Sensitivity and Specificity
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Sensitivity and specificity describe the intrinsic performance of a diagnostic or screening test. They are properties of the test itself and do not change with disease prevalence.
| Disease Present | Disease Absent | |
|---|---|---|
| Test Positive | True Positive (TP) | False Positive (FP) |
| Test Negative | False Negative (FN) | True Negative (TN) |
- Sensitivity = TP / (TP + FN) = probability of a positive test given disease is present
- Specificity = TN / (TN + FP) = probability of a negative test given disease is absent
- High sensitivity: good at ruling OUT disease (SnNOut: if Sensitivity is high, a Negative result rules Out)
- High specificity: good at ruling IN disease (SpPIn: if Specificity is high, a Positive result rules In)
- Sensitivity and specificity are usually inversely related when adjusting a test threshold
Problem: A NIPT screen has 99% sensitivity and 99.5% specificity for trisomy 21. In a population where the prevalence is 1/500, what happens when 10,000 pregnant individuals are screened?
| T21 Present (20) | T21 Absent (9,980) | |
|---|---|---|
| Screen Positive | 20 x 0.99 = 19.8 | 9,980 x 0.005 = 49.9 |
| Screen Negative | 0.2 | 9,930.1 |
- Sensitivity: 19.8/20 = 99%
- Specificity: 9,930.1/9,980 = 99.5%
- But PPV = 19.8 / (19.8 + 49.9) = 28%: most positive screens are false positives
- Newborn screening: High sensitivity is prioritized (do not miss affected newborns), accepting lower specificity (more false positives requiring follow-up)
- NIPT/cfDNA: High sensitivity and specificity, but PPV depends heavily on maternal age (prior probability)
- Carrier screening panel detection rate: Often reported as sensitivity (e.g., "detects 90% of CF pathogenic variants in the European population")
"SnNOut": Sensitivity, Negative, rules Out. A highly sensitive test with a negative result effectively excludes the condition.
"SpPIn": Specificity, Positive, rules In. A highly specific test with a positive result effectively confirms the condition.