False positive and false negative screening results
A screening result can be wrong: a healthy person screens positive (a false positive), or a person with the disease screens negative (a false negative).
Remember
A positive screen in someone without the disease is a false positive, and a negative screen in someone who has it is a false negative. A positive screen usually leads to further testing, and a negative screen reduces the chance of disease but does not rule it out.
Memory aid
the second word is what the test said and the first word is whether the test was right
In a false positive, the second word shows that the test said positive and the first word shows that the test was wrong.
Screening looks for disease or risk among apparently well people. A screening test shows the risk that a person has the disease. It is not diagnostic: a definitive diagnostic test (a more accurate test that confirms or rules out disease) is what settles it.
When the test result (positive or negative) is crossed with the truth (disease or no disease), the outcomes can be set out in a simple table. A true positive is a positive result for a person with the disease, and a true negative is a negative result for a healthy person.
A false positive is a positive result in a person who does not have the disease. The test indicated a high chance of disease that was absent in reality, so the person is referred for further tests that reveal they are healthy. The cost is anxiety, extra investigations and sometimes harm from the further testing. The word false describes a mismatch between the result and reality, not a mistake by the person being screened, and it can happen even when the test is performed correctly.
A false negative is a negative result in someone who has the disease. The person is falsely reassured and is not referred, so diagnosis is delayed. That delay may lead to harm from the worse outcomes of more progressed disease. A negative screen reduces the chance of disease but does not rule it out, especially when symptoms or other concerns are present.
Sensitivity is the proportion of people with the disease who test positive, and high sensitivity gives few false negatives. Specificity is the proportion of people without the disease who test negative, and higher specificity means fewer false positives. Where the disease is rare, most people tested are healthy, so even a modest false-positive rate produces many false positives.
Key facts
- False positive
- a positive screening result in someone who does not have the disease.
- False negative
- a negative screening result in a person who has the disease, so the person is not referred for further investigation.
- Screening result
- shows risk rather than diagnosing disease, so a positive screen usually needs a definitive diagnostic test.
- Cost of a false positive
- anxiety, extra investigations and sometimes harm from the further testing.
- Cost of a false negative
- a delay in diagnosis, with possible harm from the worse outcomes of more progressed disease.
- Sensitivity and false negatives
- high sensitivity gives few false negatives.
- Specificity and false positives
- higher specificity means fewer false positives.
Exam traps
- One person's false positive result does not show that a test has low sensitivity, because sensitivity is the proportion of people with the disease who test positive, measured across a population.
- A positive result in a person who is later shown not to have the disease is not a true positive, because a true positive is a positive result in someone who genuinely has the disease.
- A negative screening result in a person who has the disease is not a true negative, because a true negative is a negative result in a person who does not have the disease.
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