Quantitative measurement of properdin in normal human serum by electroimmunoassay and single radial immunodiffusion

Properdin in normal serum was measured by electroimmunoassay (EIA) and single radial immunodiffusion (SRID). Fresh sera gave much lower properdin values in SRID in gels containing Mg2+ ions. Storage of sera at 4 degrees C resulted in a gradual increase of the properdin values measured by SRID but not of those of the EIA. With 10 mM of EDTA in the gels no difference between the properdin values obtained by the different methods was found. Evidence is presented that immunodiffusion values of properdin might be affected by precipitation of a C3-properdin complex in gels containing Mg2+ ions after the activation of the properdin system by agarose.