Sensitivity and Specificity
Image by: Brett Dashevsky
What are Sensitivity and Specificity?
Sensitivity and specificity are the two measurements used to determine the accuracy of a screening test.
Sensitivity measures a test’s ability to correctly identify patients who have a disease.
Specificity measures a test’s ability to correctly identify patients who do not have the disease.
Let's Break it Down
Sensitivity: An Example
Let’s think about a group of 100 people who are getting screened for a disease. If a test accurately identifies 90 people who have the disease, we say that the test is 90% sensitive.
Specificity: An Example
Let’s take the same 100 people from above who are getting screened for a disease. If a test accurately identifies 70 people who do not have the disease, the test is 70% specific. (We know, the math doesn’t exactly add up: this is just an explanation)!
In Real Life Examples?
All screening tests have unique sensitivities and specificities. Some of the most common screening tests include PAP Smears, Mammograms, HIV/AIDS tests, COVID-19 PCR tests and COVID-19 Antigen tests.
A Specific and Sensitive Matter
Sensitivity and specificity help us understand a test’s strengths and weaknesses. Together, they signify the strength of a test by its ability to accurately identify two groups: people who have a condition and people who don’t have a condition. These measurements also illuminate a screening test’s flaws, commonly known as false negatives and false positives. These flaws help us understand where providers are most likely to find misdiagnoses.
Don’t be so negative...sike?!
Let’s say that a test is 90% sensitive. So, the test will miss 10% of the tested population who actually have the disease. People in this 10% who are misclassified as having the disease are called false negatives.
Do two negatives make a positive?
A test with 70% specificity will identify 30% of the population as having the disease when they actually don’t. People in this 30% who are misclassified as having the disease are called false positives.
Why do I sometimes only see 1 number?
Many news headlines only report a test’s sensitivity because one number is typically more digestible for readers. However, remember that sensitivity only tells half the story when it comes to screening tests!
Let’s split the difference?
Our recommendation is to think of what each term really means. Sensitivity is your ability to detect something, so a more sensitive test is more likely to catch a disease. On the other hand, specificity describes the ability to only identify the condition you are investigating. Therefore, a more specific test will be less likely to identify healthy people as having a disease.
Providers rely on sensitivity and specificity to rule a disease in or out. Highly sensitive tests rule out a disease and highly specific tests rule in a disease. Therefore, if a highly sensitive test, like a mammogram, is negative, you rule out breast cancer. If a highly specific test, like a Covid-19 PCR test, is positive, you rule in Covid-19.
Want an easy way to remember this? SpIN and SnOUT:
a specific test is positive → rule IN the disease
a sensitive test is negative → rule OUT the disease
Outside the Huddle
Sensitivity, Specificity, and Predictive Values: Foundations, Pliabilities, and Pitfalls in Research and Practice | Frontiers in Public Health
Understanding medical tests: sensitivity, specificity, and positive predictive value | Health News Review
Assessment of Diagnostic and Screening Tests | UNC Gillings School of Global Public Health