Homogeneous determination of C-reactive protein in serum using liposome immune lysis assay (LILA)

Recently, we have developed an analytical system based on a complement dependent liposome immune lysis assay (LILA) to measure antibody against protein antigens. This paper describes the application of LILA to measure protein antigens by inhibition reaction. We choose human C-reactive protein (CRP), one of acute phase reactants, for a model of antigen determination. CRP antigen is coupled to the carboxyfluorescein entrapped multilamellar liposomes by using N-hydroxysuccinimidyl 3-(2-pyridyldithio) propionate and dithiothreitol, and a specific lysis of liposomes is achieved upon the addition of anti-CRP antibody in the presence of complement. Inhibition of this assay can be observed by the addition of competing amounts of CRP in human serum. This inhibition assay is simple, fast, highly sensitive, reproducible and homogeneous. The assay covers the ranges 10-500 micrograms/l. In an experiment using 50 samples, the results by LILA correlated well with those by single radial immunodiffusion and enzyme immunoassay (correlation coefficient: 0.99 and 0.93, respectively).