C1 (C1q) PROTEIN

CLINICAL UTILITY:

C1, the first component of the complement system, is a trimolecular complex consisting of C1q, C1r and C1s. Measuring the level of C1q in a patient’s serum is one means of differentiating between hereditary angioedema (HAE) and acquired angioedema. In HAE there is a genetic deficiency in the control protein, C1 esterase inhibitor, which normally blocks the enzyme activity of C1r and C1s.

Because the inactivator is deficient, the downstream early complement components of the classical pathway are activated by C1r and C1s. Thus, in HAE, serum levels of C4 tend to be very low, but the level of C1q protein is normal. (See C1 esterase inhibitor section.) The acquired form of angioedema with low C1 esterase inhibitor levels is characterized by immune complex activation with very low levels of C1q protein and decreases in C4 and C1 esterase inhibitor. In some cases lymphomas are responsible for acquired C1 esterase inhibitor deficiency.

METHOD DESCRIPTION:

Single radial immunodiffusion (RID) is the method used to quantitate the serum level of C1q protein.

REFERENCE RANGE:

12-25 mg/dl.

SPECIMEN REQUIREMENTS:

0.5 ml serum. Send on dry ice.