We previously described a simplified quantitative hemolytic assay for classical pathway (CP) hemolytic function in serum that has been shown to correlate with the 50%
hemolytic complement (CH50) assay. In the present study, we used this assay to compare CP functions; plasma levels of C3, C4, and
C3dg; and ratios of
C3dg to C3 in healthy individuals and patients with
systemic lupus erythematosus (SLE) or
rheumatoid arthritis (RA) with different degrees of complement activation. A significant depression in CP function and levels of C4 and C3 and increased
C3dg levels and
C3dg/C3 ratios were observed in the SLE patients. In patients with RA, CP function was normal, whereas C3, C4, and
C3dg levels and the
C3dg/C3 ratio were elevated. The SLE results are compatible with systemic
complement consumption, whereas the RA data suggest an
acute-phase reaction with a normal C3 catabolic rate. To facilitate the handling of patient samples, we also developed a method to restore the hemolytic function of
EDTA-plasma by transferring it to
Veronal-buffered saline containing the
thrombin inhibitor
lepirudin. This process inhibits coagulation and enables complement activation, allowing a longer time lag between sample harvesting and testing. These results, combined with previous correlation studies, suggest that the CP hemolytic assay can effectively replace the CH50 assay for routine SLE differential diagnosis and monitoring of disease activity.