Joseph Lamb, MD
Joseph Lamb, MD, director of intramural clinical research at Metagenics, Inc., in Gig Harbor, Washington. He has authored numerous articles on nutritional supplements in scientific publications such as Nutrition Research.
If you take vitamin and/or herbal supplements, you probably already know that these products can interact—sometimes harmfully—with other supplements and medications.
What few people realize: Taking such supplements may also interfere with a wide variety of laboratory tests, including blood work and urine tests.
More than half of the adults in the US take one or more dietary supplements every day. And nearly 7 billion lab tests are given each year. In most instances, supplements do not affect the results of lab tests.
However, case reports in the medical literature indicate that certain supplements can…
Produce a “false-positive” or “false-negative” test result that may lead to additional tests and unnecessary treatments.
Change your body chemistry in a way that the test accurately reflects—but leads to incorrect treatment if your doctor doesn’t know that the supplement is causing the change.
Commonly used supplements that may affect certain lab tests…
• Vitamin C. This vitamin can produce a false-negative reading for a fecal occult blood test, which detects digestive tract bleeding, a possible sign of colon cancer. Vitamin C can interfere with the test’s chemical reaction, which signals the presence of hidden blood in the stool. (A fecal occult blood test may be done yearly if you have a family history of colon cancer.)What to do: Three days before testing, stop taking vitamin C, including multivitamins.
Also important: An iron supplement can produce a false-positive reading on a fecal occult blood test. Excess iron in red blood cells can be mistaken for blood in the stool. Iron supplements also can skew the results of iron tests.
If you are scheduled for a fecal occult (or iron) blood test, be sure to tell your doctor if you’re taking iron.<
• Riboflavin. High doses of this B vitamin (usually above the recommended daily intake of 1.3 mg for men, 1.1 mg for women) can turn your urine bright yellow, potentially interfering with any urine test that uses a dipstick indicating a color change.These tests include a urine protein test (to monitor kidney function)…a urine glucose test (to monitor blood sugar levels)…a urine ketone test (for diabetes)…a urine pH test (to monitor the body’s acid/alkaline balance)…and a urinalysis itself (which doesn’t use a dipstick but may evaluate the color of the urine).
What to do: Riboflavin is quickly cleared from the body. Don’t take a supplement or multivitamin with riboflavin the night before or the morning of a urine test—riboflavin can be safely resumed afterward.
• Folic acid. The B vitamin folic acid works in conjunction with another B vitamin (B-12) in many body functions, including cell division. However, a high intake of folic acid can mask one of the telltale laboratory signs of vitamin B-12 deficiency—abnormally large red blood cells—by making the cells appear normal. A deficiency of vitamin B-12 can cause anemia…neuropathy…and memory loss and other mental difficulties.
What to do: If you are being tested for a B-12 deficiency and take a large dose of folic acid (above 800 mcg) in a supplement or multivitamin, tell your doctor.
Also important: Inform your doctor if you are taking the drug L-methylfolate (Deplin). This is a high-dose folate supplement that’s sometimes prescribed with an antidepressant to help produce neurotransmitters that regulate mood (including serotonin, norepinephrine and dopamine).
• Calcium supplements. Undigested or unabsorbed calcium in the intestines may cause an artificially high reading on a bone-density scan.What to do: Avoid calcium supplements and multivitamins that contain calcium for 48 hours before a bone-density scan.
• Vitamin E, fish oil, ginkgo biloba, hops extract, red clover, ginger and garlic. These supplements have blood-thinning properties that can cause problems for people taking the anticoagulant drug warfarin (Coumadin). This medication is prescribed to thin the blood in individuals at risk for blood clots, such as those with heart disease.To make sure that the medication is working, patients should regularly have their Prothrombin Time and International Normalized Ratio checked. These tests measure the amount of time it takes for blood to clot. Because the supplements listed above can thin the blood, they can change the results of the tests.
What to do: If you take (or are about to be prescribed warfarin), be sure to tell your doctor if you use one of these blood-thinning supplements. Your doctor will need to closely monitor your prothrombin time and adjust your medication dose accordingly.
• Dan Shen and Chan Su. These are Chinese herbs. Dan Shen is used for heart and circulatory problems and Chan Su for sore throats and chest congestion. Both herbs can interfere with the test to determine if a patient’s blood level of digoxin, a drug prescribed for heart failure or arrhythmias, is within the normal range. This interference can be very dangerous, because too little digoxin is not therapeutic, while too much can be toxic.What to do: If you’re taking digoxin, don’t use either of these herbs.
The best strategies to help avoid a false or misinterpreted lab test result caused by a supplement…
Tell your practitioner about all the supplements you take. When you visit your doctor, bring along a written list of your supplements that can be photocopied. If a supplement contains a unique, multi-ingredient formula, photocopy the package insert or bottle label and attach it to your list.Mistake to avoid: Do not bring a bag filled with your supplements. If your doctor is writing down a list of your supplements, he’ll have less time to attend to other, equally important aspects of your care.
Take quality supplements. They are less likely to interfere with lab tests. A poorly manufactured supplement may include ingredients not listed on the product label or the amounts could be less (or more) than what is on the label.To ensure that you’re taking a high-quality product…
Look for a “GMP Certified” product. A GMP certification on the label means that “good manufacturing practices” have been verified by an independent third party that checks such important features as the product’s strength, composition and purity.
Do not purchase anything without a complete listing of ingredients. Avoid all products with only vague ingredient listings such as “proprietary blend”—neither you nor your practitioner should be in the dark about the contents or dosages of any supplement you’re taking.