Colloid osmotic pressure (COP) or oncotic pressure is a measure of water attracting force, which is due to important proteins such as albumin within the body. This is often affected with significant blood loss, loss of protein (either from lack of production or loss via the gastrointestinal tract, liver or kidneys), vascular permeability, or following massive fluid therapy. Rapid measurement in critically ill patients is important to help determine the best method of treatment (e.g., the use of crystalloids versus colloids). In this podcast, VetGirl evaluates the accuracy of testing COP on whole blood as compared to plasma.