Increased pleural fluid adenosine deaminase (ADA) activity is classically associated with tuberculous pleuritis. However, increased activity can also occur in a number of other diseases and this may negatively affect the diagnostic utility of ADA measurements and decrease its specificity for the diagnosis of tuberculosis (TB). The presence of ADA in pleural fluids reflects the cellular immune response in the pleural cavity and in particularly, the activation of T lymphocytes. Different disease entities are typically associated with the presence of particular types of leukocytes.
Objective: To determine whether the combined use of ADA activity and differential cell counts would provide a more efficient means for diagnosing tuberculous pleurisy than the use of ADA levels alone.
Methods: Biochemistry, cytology, and microbiology studies were performed on 472 consecutive pleural fluids. ADA and differential cell counts were determined on all exudative effusions.
Results: ADA activity in tuberculous effusions was significantly higher than in any other diagnostic group (p<0.005). At a level of 50 U/L, the sensitivity, specificity, positive predictive value (ppv), negative predictive value (npv), and efficiency for the identification of TB were calculated at 91%, 81%, 84%, 89%, and 86%, respectively. When the additional requirement of a lymphocyte neutrophil ratio of 0.75 or greater was included, the sensitivity, specificity, ppv, npv, and efficiency for the identification of TB were calculated at 88%, 95%, 95%, 88%, and 92%, respectively.
Conclusion: ADA, especially when combined with differential cell counts and lymphocyte/neutrophil ratios, remains a useful test in the diagnosis tuberculous pleuritis.