Tuberculin skin test reaction and body mass index in old age home residents in Hong Kong.

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 (> or =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.