the month, so the day matters as much as the hour. The hormone timing guide sets out which day to pick.

3. Which supplements should you pause?

This is the step most people miss, and it is the one that most often produces a strange result.

Biotin is the big one. High-dose biotin, the kind sold for hair and nails, interferes with the lab machinery itself. The US Food and Drug Administration has warned that biotin can skew immunoassay results, including thyroid tests. The reading is wrong, not your thyroid. Leave it off for 48 hours.

Iron and B12 are the other two.An iron tablet lifts serum iron for hours, so a morning dose can make stores look better than they are. A B12 supplement does the same to a B12 reading. Skip that morning’s dose and take it after you have posted the sample.

Everything else can usually stay. The full supplement list goes through the rest, and the thyroid hub covers what biotin does to a TSH result in more depth.

4. Alcohol, coffee and training: the night before

Training is the surprise. A hard session leaks muscle enzymes into your blood. In a 2008 study in the British Journal of Clinical Pharmacology, fifteen healthy men did one hour of weights. Their liver enzymes stayed raised for at least seven days. Leave 48 hours after anything heavy or unfamiliar. A walk is fine.

Alcohol needs a longer gap than most people give it. It lifts liver markers and triglycerides, and NHS pharmacy guidance notes that GGT can stay up for weeks after heavy drinking. Two clear days is the sensible minimum.

Coffee depends on the test. For a fasting test, water only, even black. Caffeine lifts cortisol and blunts insulin sensitivity for a few hours. For thyroid, cholesterol or HbA1c, your usual cup will not change the number.

5. Should you stop your medicines?

No, not on your own. Prescribed medicines stay unless the clinician who prescribed them says otherwise. The NHS notes that you may be asked to stop certain medicines, such as anticoagulants, before some tests. That instruction comes from them, not from a guide.

One timing note if you take levothyroxine. NICE advises measuring TSH every 3 months after you start or change a dose, until the level settles. Testing a week after a change reads the change, not the dose. The levothyroxine monitoring guide has the schedule.

6. Marker by marker: what each one needs

Most panels mix markers with different rules. Prepare for the strictest one on your list and the rest look after themselves.

• Triglycerides, glucose: fast 9 to 12 hours, water only.
• Total, LDL, HDL cholesterol, ApoB: fasting preferred, not essential.
• Testosterone, SHBG, cortisol: morning sample, 7am to 11am.
• TSH, free T4, free T3: no fast, but no high-dose biotin for 48 hours.
• Iron, ferritin, transferrin:morning sample, and skip that day’s iron tablet.
• Vitamin D, B12, folate: no fast, but pause the matching supplement that morning.
• ALT, ALP, creatinine, CRP: no fast, but no hard training for 48 hours.
• HbA1c: nothing at all. It reads the last three months, so today changes nothing.

Read your own results against your own lab’s printed range. The reference ranges guide explains why two labs can print different numbers for the same marker.

7. What if you got it wrong?

Usually nothing. You ate breakfast, so the triglycerides read high and everything else is fine. You trained on Saturday, so ALT looks raised and the thyroid panel is untouched.

The fix is to know which marker was affected, not to bin the whole result. Note what happened when you register your kit. A result read with that context is far more useful than a result read blind.

One marker outside its range does not on its own mean something is medically wrong. If a result sits outside the printed range, or your symptoms carry on, speak to a qualified clinician or your GP.

8. Which test are you preparing for?

Helvy’s General Energy & Wellness panel (£149) mixes fasting markers with timed ones, so a morning sample after an overnight fast covers the whole set. Advanced Heart Health (£159) is the one where fasting earns its keep, because triglycerides sit in it.

All are home finger-prick tests, processed by UKAS-accredited UK labs. If you have never taken one, the step-by-step collection guide walks through the sample itself.

Not sure which one fits? The build-my-test tool walks you through it, and the heart health hub covers the fasting markers in more depth.

Frequently asked questions

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