Effect of aging of serum on quantitation of complement component C3.

Concentrations of the third component of complement (C3, f31C globulin) in serum may vary considerably in patients with certain diseases. Its accurate measurement is therefore important medically. For some time it has been recognized that the C3 in serum stored at room temperature breaks down into two proteins, f31A and a2D. Monospecific antisera have been made to the three antigenic determinants of C3, designated A, B, and D. An anti-C3 supplied commercially is monospecific for the A determinant, which is shared by C3 and its breakdown product $1A. With C3 breakdown and formation of fl1A, depletion of antibody to the A determinant is increased to 1.6 times the depletion by native C3. Therefore, the previous history of a serum can affect the “apparent” concentration of C3, with values spuriously increased to a variable degree, in serum that has been stored. The effect of time, temperature, and conditions of storage on this increase are discussed.