Effects of Prevalence on Diagnostic Tests
Interpretation
Look at these three clinical settings with different prevalence of breast cancer. Can you see that the implications of a breast lump found on physical examination (positive predictive value) will change depending on the pre-test risk (or prevalence) of breast cancer?
| Setting | Prevalence |
PPV |
| Family Physician’s Office |
1% | 7.5% |
| Breast Screening Clinic | 3% | 20% |
| Surgical Clinic | 50% | 89% |
In different clinical settings the implications of a normal examination are also different, as shown below with different negative predictive values. Can you see that in some high risk settings like the surgical clinic, even a normal physical examination may not be enough to “rule-out” breast cancer?
| Setting | Prevalence |
NPV |
| Family Physician’s Office |
1% | 99.8% |
| Breast Screening Clinic | 3% | 99.0% |
| Surgical Clinic | 50% | 82% |