Immunoradiometric assay for human serum amyloid P component.

Human serum amyloid P component (SAP) is of increasing interest for its possible pathogenic role in amyloidosis and Alzheimer's disease, and as a therapeutic target in these conditions. We have developed and validated a robust and reproducible immunoradiometric assay (IRMA) for human SAP in serum, plasma and cerebrospinal fluid, and characterized the notable stability of human SAP immunoreactivity during storage of undiluted serum at 4°C and 37°C as well as frozen at -30°C. SAP values were also stable after repeated freeze thawing of highly diluted serum samples. The 100 fold dynamic range of the assay, 0.5-50 μg/L, encompassed all values seen in blood and cerebrospinal fluid, when tested at suitable dilutions, from both normal healthy individuals and patients, including subjects receiving the SAP-depleting drug, CPHPC. Furthermore by comparing the IRMA values in the presence and absence of calcium, the new assay revealed interference due to the binding of CPHPC by SAP, which was markedly enhanced in heparinized plasma. It is therefore essential that SAP assays in samples from patients on CPHPC be conducted in the absence of free calcium, in order to completely abrogate interference and determine the actual total SAP concentration. Estimates by the IRMA of SAP concentration in 49 serum samples from amyloidosis patients corresponded closely with those obtained by the established standard electro-immunoassay method and by a newly developed commercial ELISA kit (Hycult Biotechnology).