Immunologic tests of value in diagnosis. 2. Complement.

Laboratory tests are available to assess the function of the complement pathway and to measure levels of individual complement components. The pattern of complement abnormalities is often helpful in suggesting diagnostic possibilities. For example, when when total hemolytic complement, C3, and C4 are all decreased, one of the rheumatoid diseases is likely. In addition, complement levels in spinal and synovial fluid may provide helpful diagnostic clues.