How to prepare for a fasting blood test

Preparation is the cheapest way to improve a result

A badly prepared sample does not produce a slightly worse result. It produces a result that is wrong in ways nobody can see from the report, which is how people end up being treated for something that was not there.

None of this costs anything.

The fast itself

• Eight to twelve hours, water only. Longer is not better — a very long fast can itself alter results.
• Water is allowed and encouraged. Being properly hydrated makes the draw easier and keeps the haemogram from reading falsely concentrated.
• Black coffee breaks the fast for this purpose. So does tea with milk, a splash of juice, and a mint. If it has calories, it counts.
• No chewing gum, which stimulates digestion.
• No smoking on the morning of the test.

Which tests actually need it: glucose, insulin, triglycerides and anything derived from them. HbA1c does not. Many lipid guidelines now accept non‑fasting samples — but if you are repeating a panel, match what you did last time, because comparability matters more than which convention you follow.

The day and evening before

• Alcohol — leave at least 24 hours. It raises triglycerides and liver enzymes, and a Friday night shows up in a Saturday morning panel.
• No heavy or unusual exercise for 24 to 48 hours. Strenuous training raises AST, ALT and CK, and can nudge several other markers. An unexpected liver result after a hard week is often the week, not the liver.
• Eat normally otherwise. A sudden "clean eating" day before the test gives you a picture of that day, not of your life. The point is to measure where you actually are.
• Sleep as you normally do. A badly broken night affects glucose and cortisol.

Timing, and why it is not optional for some tests

Several hormones follow a daily rhythm, and the hour of the draw changes the number more than most interventions do.

• Testosterone — morning, generally before 10am, and the same hour when you repeat it.
• TSH — higher early, falling through the day. Consistency matters more than the exact hour.
• Cortisol — highest shortly after waking.

Book the same slot for repeats. It is free and it removes a whole source of noise.

Medicines and supplements

Do not stop prescribed medicines to prepare for a test unless the person who prescribed them told you to. Ask first.

Two specific things worth raising:

• Biotin. High‑dose biotin — common in hair, skin and nail supplements — interferes with many immunoassays and can produce strikingly wrong thyroid and hormone results. Guidance generally suggests stopping it for a few days before testing. Tell the laboratory you were taking it.
• Creatine. Raises creatinine without kidney damage, which lowers the estimated filtration rate. Say if you take it.

Bring a list of everything you take, including supplements. "Nothing" is rarely true and the gaps cause misreadings.

At the laboratory

• Sit quietly for five minutes before the draw. Standing and rushing both shift some results.
• Mention if you are unwell. An infection in the past two weeks invalidates hs‑CRP and ferritin, and distorts thyroid tests. Better to rebook.
• Mention a recent procedure, injury or vaccination.
• Ask for the PDF, not just portal access.

The one that matters most

Use the same laboratory every time. Different analysers, different assays, different reference ranges. A ten per cent change after switching laboratory may be entirely the laboratory, and you will have no way to tell.

For a one‑off question, use whoever is nearest. For anything you intend to track over a year, pick one and stay.

The boundary

Good preparation makes a result trustworthy. It does not tell you what the result means — that depends on your history, your medicines and the rest of the panel, and belongs in a conversation with a physician.