Investigation of the patient with pleural effusion (2)

We would like to thank Drs Chua, Groth and Jolobe for their letters concerning our article on respiratory investigations in pleural effusion, highlighting the use of pleural fluid adenosine deaminase (ADA) as a potential diagnostic test for suspected TB pleuritis. We agree that pleural fluid ADA is an important, useful and inexpensive diagnostic test for the investigation of patients with a moderate to high probability of TB pleuritis, on the basis of the evidence quoted. Pleural fluid ADA was not included in our article due to varying availability in the UK of this test, and constraints on the article length. The majority of chest physicians practise in low prevalence areas in the UK, although there are some areas where this is not the case. Pleural fluid ADA is associated with false positive results (eg pleural infection, rheumatoid pleuritis and malignancy) and in areas of low prevalence such as much of the UK, this limits ADA to being a ‘rule out’ test. We would suggest ADA is restricted to use in lymphocytic effusions in which there is at least a reasonable pretest probability of TB pleuritis. In addition, the diagnosis of TB pleuritis using markers of immune cell activation (ie ADA or interferon-gamma releasing assays) do not achieve specimens on which microbiological sensitivity may be tested. Although data from the USA suggest that the pattern of resistance in TB pleuritis reflects that seen in local pulmonary TB, this may not be helpful in the assessment of resistance in immigrants or recently returned travellers.¹In these circumstances, we would suggest pleural biopsy for histology and culture remains the gold standard diagnostic test, conducted either using Abram's needles or under image/thoracoscopic guidance. We would like to thank the authors for their interest in our article.

• © 2009 Royal College of Physicians