False Positive and False Negative
A result that says you have something when you do not, or misses something when you do.
What it means
False positive — the test says you have it and you do not. Leads to worry, further testing, and sometimes unnecessary treatment.
False negative — the test says you do not have it and you do. Leads to false reassurance and delay.
Every test produces both. There is no way to eliminate them, only to shift the balance between them.
The proportion of positives that are false depends heavily on how common the condition is. Testing a large group of people who mostly do not have a condition produces a lot of false positives, however good the test is. That is a mathematical property of screening, not a failure of it.
In practice
In screening information leaflets, and in explanations of why a result needs repeating.
Very much in evidence in home testing kits, where results arrive with no context at all.
Why it matters
Do not act on a single unexpected positive result. Ask for it to be confirmed, particularly if you have no symptoms.
Do not be falsely reassured by a negative result if your symptoms persist. This is the more dangerous of the two, and it is the reason every page on this site says symptoms override a past clear test.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026