Adenosine deaminase (ADA) is an enzyme associated with disorders that induce cell-mediated responses. It is a useful test in CSF for the diagnosis of tubercular meningitis, and can be performed in any laboratory as it is a simple, inexpensive, rapid and a fairly specific method.
However, raised levels of ADA in CSF are not specific to meningitis and this should be kept in mind when interpreting the test. Raised levels may also occur in other conditions particularly in CNS disorders such as lymphoma with meningeal involvement, neurobrucellosis, subarachnoid haemorrhage and sarcoidosis. False negatives may also occur.
Several studies have reported a sensitivity and specificity for diagnosis of tuberculous meningitis of approximately 90%, or even higher, with a cut-off value between 5 and 10 IU/L.
On the other hand, serologic assays for TB (ELISA) have been notoriously non-specific in the diagnosis of TB.