Worked Example
Suppose the fecal occult blood (FOB) screen test is used in 2030 people to look for 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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The small positive predictive value (PPV = 10%) indicates that many of the positive results from this testing procedure are false positives. Thus it will be necessary to follow up any positive result with a more reliable test to obtain a more accurate assessment as to whether cancer is present. Nevertheless, such a test may be useful if it is inexpensive and convenient. The strength of the FOB screen test is instead in its negative predictive value - which, if negative for an individual, gives us a high confidence that its negative result is true.
Read more about this topic: Positive Predictive Value
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