False-Negative Stool Occult Blood Tests Caused by Ingestion of Ascorbic Acid (Vitamin C)

Abstract

In a female patient with unexplained anemia, ascorbic acid ingestion and apparent false-negative occult blood tests were related. When she stopped ascorbic acid, her stools became strongly reactive ("4+") by three tests for occult stool blood; this association was observed repeatedly. A test developed to measure stool occult blood in the presence of ascorbic acid remained reactive throughout this observation, and the observation was confirmed by in-vitro studies. Current tests for occult blood depend on the pseudoperoxidase activity of heme and are inhibited by low levels of ascorbic acid. Reducing substances chemically similar to ascorbic acid also inhibits occult blood tests; oxidized ascorbic acid and sulfhydryl reducing agents do not inhibit them at physiologic levels.

Diagnostic accuracy of faecal occult blood tests used in screening for colorectal cancer: a systematic review

“…A number of variables may influence guaiac test results that do not influence the results of immunochemical tests. These include ingestion of animal haemoglobin/ myoglobin in red meat, fruits and vegetables high in peroxidase activity, and aspirin or other medication that may produce gastrointestinal bleeding, each of which may cause false-positive results, 95 and high doses of vitamin C 96 and faecal dehydration 97 which may cause false-negative results. Insufficient data were available to support investigation into the impact of study design, type of test, patient spectrum, reference standards used, the threshold used to define a positive result and test-specific details (e.g.…”

Criteria and Recommendations for Vitamin C Intake

“…39 Diarrhea or abdominal bloating can occur when several grams are taken at once, although there are no indications for such doses. With guaiacbased tests, false-negative results for detecting stool occult blood occur with intake of 250 mg of vitamin C. 40 Vitamin C intake from all sources should be less than 250 mg for several days prior to stool testing. Vitamin C also can cause false-negative test results in detecting gastric occult blood.…”

Establishment of Dietary Reference Intakes for Dietary Antioxidants

“…Vitamin C intakes of 250 mg/day or higher have been associated with false-negative results for detecting stool and gastric occult blood (Gogel et a!., 1989;Jaffe et al, 1975). Therefore, high-dose vitamin C supplements should be discontinued at least 2 weeks before physical exams because they may interfere with blood and urine tests.…”