Further, they are rendered ineffective by organic matter in wounds like blood and exudate and are thus not useful in open wounds.

They detected dried blood cells and possibly fibrin in a state of degradation from maturity, suggesting an established blood clot of more than a few days' age.

However, pleural fluid testing is not perfect, and the final decision about whether a fluid is a transudate or an exudate is based not on chemical analysis of the fluid, but on accurate diagnosis of the disease that produces the fluid.