What complement components help in diagnosis?
• Complement components ( C3 & C4 ): measured by nephelometry. Low levels of both C3 and C4 indicate classic complement pathway activation usually by immune complexes. Alternative complement pathway activation is indicated by low levels of C3 with normal C4. A normal C3 but low C4 suggests heterozygous C4 deficiency or low level complement pathway activation (C1 INH deficiency, etc). Medicare cost for C3/C4 test is $14.81 each.
• CH50 (total hemolytic complement assay): the test name describes how the measurement is carried out, by assessing the ability of serum complement to hemolyze sheep RBCs that have been sensitized with rabbit IgM. Only in the presence of all nine complement components will antibody activate complement, form the membrane attack complex, and lead to cell lysis. As such, it is a good screen for complement deficiency. A CH50 level of 0 or “unmeasurable” is suggestive of a hereditary homozygous complement deficiency and should prompt testing for individual complement levels (C2 is the most common homozygous deficiency, followed by C4 and C1q). It is not a good disease activity marker as in active inflammation its level can be either low, normal, or high, reflecting the end result of balance between production and consumption of the complement components.
