OBJECTIVES: To study the relationship between body mass index (BMI) and tuberculin skin test (TST) reaction in predicting the development of active tuberculosis (TB).
DESIGN: A follow-up study.
SETTING: Old age homes.
PARTICIPANTS: Three thousand six hundred five residents who took part in a screening program for TB and had two-step TST using two units of the tuberculin PPD-RT23.
MEASUREMENTS: Rate of development of active TB in these residents over an average follow-up period of 2.5±1.25 years.
RESULTS: After one-step and two-step testing, 46.3% and 69.6% of residents, respectively, had positive TST reactions (≥10 mm). Thirty-four residents developed active TB (323 per 100,000 person-years) during follow-up. The only significant risk factors associated with development of active TB were positive TST according to one-step testing (adjusted odds ratio (OR)=2.91, 95% confidence interval (CI)=1.26–6.74) and a BMI less than 18.5 (adjusted OR=3.15, 95% CI=1.45–6.86). Residents with a BMI less than 18.5 and a negative TST also had greater risk of active TB than residents with a BMI greater than 18.5 and negative TST (adjusted OR=4.36, 95% CI=1.04–18.3), whereas those with a positive TST had the highest risk (adjusted OR=10.2, 95% CI=2.63–39.4). Two-step testing increased the sensitivity but reduced the specificity of TST in identifying active TB on follow-up.
CONCLUSION: In the elderly population, interpretation of TST should take into consideration the BMI of the individual. A positive TST according to one-step but not two-step testing was useful in predicting the development of active TB on follow-up.