The etiology of pleural effusions is a classic internal medicine question and as always a very popular morning report topic. ‘Light’s Criteria’ suggest an effusion is an exudate if it has any of:
- pleural protein / serum protien >0.5
- pleural LDH/ serum LDH >0.6
- pleural LDH > 2/3 upper limit of normal serum LDH
which is nicely detailed in the article:
Pleural Effusion. Light RW; NEJM 2002 vol 36 no. 25 1971
Conversely, it has been suggested that the etiology of a pleural effusion may be better determined using liklihood ratios of various biochemical parameters. Theoretically this would better incorporate the pretest probability of an exudative and the absolute level of additional parameters including cholesterol, bilirubin and albumin which have some evidence as individual tests. Further, this avoids dichotimizing an effusion into transudate / exudate based on a single cutoff value. If intereted see:
Multilevel Likelihood Ratios for Identifying Pleural Exudatie Pleural Effusions. Heffner JE, Sahn S and LK Brown. Chest 2002 (121) 6: 1916-1920.
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