Osmotic pressure of the serum proteins.

The osmotic pressure of the serum proteins (colloid osmotic pressure [COP] or "oncotic" pressure) is only one of the four Starling forces (plus the capillary permeability coefficient) which affect the net filtration of fluid from the capillaries. The COP will vary with the concentration of total serum proteins, but more so with the specific pattern or composition of the protein components, especially albumin. The use of formulas utilizing total protein (or albumin/globulin) to calculate COP is not warranted. COP should be determined; this is easy at the present time with the advent of a compact commercial instrument. Generalized or localized edema (e.g., pulmonary edema or ascites) has been associated with low serum albumin and low COP values. This is not always so since cases of analbuminemia do not necessarily exhibit edema. The study of COP is warranted but precautions are necessary in proper interpretation of the causes of "edema,"--the Starling forces and hemodynamic factors, capillary permeability, lymphatic return, etc., are all involved in the phenomenon.