The two numbers that tell you how reliable a test actually is.
Every medical test aims to tell us if you have a specific condition. No test is perfect, so we rely on two statistical measures to understand their accuracy. Sensitivity tracks how well a test catches everyone who actually has the disease. A test with high sensitivity rarely misses a positive case.
A highly sensitive test is designed to cast a wide net. We use these tests when it is critical not to miss a diagnosis, such as for infectious diseases. If a test is 99% sensitive, it will correctly identify 99 out of 100 sick people. The small group missed by the test represents a false negative.
Specificity measures how well a test correctly identifies people who do not have the condition. A test with high specificity is very good at ruling out disease in healthy patients. This keeps us from telling you that you are sick when you are actually healthy.
When a test lacks high specificity, it can lead to a false positive. This happens when the test says you have a condition that you do not actually have. High specificity prevents the stress, costs, and potential side effects of unnecessary follow-up treatments.
Most diagnostic tools require a balance between these two numbers. We often prioritize sensitivity for initial screening tests to ensure safety. Once a screen identifies a potential issue, we use a second test with higher specificity to confirm the diagnosis definitively.