Blood tests and supplements are connected in two important ways. First, testing is often the best way to find out whether you actually need a supplement. Second, some supplements can interfere with test results, leading to wrong answers and sometimes wrong treatment. Both are worth understanding before your next checkup.
Part 1: Tests that show whether you need a supplement
Vitamin D: 25-hydroxyvitamin D
This is the standard test for vitamin D status. It reflects vitamin D from sun, food and supplements over the past few weeks. It is especially useful if you are at higher risk of being low, such as people with darker skin, older adults or those who rarely go outside. See how much vitamin D should you take?.
Vitamin B12
A B12 blood test can identify a deficiency. If the result is borderline, doctors sometimes order extra tests, such as methylmalonic acid, which can reveal a problem the standard test misses. See vitamin B12 after 50.
Iron: ferritin and a complete blood count
Ferritin measures your iron stores, and it often drops before anemia shows up on a complete blood count. Doctors may also check iron, transferrin saturation and other markers. Testing matters before taking iron, because too much iron can be harmful for some people. See iron supplements: who needs them.
Folate
Usually checked along with B12 if you have certain types of anemia or symptoms of deficiency.
Magnesium
Blood magnesium can be checked, but it does not always reflect what is in your cells, because most magnesium is stored in bones and tissues. A normal blood result does not completely rule out low intake.
Thyroid tests
Not a nutrient test, but often ordered when people feel tired. Thyroid problems can mimic nutrient deficiencies, so it is worth checking before assuming you need a supplement.
Part 2: Supplements that can throw off results
Biotin: the big one
Biotin, a B vitamin heavily marketed for hair, skin and nails, is the most important supplement to know about here. High doses can interfere with a type of lab test called an immunoassay, which is used for many common tests. Depending on the test, biotin can cause falsely high or falsely low results. Tests that can be affected include:
- Thyroid tests, which can look like an overactive thyroid when it is not.
- Troponin, a heart test used to diagnose heart attacks. A falsely low result could cause a heart attack to be missed.
- Some hormone and vitamin D tests.
The FDA has issued a safety communication about biotin and lab tests. Many hair and nail products contain 5,000 to 10,000 micrograms of biotin, far more than the 30 micrograms most adults need.
What to do: Tell your doctor and the lab if you take biotin. Many labs advise stopping biotin for at least a few days before a blood test. Ask your doctor for specific timing.
Vitamin C
Very high doses of vitamin C can interfere with some glucose tests and stool tests for hidden blood. If you take large doses, mention it.
Creatine
Creatine can slightly raise creatinine, a blood marker used to estimate kidney function. That can make kidneys look like they are working less well than they are. If you take creatine, tell your doctor so they can interpret the result correctly. See creatine.
Recently started supplements
If you started a supplement shortly before a test, your level may look normal even though your long-term status is low. Let your doctor know when you started.
How to get the most from your tests
- Bring a full list of supplements, including doses, to every appointment.
- Ask whether you should stop anything before a scheduled blood test.
- Ask for a copy of your results and the reference ranges.
- Retest after supplementing if you were treating a deficiency, so you know it worked and when to stop or reduce the dose.
Our list of questions to ask your doctor about supplements can help you prepare.
A note on at-home and online tests
Many companies sell at-home test kits and broad nutrient panels. Some are useful and use certified labs. Others test things with little medical value and then recommend their own supplements. Be cautious of tests sold together with the products they recommend. Discuss any results with your doctor.
The bottom line
Testing is the most reliable way to know whether you need vitamin D, B12, iron or folate. And if you take supplements, especially biotin, tell your doctor and the lab, because some products can make results misleading. MedlinePlus has clear explanations of individual lab tests if you want to understand your results.