How are false positives and false negatives in blood testing caused?

False positives can be caused by 1) substances or conditions that cause bleeding, such as bleeding gums following a dental procedure or drugs that may cause gastrointestinal bleeding, such as anticoagulants, aspirin, steroids, and large doses of iron preparations; 2) other sources of hemoglobin, such as the ingestion of red meat within three days before testing; and 3) other substances that will react with the test, such as the ingestion of fish, turnips, horseradish, or drugs such as colchicines and oxidizing drugs (for example, iodine and boric acid). Your doctor may have you stay off your medications and follow dietary restrictions before collecting the samples.

False negatives may be caused by large doses of vitamin C and by not collecting multiple samples (because many conditions only produce blood intermittently).

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