Critical aspects of the 125I-C1q binding assay for detecting immune complexes in tumor patients

Clinical studies on C1q binding activity in tumor patients showed contradictory results and it was suggested that methodical problems might be partly responsible for those discrepancies. Therefore, precision of the C1q binding assay when testing tumor sera, influences of assay modifications on test results and possible interfering substances were investigated. Compared to aggregated human IgG and BSA: anti-BSA complexes as standards the C1q binding material in tumor sera was more unstable after freezing-thawing and distinctly more susceptible to methodical influences like testing with different 125I-C1q preparations or variations of PEG concentration and ionic strength of PEG solution. Addition of heparin and fibrinogen influenced the C1q binding of some tumor sera more than the C1q binding of standards and normal sera. Thus, instability and sensitivity to methodical influences of the C1q binding material have to be considered, when clinical studies using the C1q binding assay to detect immune complexes in tumor sera are interpreted.