Complement studies of sera and other biologic fluids.

The complement system consists of at least 15 proteins whose sequential activation, split products, and interaction with other plasma proteins and cells are important in inflammation. Measurement of complement is useful in many rheumatic and immunologic diseases. Most attention has focused on low levels usually due to immune complex disorders. A logical starting point is measurement of the total hemolytic complement, CH50; if this is low, one can see which component is involved. If only one component concentration is decreased, an inherited defect may be present; multiple low levels usually reflect an acquired process. Serial levels of CH50, C4, and C3 are particularly useful in monitoring patients with systemic lupus erythematosus and vasculitis. Complement may also be involved in effusions.