TO THE EDITOR:
Bailey and colleagues [1] studied risk factors for tuberculin positivity in hospital employees and found that the employees' postal zones of residence had the strongest statistical association with tuberculin skin test conversion.
Several years ago, in an effort to define the baseline levels of tuberculosis in a large urban population and to quantitate the risk factors for a positive tuberculin test result, we studied skin test results of more than 50 000 New York City Board of Education employees [2]. Along with reaction size and history of a positive reaction, we determined age, race or ethnic group, sex, and socioeconomic status using a specific index for demographic characteristics (ZIP code of residence) as a surrogate for socioeconomic status. (Rezide ZQ rating, Claritas, Inc., Washington, DC) [3].
Before examining our data, we had expected to find that socioeconomic status had a stronger relation to a positive tuberculin test result than did race or ethnic group; however, a multivariate analysis showed a stronger correlation between race and a positive test result.
Tuberculous infection is thought to reflect conditions of past decades because most persons with a positive reaction have been previously infected. Infected persons who currently live in areas of higher socioeconomic status may have previously lived in areas that have lower status and greater rates of tuberculosis, where they presumably acquired the disease. We speculated that this factor explained the strong correlation between race and a positive skin test result; that is, some racial or ethnic groups may have lived in areas of lower socioeconomic status when they became infected. In other words, our study's ascertainment reflected current socioeconomic status, not the status at the time of infection. We adopted this theory to explain the findings of our investigation.
It is interesting that Bailey and colleagues [1] found that only the proportion of persons living below the poverty level within their postal zone of residence showed a statistically significant association to tuberculin conversions. This finding tends to substantiate the theory we used to explain our earlier findings.
Lee B. Reichman, MD, MPH
New Jersey Medical School National Tuberculosis Center
Newark, NJ 07107
1. Bailey TC, Fraser VJ, Spitznagel EL, Dunagan WC. Risk factors for a positive tuberculin skin test among employees of an urban, midwestern teaching hospital. Ann Intern Med. 1995; 122:580-5.
2. Reichman LB, O'Day R. Tuberculosis infection in a large urban population. Am Rev Respir Dis. 1978; 117:705-12.
3. Rezide—The National Encyclopedia of Residential ZIP Code Demography. Washington, DC: Claritas, Inc.; 1974.
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