Worked Example
Suppose that a fecal occult blood (FOB) screen test is used in 2030 people to detect bowel cancer:
| Patients with bowel cancer (as confirmed on endoscopy) |
||||
| Condition Positive | Condition Negative | |||
| Fecal Occult Blood Screen Test Outcome |
Test Outcome Positive |
True Positive (TP) = 20 |
False Positive (FP) = 180 |
Positive predictive value
= TP / (TP + FP) = 20 / (20 + 180) = 10% |
| Test Outcome Negative |
False Negative (FN) = 10 |
True Negative (TN) = 1820 |
Negative predictive value
= TN / (FN + TN) = 1820 / (10 + 1820) ≈ 99.5% |
|
| Sensitivity
= TP / (TP + FN) = 20 / (20 + 10) ≈ 67% |
Specificity
= TN / (FP + TN) = 1820 / (180 + 1820) = 91% |
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In this setting, with NPV = 99.5%, a negative test result may provide some reassurance that the subject is unlikely to have cancer. This high NPV value would be particularly notable if the cancer were relatively common. For example, if 5% of people in the population had bowel cancer, then a NPV of 99.5% would indicate that a person with a negative test result has much lower than the average population risk for bowel cancer. However if the prevalence of bowel cancer were 0.5%, a negative test result in this setting would be uninformative.
Read more about this topic: Negative Predictive Value
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