Adenosine desaminase is an enzyme that is necessary for the maturation and differentiation of lymphoid cells. It is useful surrogate marker for TB in pleural and peritoneal fluids.
ADA testing is not widely available but can be done relatively easily and cheaply if a spectrophotometer is available2. Kits can be purchased to perform the test (see MSF Medical catalogue, volume 4).
ADA is typically greater than 50 U/litre in TB pleural effusions. Pleural effusions with an ADA level below 40 U/litre are much less likely due to TB. The specificity is increased when ADA is greater than 50 and the lymphocyte-neutrophil ratio is greater than 0.753.
A meta-analysis suggests the optimal cut off values of greater than 39 U/litre4 likely to be due to TB. However, the sensitivity of ADA in peritoneal fluid is substantially lower in patients with cirrhosis.
– HIV-infected patients and patients already on TB medications may have lower levels of ADA.
– ADA is generally not a good test in cerebrospinal fluid.