The British Thoracic Society (Hooper C, Lee YCG, Maskell N. Investigation of a unilateral pleural effusion in adults. Thorax 2010; 65: Supplement 2, ii4-ii17), recommends that Light’s criteria are used for patients not receiving (or recently receiving) diuretics. Serum and effusion total protein and LDH should be measured. Fluid is an exudate if any of the following criteria are met:
- Fluid:serum protein ratio >0.5
- Fluid LDH >67% of the upper limit of normal for serum (i.e. >166 U/L in our laboratory)
- Fluid:serum LDH ratio >0.6
An alternative is to use the serum-effusion albumin gradient ( SEAG). Serum albumin and effusion albumin are measured and then the effusion albumin value is subtracted from the serum albumin value.
- SEAG <12g/L is an exudate,
- SEAG >12g/L is a transudate
This has been found to be useful in classifying patients on diuretics or who have recently been on diuretics.
Measure the fluid total protein:
- Levels below 25-30 g/L classed as “transudates” (cirrhosis, CCF, nephrotic)
- Levels above 25-30 g/L classed as “exudates” (inflammation or infection)
An alternative to the exudate-transudate concept is Serum Ascites Albumin Gradient (SAAG). Serum albumin and ascitic fluid albumin are measured.
- SAAG = <11 g/L (low albumin gradient) associated with inflammation and infection
- SAAG = > 11 g/L (high albumin gradient) associated with portal hypertension