Association Between TB Skin Test Result and TB Disease
Association Between TB Skin Test Result and TB Disease
We analyzed reports of persons with culture-confirmed TB in the National Tuberculosis Surveillance System of the Centers for Disease Control and Prevention (CDC) during January 1, 1993 through December 31, 2010. The analysis included persons with a documented TST result, anatomical site of disease, HIV status, and birthplace (US- or foreign-born). Cases of pulmonary TB without a chest radiograph result were excluded to allow for evaluation of the association between radiograph findings and TST result. Reports from California were also excluded because HIV status was not routinely reported from that jurisdiction prior to 2011.
Based on CDC guidelines for the classification of TST reactions, the TST result was divided into categories of 0–4 mm, 5–9 mm, 10–14 mm, and ≥ 15 mm. A TST result of 0–4 mm was considered negative and a result ≥ 5 mm was considered positive. Pearson's chi-square statistic was used to assess differences in the distribution of TST results for sociodemographic and clinical characteristics. Clinical presentation of disease was defined as one of the following mutually exclusive categories: miliary disease, combined pulmonary and extrapulmonary disease, extrapulmonary only disease, and pulmonary only disease which was further divided into non-cavitary pulmonary disease and cavitary pulmonary disease. A designation of miliary disease was based on either clinical impression or a miliary radiographic pattern on either chest radiograph or CT scan.
Multinomial logistic regression was used to examine the association between TST result category and clinical presentation of disease category and to calculate odds ratios and 95% confidence intervals. Non-cavitary pulmonary disease was the largest clinical presentation category and was used as the referent outcome category. A TST of 0–4 mm (negative) was used as the referent category for TST result. Persons with non-cavitary pulmonary disease with a TST of 0–4 mm served as the comparison group to calculate odds ratios for each of the respective clinical presentation/TST result category combinations (e.g., cavitary pulmonary disease with TST ≥ 15 mm or miliary disease with TST 10–14 mm were all compared to non-cavitary pulmonary disease with a TST of 0–4 mm). We examined the following covariates for effect modification or confounding: sex, age, race and ethnicity (self-designated), HIV status, birthplace, incarceration at the time of diagnosis, homelessness in the 12 months prior to diagnosis, and excessive alcohol or illicit drug use in the 12 months prior to diagnosis. Finally, we conducted an additional analysis restricted to persons with exclusively pulmonary disease who had a documented sputum smear result at baseline. Multivariate logistic regression was used to calculate odds ratios and 95% confidence intervals to quantify the odds of having a positive sputum smear (vs. negative) result for acid-fast bacilli (AFB) for each TST category (TST 0–4 mm referent).
As data were collected as part of routine TB surveillance by the CDC, this analysis was not considered research involving human subjects, and institutional review board approval was not required.
Methods
We analyzed reports of persons with culture-confirmed TB in the National Tuberculosis Surveillance System of the Centers for Disease Control and Prevention (CDC) during January 1, 1993 through December 31, 2010. The analysis included persons with a documented TST result, anatomical site of disease, HIV status, and birthplace (US- or foreign-born). Cases of pulmonary TB without a chest radiograph result were excluded to allow for evaluation of the association between radiograph findings and TST result. Reports from California were also excluded because HIV status was not routinely reported from that jurisdiction prior to 2011.
Based on CDC guidelines for the classification of TST reactions, the TST result was divided into categories of 0–4 mm, 5–9 mm, 10–14 mm, and ≥ 15 mm. A TST result of 0–4 mm was considered negative and a result ≥ 5 mm was considered positive. Pearson's chi-square statistic was used to assess differences in the distribution of TST results for sociodemographic and clinical characteristics. Clinical presentation of disease was defined as one of the following mutually exclusive categories: miliary disease, combined pulmonary and extrapulmonary disease, extrapulmonary only disease, and pulmonary only disease which was further divided into non-cavitary pulmonary disease and cavitary pulmonary disease. A designation of miliary disease was based on either clinical impression or a miliary radiographic pattern on either chest radiograph or CT scan.
Multinomial logistic regression was used to examine the association between TST result category and clinical presentation of disease category and to calculate odds ratios and 95% confidence intervals. Non-cavitary pulmonary disease was the largest clinical presentation category and was used as the referent outcome category. A TST of 0–4 mm (negative) was used as the referent category for TST result. Persons with non-cavitary pulmonary disease with a TST of 0–4 mm served as the comparison group to calculate odds ratios for each of the respective clinical presentation/TST result category combinations (e.g., cavitary pulmonary disease with TST ≥ 15 mm or miliary disease with TST 10–14 mm were all compared to non-cavitary pulmonary disease with a TST of 0–4 mm). We examined the following covariates for effect modification or confounding: sex, age, race and ethnicity (self-designated), HIV status, birthplace, incarceration at the time of diagnosis, homelessness in the 12 months prior to diagnosis, and excessive alcohol or illicit drug use in the 12 months prior to diagnosis. Finally, we conducted an additional analysis restricted to persons with exclusively pulmonary disease who had a documented sputum smear result at baseline. Multivariate logistic regression was used to calculate odds ratios and 95% confidence intervals to quantify the odds of having a positive sputum smear (vs. negative) result for acid-fast bacilli (AFB) for each TST category (TST 0–4 mm referent).
As data were collected as part of routine TB surveillance by the CDC, this analysis was not considered research involving human subjects, and institutional review board approval was not required.
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