Pleural Fluid Analysis: Standstill or a Work in Progress?

Figure 1

Recommended algorithm for investigation of pleural effusion. The use of Light’s criteria is recommended when a thoracocentesis revealed a protein level between 25 and 35 g/L to narrow down the differential diagnosis by determining whether a pleural effusion is transudative or exudative. NT-proBNP should be measured when a suspected cardiac effusion meets the exudative criteria. Determining causes of an exudative effusion is more challenging, and routine test, including biochemical measurement (i.e., pH and glucose), differential cell counts, cytology, and routine microbiology test are diagnostically useful. Pleural fluid pneumococcal antigen has been shown to be superior than urinary antigen to identify bacterial-induced pleural effusion. Tumour marker such as SMRP has a good diagnostic value to diagnose mesothelioma, however, the diagnostic utility of other tumour markers remains limited. Immunocytochemical evaluation of pleural fluid specimen is helpful in labelling different tumour markers. Other biological markers to differentiate parapneumonic/infective and malignant effusion remain elusive, expensive, and not widely available. Testing of pleural fluid ADA is an inexpensive and efficacious method for diagnosing tuberculous effusion, regardless of the patient’s immune status. Other tuberculosis-related inflammatory markers are available but are not superior to the latter. (PF: pleural fluid, black continuous line: strongly recommended and routinely practised, blue continuous line: not strongly recommended and not routinely practised, red dotted line: complementary diagnosis with other nonpleural tests.)