Review Question - QID 103966

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.

Jerant et al. discuss the use of the tuberculin skin test in screening for individuals with latent tuberculosis infection. They note that different thresholds are used for different populations, based on their tuberculosis risk. This strategy takes into account the variability in PPV and negative predictive value (NPV) of a test based on the prevalence of a disease within the population being tested.

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.

Incorrect Answers:
Answer 2: Making this change will increase the NPV of the test.
Answer 3: Making this change will increase the NPV of the test.
Answer 4: Making this change will increase the sensitivity of the test.
Answer 5: Making this change will decrease the specificity of the test.