- A ratio pleural fluid protein to serum protein of >0.5
- A ratio of pleural fluid lactate dehydrogenase (LDH) to serum LDH of >0.6
- A pleural fluid LDH > ⅔ the normal serum upper limit of LDH
An exudate will have at least one of these criteria, whereas a transudate will have none.
However, there's a catch. Apparently, this still mislabels 25% of transdates as exudates. The recommended solution: in cases where you still think it's likely to be a transudate (e.g. in cardiac failure), measure the serum and pleural fluid albumin levels. A difference of 12 g/L (1.2 g/dL) indicates a transudate, and you can now ignore the other criteria.
There is significant new research in this field, however (measuring fluid cholesterol levels, etc.), but for now, the above has stood the test of time.
Why is this all necessary? The best single thing to know about a pleural effusion is whether it is an exudate or a transudate. An exudate implies some local pathology in the lung or pleura (e.g. pneumonia), whereas a transudate implies some systemic factors are at play (e.g. altered aldosterone levels in cardiac failure, causing greater salt and water retention).
Thus, distinguishing between an exudate and a transudate helps us know where to look for its underlying cause. This is almost always the reason we tap a pleural effusion. Apart from when it is so massive as to cause significant dyspnoea from lung compression, a pleural effusion is of little direct clinical consequence. Rather, it's a clue to helping us find pathology elsewhere - nearby or far.