Editorials

Adenosine Deaminase in the Diagnosis of Tuberculous Pleural Effusion

Although pleural effusion is a common clinical manifestation, the differential diagnosis of the cause of pleural effusion is often challenging, especially in the early differentiation of tuberculous pleurisy (TP) from other pleural effusion. The aim of this study was to evaluate the performance of commonly used laboratory tests for the early diagnosis of difficult cases of pleural effusion.

Patients with undiagnosed pleural effusion were enrolled and subjected to five laboratory tests including thoracoscopy, pleural fluid adenosine deaminase assay (ADA), serum tuberculosis antibody test (TB-antibody), tuberculin skin test (TST), and T-SPOT.TB assay. The diagnosis of TP was established based on pleural histology and mycobacterial culture. The different tests were compared for diagnostic performance.

A total of 106 patients were enrolled; their mean age was 53 years and 70.8% were male. Seventy-two (68%) of them were confirmed to have TP. When used individually, the five laboratory tests showed highly variable performance parameters, including sensitivity ranging from 46% to 92% and specificity ranging from 33% to 82%. When used in different combinations, thoracoscopy combined with TST or TB-antibody showed the optimal performance parameters, with a sensitivity of 80.8% and a specificity of 85.7%.

The results of this study suggest that the combination of thoracoscopy with TST or TB-antibody test is the best choice for the early diagnosis of difficult cases of TP in high TB burden countries.

Adenosine deaminase (ADA) catalyzes the deamination of adenosine to inosine. The activity of ADA in body fluids has clinical utility in the assessment of suspected tuberculosis.

The conversion of adenosine to inosine was monitored in 96-well microplates as a continuous decrease at 265 nm for 15 min at ambient temperature. Analytical precision, sensitivity, linearity, accuracy, and enzyme stability were validated. Reference intervals were established from > 120 tuberculosis-negative pleural, peritoneal, and cerebrospinal fluid samples.

The molar extinction coefficients of adenosine and inosine at 265 nm were 12,715 and 4,918 l/mol.cm and their difference was used to calculate ADA activity. Maximum within-day imprecision was < 11% and maximum total precision was < 19%. Analytical sensitivity was 0.5 U/l and the assay was linear to 40 U/l. ADA recovery was 96–110% over an activity range of 11.7–25.3 U/l. ADA was stable for 1, 7 and 30 days at ~ 25 °C, 4–8 °C, and − 20 °C storage, respectively. Upper reference limits were 9.4, 7.3, and 1.5 U/l for pleural, peritoneal, and cerebrospinal fluid, respectively.

The microplate-based kinetic ADA assay has favorable performance characteristics. This method eliminates the need for assay calibration and allows 96 samples to be tested simultaneously.

The conventional bacteriologic methods used for diagnosing pleural tuberculosis are less sensitive and time consuming. The objective of this study was to develop nonbacteriologic methods and to assess their potential utilities for the rapid diagnosis, especially in smear/culture-negative patients. One hundred forty patients with pleural effusion were investigated for tuberculous etiology by bacteriologic methods. Mycobacterium tuberculosis in the pleural fluid specimens was isolated in 11 patients. To establish a tuberculous etiology in the remaining 129 patients, we performed the following assays: a) estimation of tuberculosis-associated glycolipid antigen (TBGL) by a modified indirect enzyme-linked immunosorbent assay (ELISA), b) an immunocytochemical method for the demonstration of TBGL antigen in the Cytospin smears, and c) detection of mycobacterial DNA by polymerase chain reaction (PCR). Estimation of TBGL antigen by ELISA showed 100% specificity and overall 85.5% sensitivity. Immunocytochemistry could be applied only in those samples with adequate number of macrophages. PCR carried sensitivity and specificity of 87% and 93%, respectively. Estimation of TBGL antigen in pleural fluid specimens by ELISA has a definite role in establishing tuberculous etiology, particularly in those patients in whom bacteriologic methods did not demonstrate M. tuberculosis and also in those in whom a distinction between tuberculous and nontuberculous etiology was not possible based on the clinical and radiologic features of the thorax.