Light's criteria

Light's criteria are a simple set of laboratory rules that act like a sorting gate at the very start of working up a pleural effusion: do the test, and the fluid is funneled into one of two streams — transudate or exudate — each leading to a different list of causes.

The rules compare the chemistry of the pleural fluid with the same chemistry in the blood. The fluid is called an exudate if any one of three conditions is met: the ratio of fluid protein to blood protein is above about 0.5; the ratio of fluid LDH (an enzyme released by injured cells) to blood LDH is above about 0.6; or the fluid LDH is above roughly two-thirds of the upper normal blood value. If none of the three is met, the fluid is a transudate.

The criteria are deliberately tuned to almost never miss an exudate — which is the more dangerous category to overlook — but at the cost of occasionally mislabeling a transudate as exudative, especially in patients already on diuretics. When the clinical picture and the result disagree, clinicians look at additional measures rather than trusting the label blindly.