DISCUSSION

We describe a significant increase in TST conversions among HCWs in the Grady Health System temporally associated with use of 50-dose vials of Tubersol tuberculin, most of which were presumed to be false-positive results. After switching back to 10-dose vials, TST conversion rates returned to the previous baseline rate. To our knowledge, this is the first report of an increase in TST conversions temporally associated with a change to larger-volume vials of PPD. Although an increase in TB transmission in the Grady Health System and/or surrounding community cannot be entirely excluded, we found no evidence to support occult TB transmission. Similarly, no evidence was found of increased exposure to environmental mycobacteria, which could also potentially lead to false-positive TST reactions. The mechanism for the increase remains unclear, but these results occurred with multiple lots of PPD and is thus unlikely to be a batch effect.

Larger areas of induration following TST administration using larger PPD vials has been reportedReference Ruffino-Netto, Kritski, Teixeira, Dos Santos-Loredo, De Souza and Trajman ⁵ and could theoretically result from the properties of the vials themselves. There may be less adsorption of PPD in larger glass vials with less contact surface area between glass and solution.Reference Landi, Held, Hauschild and Hilsheimer ⁶ Additionally, a vial containing more doses of PPD may have more exposure to air and spend more time at room temperature, both of which have been associated with reduced concentrations of preservative.Reference Held and Landi ⁷ However, studies examining these factors are decades old, and it is unclear whether any observed differences in PPD potency had a measureable impact on human TST results.

Proponents of annual TB screening programs often cite their ability to recognize TB transmission not identified through other infection control programs. However, as infection control procedures in the United States have improvedReference Blumberg, Watkins and Berschling ⁸ and the incidence of TB in the United States has declined,Reference Yuen, Kammerer, Marks, Navin and France ⁹ serial TB screening programs targeting low-risk HCWs have increasingly become a source of false-positive test results.Reference Blumberg and Ernst ³ Annual screening for low-risk HCWs prevents active TB disease at an incremental cost of more than US$1.5 million per case prevented compared to testing only those with known TB exposure.Reference Mullie, Schwartzman, Zwerling and N’Diaye ¹⁰ Here, we demonstrate that false-positive TST conversions can result in expenditures of hospital resources beyond the cost of a low-yield LTBI testing and treatment program.Reference Blumberg and Ernst ³ An increase in false-positive results occuring over a short period of time in a population with a very low baseline rate of test conversions can give the appearance of institutional TB transmission, leading to unnecessary investigations by hospital epidemiology and infection control personnel.

This study shows the possibility of false-positive TST conversions when using 50-dose vials of Tubersol tuberculin, and our results suggest that using smaller vials may help limit false-positive results. These results also provide another demonstration of the potential pitfalls of serial TB screening in a low-risk population. When the prevalence of a disease in the population is low (e.g., <1%) and the performance characteristics of screening tests are poor,Reference Lewinsohn, Leonard and LoBue ⁴ most positive tests are falsely positive.Reference Blumberg and Ernst ^{3 ,} Reference Mullie, Schwartzman, Zwerling and N’Diaye ¹⁰ Annual TB screening programs should be modified to eliminate automatic annual LTBI testing for low-risk, previously negative HCWs and could instead be structured to carefully evaluate individual exposure risks and personal health history.