Comparative study of circulating immune complexes quantity detection by three assays--CIF-ELISA, C1q-ELISA and anti-C3 ELISA.

The assessment of the soluble immune complexes (IC) in human sera is traditionally performed by the C1q binding assay. In the present study, a novel method for the quantity of immune complexes was reported. The methodology was based on measuring their deposition on solid-phase C3 binding glycoprotein (CIF), using an enzyme-linked immunosorbent assay. We also used ELISA that employed anti-C3 antibodies to determined the quantity of immune complexes. The three assays were evaluated for their performance characteristics on the same specially prepared samples: 55 normal sera, 99 sera from RA, 88 sera from SLE, and 27 sera from PSS. The results were compared by reference to a common standard-heat aggregated IgG that possesses many activities of immune complexes. Three of the tests used displayed almost the same specificity (over 95%), while their relative sensitivity varied depending on the disease sera tested. The sensitivity of the assays used was recorded highest for C1q ELISA-28.97% of positive sera, followed by CIF-ELISA-19.63% and lowest for anti-C3 ELISA-17.29%. A well-expressed correlation was found between CIF-ELISA and anti-C3 ELISA data (r=0.42), and a week correlation was noted when comparing CIF-ELISA and C1q ELISA IC levels detected (r=0.28). When the correlation coefficients were calculated individually for each disease category, they were clearly different, and that reflected indirectly in different sensitivities of the test for various disease categories. We also found that the results from the simultaneous performance of the tests demonstrated low percentage positive results when three or two assays were used. This is most probably due to the different assay abilities to detect IC with different sizes and composition, which shows that a small part of IC in the tested sera can be detected simultaneously by more than one assay. On the basis of the results obtained, we concluded that optimal screening for IC could be achieved by parallel application of several different methods.