Cuttoff values of biochemical tests on pleural fluid: their usefulness in differential diagnosis of 1,040 patients with pleural effusion

The aim of biochemical pleural fluid testing is to reach an etiological diagnosis of the pleural effusion. We assessed the utility of considering cuttoff points for the wide range of analytes used to investigate pleural fluid. Among 1,040 patients with pleural effusion, we sought the etiologies of those fluids which showed any of the following characteristics: red blood cell count = 10 x 109/L, leukocytes = 10 x 109/L, percentage of neutrophils or lymphocytes >50%, protein = 50 g/L, glucose = 60 mg/dL, pH = 7.2, lactate dehydrogenase = 1,000 U/L, adenosine deaminase = 40 U/L, amylase = 100 U/L or cholesterol = 60 mg/dL. Some of the more prominent findings were: a) a sixth of transudates were blood-tinged or contained predominantly neutrophils; b) a groosly bloody fluid suggests malignant disease, trauma, or pulmonary embolization; c) nearly 90% of fluids containing = 10 x 10(9) leukocytes/L were parapneumonics; d) 73% of tuberculous pleural fluids had protein > or = 50 g/L, e) tuberculosis and parapneumonics explained more than 90% of fluids with high adenosine deaminase content; f) one third of amylase-rich pleural effusions were malignant; g) a low pleural glucose or pH levels indicate that patient probably has a parapneumonic, tuberculous or malignant etiology; y h) the diagnostic yield of pleural fluid cytology in malignant effusions was 57%, a percentage which raised to 94% in those with low glucose fluid level. Cuttof values of biochemical pleural fluid tests may greatly support particular causes of pleural effusions.