Influence of storage time on pleural fluid adenosine deaminase activity

Storing pleural fluid samples for research purposes is a common practice, but whether adenosine deaminase (ADA), an enzyme used for the diagnosis of tuberculous pleuritis, is stable over long periods of time is unknown.

We evaluated the stability of pleural ADA concentrations in 223 samples frozen at -800C as compared to values obtained immediately following the initial thoracentesis. Sample storage time ranged from several months to slightly more than 10 years.

ADA activity was stable for up to 2.6 years. Afterwards, it decreased 6 to 8 U/L, enough to drop 2 (3.3%) tuberculous patients below the diagnostic ADA cutoff.

As far as ADA enzymatic activity is concerned, pleural fluid samples are viable for extended periods of time. However, some caution in interpreting results from specimens stored for > 2.6 years is prudent.