Which diagnostic feature is characteristic of exudative effusions?

Exudative effusions are characterized by a high protein content. This is due to the presence of cellular debris, inflammatory proteins, and other substances that leak into the pleural space as a result of increased vascular permeability associated with inflammation or malignancy. The high protein concentration distinguishes exudative effusions from transudative effusions, which typically have lower protein content due to mechanisms such as hydrostatic imbalance or osmoregulation rather than inflammation.

In addition to high protein levels, exudative effusions often exhibit other features such as higher levels of lactate dehydrogenase (LDH) and the presence of inflammatory cells. These characteristics reflect the underlying pathological process, distinguishing exudates caused by conditions like infections, malignancies, or autoimmune diseases from transudative effusions caused by systemic conditions such as heart failure or cirrhosis.

The other features listed do not define exudative effusions accurately: low cellularity characterizes more benign processes, fluid clarity may vary in both types of effusions, and low lipid content is not a distinguishing feature of exudates.