False-Positive Results in Medical Screening

A false-positive result occurs when a screening test indicates that a person may have a disease even though the person does not actually have it. Screening identifies people who need further assessment, so a positive screening result is not a confirmed diagnosis.

Why false positives occur

Screening tests cannot perfectly distinguish everyone with a disease from everyone without it. A result may cross the test's positive threshold even when the target disease is absent.

• Specificity is the proportion of people without the disease who correctly receive a negative result.
• The false-positive rate is the proportion of people without the disease who receive a positive result; it equals one minus specificity.
• Lowering the threshold for a positive result generally increases sensitivity, but can also increase false positives.

How disease prevalence affects interpretation

The positive predictive value is the proportion of people with positive results who actually have the disease. For a test with unchanged sensitivity and specificity, this value is lower when disease prevalence in the screened population is lower.

• Consequently, even a reasonably accurate test can produce many false positives when screening a population in which the disease is uncommon.
• Low prevalence does not itself increase the test's false-positive rate; it increases the proportion of positive results that are false.

Consequences for screening programmes

False positives can cause anxiety, additional expenditure and unnecessary investigations, some of which carry procedural risks. Appropriate confirmatory assessment helps prevent a screening result from being mistaken for a diagnosis.

• A programme must weigh the benefits of early detection against false-positive harms and the resources needed for follow-up.