Vitamin C in the Lab

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Then again, the physician may study your urine instead of your blood. Whenever glucose concentrations in the blood—stream rise extremely high—something that can happen if insufficient insulin is availablethe 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.

In fact, just two grams of vitamin C per day, taken orally, can cause a urinary sugar test to show positive results when no glucose is present. Another technique used commonly by diabetics testing themselves at home, and by many clinics, utilizes a chemical-coated strip that's dipped in the specimen. In this case, a vitamin C intake of two grams per day can produce a negative reading . . . even when glucose is present!

HIDDEN BLOOD TESTS

In 1975, researchers at the National Institute of Health reported the case of a female patient with unexplained anemia. She was examined for indications of internal bleeding, but—on four occasions—tests for occult (hidden) blood in her stools produced either negative or uncertain results. And yet the use of a different testing protocol on one of the same specimens gave strongly positive indications.

Then, during an interview, the patient disclosed that she was taking two grams of vitamin C per day as a nutritional supplement. Four days after she stopped taking the tablets, her fecal specimens yielded strongly positive results by all the lab techniques.

Another test for internal bleeding involves checking the urine for hemoglobin. This approach detects blood coming from the kidney or bladder, and it's one of the most important screenings that your urine sample will get in the laboratory. Unfortunately, vitamin C affects this test so strongly that as few as 250 milligrams, taken three times daily, can cause a false negative report.

OTHER INTERFERENCES

Although vitamin C also affects several other frequently used tests—such as those to detect certain enzymes, uric acid, and bilirubin-the changes are too slight to affect interpretation of the results. It does, however, strongly interfere with some less common tests, such as those for various drugs and for vitamin B, 12 . And as scientists continue their search for further consequences of large vitamin C intake, they'll probably find additional cases in which the supplement interferes with laboratory tests.

HOW TO DEAL WITH THE PROBLEM

In many cases, labs can either remove vitamin C and its derivatives from your samples or use methods that aren't susceptible to interference . . . but the technicians can do so only if they know that you've been taking the substance. Therefore, prior to having tests done, you should always review with your doctdr the amounts and types of vitamin supplements that you've been taking. However, to keep costs down—special laboratory work is always more expensive than are routine techniques—there's a good chance that your physician simply will advise you to stop taking the vitamins for two to three days prior to the exams. In short, by keeping well informed and acquainting your doctor with your dietary habits, you can play an active part on your health care team.

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