Exercise before a blood test: what can skew your results?

The short answer

• 7+ days

  How long AST and ALT, often called liver tests, stayed raised after one hard weightlifting session

• 14 of 14

  Men whose AST went above the normal range 3 to 5 days after that session

• 39%

  Biotin-based lab tests that gave wrong results after a week of 10 mg biotin a day

• 1 week

  Time off biotin after which every result was back to normal in that study

Key findingBefore a blood test, tell your doctor about any hard or new exercise in the past week. Also list every supplement you take, especially biotin and creatine. Follow your lab’s instructions. Don’t stop a medicine or supplement your doctor prescribed without asking.

Two ways a blood test can be thrown off

Sometimes your body really changes for a while. A hard workout makes muscles leak certain substances into your blood. A creatine supplement raises creatinine, a waste product that doctors use to check your kidneys. The test is right, but the result doesn’t mean what it usually means.

Other times, the test itself is fooled. Many hormone tests are immunoassays: a common type of lab test that uses antibodies to measure a hormone. Many of them also use biotin as part of how they work. Extra biotin in your blood gets in the way, so the result can come out falsely high or falsely low.

Exercise before a blood test: what one hard session did

AST and ALT are often called liver enzymes. But your muscles contain them too. After the session, all 14 men had AST above the normal range on days 3, 4 and 5. CK, a marker of muscle damage, went up far more.

Three other liver tests barely moved: GGT, ALP and bilirubin. They aren’t found in muscle. This is why your doctor needs to know about recent exercise.

The results didn’t peak right away. Most peaked 3 to 5 days after the session. For most of them, more than half the men were still above the normal range on day 7. After 10 to 12 days, most men still had a high ALT. The authors warn that a test at that point could wrongly suggest mild liver disease.

In 8 of the 14 men, CK rose above a level doctors use to diagnose severe muscle breakdown. None had kidney problems when checked later. The authors advise no weightlifting for at least a week before blood tests in drug trials. If you have severe muscle pain or feel very unwell after exercise, see a doctor.

So, can you exercise before a blood test?

This study tested one hard session in men who were new to lifting. People who lift often may react less. The study didn’t test walking or other light exercise. None of the studies here measured cholesterol, either.

So if you can, avoid hard or new kinds of exercise in the days before a test. Tell your doctor about any hard session in the past week. Your lab may also give its own instructions, so follow them.

Creatine and creatinine

Creatine is a popular supplement for strength. A 2019 review pooled 6 studies. Creatine raised blood creatinine modestly, and it raised urea too. Doctors use both to check the kidneys. The authors found no sign of kidney damage at the doses and lengths of time studied.

So a higher creatinine on your report may come from the supplement, not your kidneys. Tell your doctor you take creatine. For the full story on creatine and your kidneys, read our creatine explainer.

Biotin: the supplement that fools the test

Biotin is vitamin B7. It is in many multivitamins, prenatal vitamins and hair, skin and nail supplements. Your body needs only about 30 micrograms a day. Many supplements contain 10 milligrams, which is about 333 times as much.

In this 2017 study, 6 healthy adults took 10 mg of biotin a day for a week. Then their blood was tested on 4 common lab systems. Of the 23 tests that use biotin, 9 gave wrong results. None of the 14 tests without biotin did. Tests of one design read falsely high, and tests of another design read falsely low.

Some errors were big. On one system, the thyroid test TSH fell by 94%, and all six results dropped below the normal range. That could look like an overactive thyroid in a healthy person. One heart failure test fell below what it could measure. Ferritin, which shows your iron stores, wasn’t affected on any system.

A week after people stopped biotin, every result was back to normal.

Heart, thyroid and pregnancy tests

The US Food and Drug Administration (FDA) has warned since 2017 that biotin can cause wrong results that may go unnoticed. It has had reports of biotin causing falsely low troponin results. Troponin is a blood test doctors use to check for a heart attack. In 2022, the FDA listed 17 troponin tests whose makers hadn’t yet fixed the problem.

If you have chest pain, get emergency help at once. Tell the staff if you take biotin.

A 2026 guideline from European thyroid experts agrees. Biotin can make some thyroid results falsely high. If biotin might be the cause, they advise repeating the test after stopping it, if possible. They also stress that thyroid tests are highly accurate for most people.

Biotin can also affect hCG, the pregnancy hormone. In a 2023 study, 10 men were given hCG with either biotin or a dummy pill. With a lab urine test that uses biotin, only 19% of samples came out positive on biotin, against 71% on the dummy pill. Their urine held 29 times more biotin than their blood. A blood test and a urine test without biotin still found the hormone.

That study used lab machines, not home pregnancy tests, and it didn’t include pregnant women. If you take high-dose biotin and need a pregnancy test, tell your doctor or clinic.

How long should you stop biotin before a blood test?

There is no single rule. It depends on the dose and on the test your lab uses. In the 2017 study, a week off was enough at 10 mg a day, and the authors suggest a week where possible. The thyroid guideline and the FDA don’t give a number.

So ask your doctor or lab how long to pause it. Some people take high doses of biotin for a medical condition. If your doctor prescribed it, don’t stop without asking.

Claims and evidence

• ClaimA workout the day before won’t change anything.

  EvidenceAfter one hard weightlifting session, AST and ALT stayed raised for at least 7 days in 14 healthy men.

• ClaimA high AST or ALT always means liver trouble.

  EvidenceMuscles release AST and ALT too. In the weightlifting study, GGT, ALP and bilirubin, which aren’t found in muscle, barely changed.

• ClaimBiotin is just a vitamin, so the lab doesn’t need to know.

  EvidenceThe FDA warns that biotin can cause wrong results that may go unnoticed. In one study, 9 of 23 biotin-based tests were thrown off.

• ClaimIf creatine raises creatinine, it must be harming your kidneys.

  EvidenceA review of 6 studies found that creatinine rose modestly, with no sign of kidney damage at the doses studied.

What this means for you

What the research can’t tell you yet

• Small studies. The exercise study had 15 men, the biotin study 6 adults and the pregnancy-hormone study 10 men.
• Who was studied. The exercise and hCG studies were in men only. People who lift often may react less than beginners.
• Changing tests. Many test makers have changed their tests since these studies, so the risk depends on your lab.
• Not tested here. No study here looked at light exercise, at cholesterol or blood sugar tests, or at the shortest safe break from biotin.

How Bodyvault uses this research

If you log biotin, Bodyvault Pro warns that it can make thyroid, troponin and hCG results falsely high or low. It asks you to tell your lab and doctor, and Coach gives the same reminder. Creatine adds a note to creatinine and eGFR results, explaining that it can raise creatinine without kidney harm. Bodyvault doesn’t yet link a recent hard workout to a raised AST, ALT or CK result, so mention it to your doctor yourself.

General information about fitness and nutrition, not medical advice. If you have a health condition, are pregnant or take medication, talk to your doctor before changing your diet or training.