Pleural fluid adenosine deaminase (ADA) activity can be helpful in a differential diagnosis of an exudative pleural effusion because it is increased in a tuberculous pleural effusion. The ADA activity is determined mainly by the lymphocyte function. Age-associated immune decline is characterized by a decrease in T-lymphocyte function. For that reason, the pleural fluid ADA level would be lower in older patients with exudative pleural effusion. This study focused on the influence of age on the pleural fluid ADA activity in patients with exudative pleural effusion.

A total of 81 patients with exudative pleural effusion were enrolled in this study. In all patients, the pleural fluid ADA activity was measured using an automated kinetic method.

The mean age of the patients was 52.7±21.2 years. In all patients with exudative pleural effusion, the pleural fluid ADA activity revealed a significant difference between young patients (under 65 years of age) and old patients (at or over 65 years of age) : 82.8 ± 48.0 IU/L in young patients Vs. 57.7±37.2 IU/L in old patients (p<0.05), and showed a negative correlation with age (r=-0.325, p<0.05). In the 60 patients with a tuberculous pleural effusion, the pleural fluid ADA activity revealed a significant difference between the young and older patients : 103.5±36.9 IU/L in young patients Vs. 72.2±31.6 IU/L in old patients (p<0.05), and showed a negative correlation with age (r=-0.384, p<0.05). In the 21 patients with non-tuberculous exudative pleural effusion, the pleural fluid ADA activity of the young patients and old patients was similar : 23.7±15.3 IU/L in young patients Vs. 16.1±10.2 IU/L in old patients (p>0.05), and did not show any correlation with age (r=-0.263, p>0.05). The diagnostic cutoff value of pleural fluid ADA activity for tuberculous pleural effusion was lower in the older patients (25.9 IU/L) than in the younger patients (49.1 IU/L) or all patients (38.4 IU/L) with exudative pleural effusion.

Tuberculous pleural effusion is an important possibility to consider in older patients with a clinical suspicion of a tuberculous pleural effusion, although no marked increase in the pleural fluid ADA activity is usually detected. For a diagnosis of a tuberculous pleural effusion in old patients, the cutoff for the pleural fluid ADA activity should be set lower.