The Effects of Vitamin C on Medical Tests

Enlarge Image Image Gallery Major interferences caused by vitamin C.  ILLUSTRATION: MOTHER EARTH NEWS STAFF

Imagine that you're about to undergo a routine physical exam and your doctor says, "Before I send you to the lab, tell me what drugs—including aspirin and antacids—you've taken in the last 48 hours." You smile and reply that you haven't needed medicine in months. After all, you've been taking good care of yourself: exercising, eating vitamin-packed food from your garden, and following wholistic health routines.

But wait, did you consider the vitamin C tablets you gulp down each morning? Well, you'd better mention them to your doctor, because that daily supplement could interfere with the accuracy of the tests!

Since 1970, when Linus Pauling first publicized vitamin C's role in helping the body fight the common cold, many individuals have made taking large doses of it part of their daily health regimen. Today, however, medical journals are alerting physicians and lab technologists to the effects of vitamin C on laboratory procedures.

Laboratory Interference 

The various lab tests are important to every doctor's investigations, and false or unexpected results can seriously hinder a physician's overall interpretation of such reports.

To understand how vitamin C can skew the results of an examination, consider the following analogy: Let's say you have a sack containing a couple of hundred marbles. You'd like to know exactly how many are in the bag, but counting them one by one would be painfully slow. So you decide to weigh the whole package, subtract the weight of the sack, and then divide the remainder by the weight of one marble. It might go like this: Suppose the group weighs 1,656 grams (metric units simplify the math), and you find that one marble weighs nine grams. By dividing 9 into 1,656, you figure out that you have 184 marbles. But what happens if there's some mud on a few of the marbles? If, suppose, there happen to be 18 grams of dirt on the collection, you'll calculate—incorrectly—that you have 186 marbles.

And as researchers are now learning, vitamin C can "muddy" the chemicals being tested in some clinical laboratory tests, giving a falsely heightened result. Furthermore, in other situations vitamin C may actually impede chemical reactions . . . which, again, would influence the test results.

Glucose Tests 

For example, during your regular physical exam, your doctor may check blood glucose to detect diabetes or hypoglycemia. Vitamin C can block relevant chemical reactions, which may lead to your registering an incorrectly low blood glucose value.

Then again, the physician may study your urine instead of your blood. Whenever glucose concentrations in the bloodstream rise extremely high—something that can happen if insufficient insulin is available—the sugar is said to "spill over" into the urine. As is the case with blood glucose exams, urinary tests can produce false results when vitamin C obstructs chemical actions.