transudate

Imagine pleural fluid that has simply been squeezed out of normal blood vessels, the way water seeps from an overfilled sponge. A transudate is thin, clear, watery fluid that accumulates not because the pleura is sick, but because the pressures balancing fluid across vessel walls have gone out of kilter.

Two main forces drive it. Raised pressure inside the veins and capillaries — typically from heart failure — pushes more fluid into the pleural space. Low levels of albumin, the blood protein that normally holds water in the vessels, let fluid escape more easily, as in liver cirrhosis or the protein loss of nephrotic syndrome. Because the vessel walls themselves stay intact, the leaked fluid is low in protein and cells.

Recognizing a transudate matters because it redirects the search away from the lung and pleura and toward the heart, liver or kidney. Treatment targets that systemic problem — diuretics, managing heart or liver failure — rather than the chest itself. The everyday distinction from a protein-rich exudate is made at the bedside laboratory using Light's criteria.