How is pleural fluid used to differentiate transudates from exudates in the evaluation of pleural effusion (fluid on the lungs)?

Updated: Dec 28, 2018
  • Author: Kamran Boka, MD, MS; Chief Editor: Guy W Soo Hoo, MD, MPH  more...
  • Print

The initial diagnostic consideration is distinguishing transudates from exudates. Although a number of chemical tests have been proposed to differentiate pleural fluid transudates from exudates, the tests first proposed by Light et al have become the criterion standards. [23]

The fluid is considered an exudate if any of the following are found:

  • Ratio of pleural fluid to serum protein greater than 0.5

  • Ratio of pleural fluid to serum LDH greater than 0.6

  • Pleural fluid LDH greater than two thirds of the upper limits of normal serum value

The fluid is considered a transudate if all of the above are absent.

These criteria require simultaneous measurement of pleural fluid and serum protein and LDH. However, a meta-analysis of 1448 patients suggested that the following combined pleural fluid measurements might have sensitivity and specificity comparable to the criteria from Light et al for distinguishing transudates from exudates [24] :

  • Pleural fluid LDH value greater than 0.45 of the upper limit of normal serum values

  • Pleural fluid cholesterol level greater than 45 mg/dL

  • Pleural fluid protein level greater than 2.9 g/dL

Clinical judgment is required when pleural fluid test results fall near the cutoff points.

Did this answer your question?
Additional feedback? (Optional)
Thank you for your feedback!