Tuberculin responses and risk of pneumonia in immobile elderly patients

Background - Delayed type hypersensitivity (DTH) response to tuberculin is an important marker of T helper 1 (Th1) mediated acquired immunity against tuberculosis. Depressed DTH responses to tuberculin are observed in immobile elderly individuals with reduced activities of daily living (ADL) scores. Immobility in older people increases the risk of pneumonia related mortality. The decline in the competence of the immune system might be a reason for the increased susceptibility to infection in the elderly and the depressed DTH response to tuberculin might be a predictor for increased risk of pneumonia. Method - The DTH responses to tuberculin were examined in 49 older patients with limited ADL of similar severity, all of whom had a past history of positive DTH responses to tuberculin. Responses with an induration diameter of ≥ 10 mm were considered positive. Reactivities of Th1 and Th2 lymphocytes taken from the peripheral blood of each subject were also examined and compared between the positive tuberculin responders and the negative tuberculin responders. The rates of pneumonia in these groups were then compared prospectively for two years. Results - The number of CD4 lymphocytes differed significantly between patients with a positive tuberculin response (n=22, mean (SE) 1018 (118) x 16⁶/l) and those with a negative response (n=27, 666 (80) x 10⁶/l, p=0.02). Likewise, the number of Th1 cells was significantly higher in positive tuberculin responders than in negative responders (276 (5.8) x 10⁶/l versus 149 (14) x 10⁶/l, p=0.01). However, the numbers of Th2 cells were similar between patients with positive and negative responses (22 (3) x 10⁶/l versus 19 (3) x 10⁶/l, p=0.41). During the follow up period new pneumonia was diagnosed in 18 (67%) of the 27 negative tuberculin responders and in seven (31%) of the 22 positive tuberculin responders. According to the Cox regression model, a hazard ratio of developing pneumonia in patients with a negative tuberculin response compared with those with a positive tuberculin response was 2.57 (95% CI 1.12 to 6.17, p=0.03). Conclusion - The diminished DTH response to tuberculin may be a predictor for increased risk of pneumonia in elderly disabled patients.