The following tests should be performed
- Pleural fluid protein, glucose and LDH and serum protein, glucose and LDH if available
- Pleural fluid pH if a specimen in a blood gas syringe is available. The sample cannot be analysed if it is not suitable for analysis on the blood gas analyser
Light’s Criteria for a transudate(1)
The ratio of pleural fluid protein to serum protein is less than 0.5
The ratio of pleural fluid LDH to plasma LDH less than 0.6
The pleural fluid LDH is less than 2/3 of the upper reference limit
Although these criteria have been re-evaluated there is no clear cut case for using anything other than Light’s criteria(2).
A pH < 7.3 is seen with emphysema, tuberculosis, malignancy, collagen vascular disease or oesophageal rupture.
Glucose < 2.2 mmol/L is associated with an emphysema, rheumatoid arthritis, tuberculosis or malignancy.
Other tests may be useful(3)
An exudates is more often associated than a transudate with:
A cholesterol level > 11.7 mmol/L
A pleural / serum Bilirubin ratio <0.6
A pleural – serum albumin gradient of < 12 g/L