Sensitivity vs Specificity
How to tell Sensitivity and Specificity apart when a board question makes them look alike.
Sensitivity: true positive rate, rules disease out when negative (SnNout); specificity: true negative rate, rules disease in when positive (SpPin).
Side by side
Sensitivity is the proportion of people with disease who test positive and helps rule out disease when a highly sensitive test is negative.
Specificity is the proportion of people without disease who test negative and helps rule in disease when a highly specific test is positive.
Sensitivity is the proportion of people with disease who test positive and helps rule out disease when a highly sensitive test is negative.
Specificity is the proportion of people without disease who test negative and helps rule in disease when a highly specific test is positive.
Sensitivity = TP/(TP+FN); a highly sensitive test, when negative, helps rule out disease (SnNOUT).
Specificity = TN/(TN+FP); a highly specific test, when positive, helps rule in disease (SpPIN).
The trap
Sensitivity is true positive rate; specificity is true negative rate.
Watch it explained
Test it nowThe repair move
Before naming the answer, force the stem through the separator: Sensitivity is true positive rate; specificity is true negative rate.
Try a free question
CoreA clinic uses an HIV ELISA antibody test as the initial screening test for HIV. The test is highly SENSITIVE (99.9%) but only moderately specific (98%). A negative result on this test is used to RULE OUT HIV. Which property of the test makes a negative result clinically useful for ruling out disease?
Show answer and explanation
Answer: A. High sensitivity (true positive rate = TP/(TP+FN)). Sensitivity = TP / (TP + FN). High sensitivity means few false negatives, so a negative test reliably rules out disease. Mnemonic: SnNout (Sensitivity, Negative rules out). Highly sensitive tests are used for SCREENING: HIV ELISA, mammography, fecal occult blood. Confirmatory testing follows with a more specific test.
Why the other choices are tempting
- B. High specificity rules IN disease when positive (SpPin), not rules out.
- C. PPV depends on prevalence; not the intrinsic property.
- D. NPV is the post-test probability of no disease given negative test; depends on prevalence.
- E. Accuracy is overall correct results (TP+TN)/(TP+FP+TN+FN).
Flashcards
Tap a card to flip it.
What clue supports Sensitivity?
Choose Sensitivity when the stem gives this pattern: Sensitivity is the proportion of people with disease who test positive and helps rule out disease when a highly sensitive test is negative.
What clue supports Specificity?
Choose Specificity when the stem gives this contrasting pattern: Specificity is the proportion of people without disease who test negative and helps rule in disease when a highly specific test is positive.
Quick answers
What is the fastest way to tell Sensitivity from Specificity?
Sensitivity: true positive rate, rules disease out when negative (SnNout); specificity: true negative rate, rules disease in when positive (SpPin).
What is the key clue for Sensitivity?
Sensitivity is the proportion of people with disease who test positive and helps rule out disease when a highly sensitive test is negative.
What is the key clue for Specificity?
Specificity is the proportion of people without disease who test negative and helps rule in disease when a highly specific test is positive.
What is the common trap with Sensitivity vs Specificity?
Sensitivity is true positive rate; specificity is true negative rate.