Health officials are considering a change be made to the interpretation of the tuberculin skin test that will change the cut-off for a positive purified protein derivative (PPD) from 10 mm to 5 mm for healthcare workers. Which of the following is a true statement regarding this potential change?
- This change will decrease the positive predictive value of the test
- This change will decrease the negative predictive value of the test
- This change will not change the negative predictive value of the test
- This change will not change the sensitivity of the test
- This change will increase the specificity of the test
This change will increase the sensitivity of the test and, assuming the prevalence of disease in healthcare workers has not changed, will decrease the positive predictive value (PPV).
The PPV of a test describes the probability that an individual will have the disease if they receive a positive test result. In the case of changing the threshold for a positive PPD, a lower cut-off will result in more positive test results–both true and false positives. The increase in false positives will be disproportionately larger than the increase in true positives when the incidence of disease is unchanged. As a result, the PPV will decrease.
The CDC guidelines identify populations at risk for contracting TB that ought to be screened. These include individuals with HIV, IV drug users, homeless persons, foreign-born persons, and healthcare workers. They report that mandated skin-testing programs should be discouraged unless the targeted groups contain substantial proportions of persons at high risk.
Illustration A is a cartoon depicting a 2x2 table with sample values and calculation of the PPV at the bottom of the image. Illustration B depicts the trade-offs inherent to defining cut-offs. This example would correlate to moving the cut-off from the middle line towards the left.