False positive

A test result that indicates a condition is present when it is actually absent. Its frequency depends on the test’s specificity and the condition’s prevalence.

Specificity (tests): The proportion of people without a condition who receive a negative test result. Low specificity allows more people without the condition to test positive.
Medical screening: Testing people without symptoms to identify those who may have a disease. Screening often accepts false positives to catch disease early, then uses follow-up tests.
False negative: A test result indicating a condition is absent when it is actually present. It is the opposite classification error: a real condition goes undetected.
Overdiagnosis: The diagnosis of a condition that would never have caused symptoms or harm during a person’s lifetime. False-positive screening can lead to diagnoses and treatment of harmless findings.
Sensitivity (tests): The proportion of people with a condition who receive a positive test result. Sensitivity describes missed cases, the counterpart to false-positive errors.
Mammography: Breast imaging, usually with low-dose X-rays, used to detect and assess breast abnormalities. An abnormal screening image can prompt follow-up even when cancer is absent.
True positive: A positive test result correctly indicating that a condition is present. It contrasts with a positive result that incorrectly signals an absent condition.
Biopsy: A procedure that removes tissue or cells for examination, usually to diagnose disease. A false-positive screen can lead to an invasive biopsy that finds no disease.
Base rate fallacy: An error in judgment that neglects how common an event is when interpreting evidence. Ignoring prevalence can make a positive result seem more conclusive than it is.
Drug test: An analysis of a biological sample for evidence of a drug or its metabolites. Cross-reactivity and other limits can produce positive results requiring confirmation.

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