Accurate blood test results are foundational to proper medical diagnosis and treatment. However, many people are unaware that common over-the-counter vitamins and supplements can significantly skew these results, leading to false positives or negatives [1, 4, 5]. Understanding which supplements to temporarily stop taking, and for how long, is a crucial step in preparing for any blood test. The primary culprits are often high-dose versions of certain vitamins that interact directly with the chemicals and processes used by the lab to run an analysis [1, 5, 8].
Biotin (Vitamin B7) and Lab Interference
High-dose biotin is arguably the most significant cause of vitamin-related blood test interference. It is a common ingredient in supplements for hair, skin, and nail health, as well as many multivitamins [1, 5].
How Biotin Causes Problems
Many immunoassay tests used in labs rely on biotin-streptavidin technology to detect various substances in the blood [1, 5]. When excess biotin is present in the blood sample, it can interfere with this binding process, causing inaccurate results [1, 5]. Depending on the test, this interference can lead to:
- Falsely low or high thyroid-stimulating hormone (TSH) levels [1, 5]
- Inaccurate thyroid hormone levels (T3 and T4) [1, 5]
- Falsely low troponin levels, a critical marker for heart attacks [1, 5]
- Skewed levels of other hormones, like cortisol and vitamin D [1, 5]
Recommended Precaution for Biotin
To avoid this significant risk, it is best to stop taking biotin supplements, including multivitamins containing high amounts, for at least 72 hours (3 days) before a blood test [1, 3]. For high-dose supplements (5-10 mg or more), some recommendations suggest a week-long break [1, 3, 9]. Always inform your doctor about any biotin use, even if you stopped taking it, as your physician needs this context to interpret your results correctly [1, 5].
High-Dose Vitamin C and Other Tests
While a powerful antioxidant, large doses of vitamin C (ascorbic acid) can interfere with certain lab procedures [1, 6].
How Vitamin C Skews Results
High amounts of vitamin C in the blood can interfere with tests that rely on oxidation-reduction reactions, potentially altering results [1, 6]. For example, studies have shown that high-dose vitamin C can:
- Falsely increase or decrease glucose readings in certain tests [1, 6]
- Affect levels of creatinine and cholesterol [1, 6]
- Interfere with certain liver function and iron panels [1, 6]
Recommended Precaution for Vitamin C
To prevent issues, avoid taking high-dose vitamin C supplements (over 500mg) for 24 hours before your blood test [1, 3]. Moderate intake from natural food sources is generally not a concern [1, 3].
Iron Supplements and Iron Panel Tests
If you are scheduled for an iron panel to check for anemia or iron overload, timing your supplement intake is critical [1, 7].
Why Iron Affects Readings
Taking an iron supplement can cause a temporary spike in your serum iron levels, which can lead to an inaccurate representation of your body's true iron stores [1, 7]. For a proper assessment of conditions like iron deficiency anemia, the measurement of ferritin (iron storage) and total iron-binding capacity (TIBC) is essential [1, 7]. A recent supplement can obscure these long-term indicators [1, 7].
Recommended Precaution for Iron
It is recommended to stop taking iron supplements for at least 24 hours before an iron test [1, 7]. However, if your doctor is checking your levels while you are on a treatment plan, they may have specific instructions for you [1, 7].
Other Common Supplements to Consider
Beyond the most common offenders, other supplements can interfere with specific tests:
- Multivitamins: Since they contain a mix of different vitamins, including biotin and vitamin C, it is generally recommended to stop taking multivitamins 24 to 48 hours before a blood draw [1, 8].
- Creatine: This popular supplement can increase creatinine levels in the blood, which can falsely suggest issues with kidney function [1, 9]. It's best to stop taking it for a week before a kidney function test [1, 3].
- Calcium: High doses of calcium can affect levels of calcium and parathyroid hormone [1, 3]. It is recommended to pause intake 24 hours before a calcium test or bone density scan [1, 9].
- Herbal Supplements: The effects of herbal supplements like St. John's Wort or Ginkgo Biloba can vary. It is always best to inform your doctor of any herbal supplements you are taking [1, 3].
Comparison Table: Vitamins and Bloodwork Interference
| Vitamin/Supplement | Potential Lab Tests Affected | Recommended Waiting Period | Reason for Interference |
|---|---|---|---|
| Biotin (B7) | Thyroid hormones, Troponin, Vitamin D | 72 hours - 1 week | Interferes with immunoassay test technology [1, 5] |
| Vitamin C (High Dose) | Glucose, Cholesterol, Iron, Creatinine | 24 hours | Acts as a reducing agent in test reactions [1, 6] |
| Iron Supplements | Iron panel (serum iron, ferritin) | 24 hours (or more for baseline) | Causes temporary spike in blood iron levels [1, 7] |
| Multivitamins | Varies (contains Biotin, Vit C, etc.) | 24-48 hours | Individual components can interfere [1, 8] |
| Creatine | Kidney function (Creatinine) | 1 week | Increases blood creatinine levels [1, 9] |
| Calcium | Calcium, Parathyroid hormone | 24 hours | Directly affects measured levels [1, 3] |
Proactive Steps for Accurate Results
Preparing for bloodwork involves more than just knowing what to avoid. Taking proactive steps can help prevent misinterpretations and the need for retesting. First and foremost, always inform your healthcare provider about all the supplements you are taking, including doses [1, 3]. This transparency allows them to correctly interpret any unexpected results. For certain tests, your doctor may ask you to fast, which involves abstaining from all food and drinks (except water) for a specified period, typically 8 to 12 hours [1, 3]. Being well-hydrated by drinking water is also important, as it helps make your veins more visible for blood drawing [1, 3]. Following these clear instructions ensures you and your doctor get the most reliable information possible.
Conclusion: Navigating Supplements and Lab Tests
For anyone on a regular vitamin or supplement regimen, it is crucial to understand what vitamins should I avoid before bloodwork. By paying close attention to specific supplements like biotin, high-dose vitamin C, and iron, and communicating openly with your healthcare provider, you can minimize the risk of inaccurate test results. The best practice is to temporarily pause non-essential supplements for a few days to a week before your appointment, depending on the supplement and dosage [1, 9]. Always consult your doctor for personalized advice based on your health history and the specific tests you are undergoing. Taking these simple precautions can make all the difference in ensuring your lab work provides a true and reliable picture of your health. For more general guidance on supplements, refer to reputable health information websites such as the National Institutes of Health [1, 6].
