Data from one study of variation between duplicate PPD tests
(37), indicated that more than half of the discordant readings occurred in persons with one test measured as zero and the other as 1-4 mm of induration.
The booster phenomenon confounds the interpretation of the PPD test, complicating TB control programs (12).
The debate about scientific soundness derives from the reliance on the PPD test, which is neither sensitive nor specific, unlike the hepatitis B antibody and surface antigen test on which the bloodborne pathogen standard is based.
Furthermore, 21st century TB control efforts continue to rely on the 19th-century PPD test and the insensitive sputum AFB smear.
Patients with a negative PPD test reacted most often to the control skin test Candida (63.5%), followed by mumps (52.2%), histoplasmosis (37.2%), tetanus (35.7%), and Trichophyton (6.1%).
The sensitivity of the PPD test varies, however, from 96% in the general population,(3) to 59% in the population infected with HIV,(4) to only 50% in critically ill patients with disseminated tuberculosis.(5) False-negative PPD test reactions may result from problems with the tuberculin test itself, such as administration of the tuberculin material used; errors in retrieving the results; or host factors, such as altered cellular immunity.
There is ongoing controversy concerning the most reliable method for separating a negative PPD test due to nontuberculous infection from negative tuberculin reactivity secondary to cutaneous anergy.
In spite of the CDC's recommendation, many health care workers who administer and interpret PPD tests believe that a BCG vaccination always results in a positive PPD test.
Such a misconception is a potentially serious breach in the surveillance of tuberculosis in general, but especially among immigrants, refugees, and migrant farmworkers, as most immigrants to the United States originate from countries where TB is 3 to 60 times more prevalent than in the United States.[5-7] Research has demonstrated that tuberculosis among farmworkers in the United States can be considered an occupational risk, and that all farmworkers are at a far greater risk for infection than are most other populations. Thus, a positive PPD test in an immigrant or farmworker is more significant than in almost any other population group.
There is some logic to the proposition that BCG produces a positive PPD test result.
Previous literature has reported that T-SPOT.TB is superior to tuberculin skin test in both sensitivity and specificity for diagnosing TB.[sup], In this study, both T-SPOT.TB and PPD tests were performed in 26 patients: 20 (76.9%) positive T-SPOT.TB and 13 (50.0%) positive PPD were detected, with a statistically significant difference.
For example, only 26 patients completed both T-SPOT.TB and PPD tests. Furthermore, all enrolled subjects were hospitalized patients, whose clinical conditions might be more severe compared to outpatients, a selection bias might limit the external validity of the study.