A sol particle agglutination assay for human chorionic gonadotrophin

Conjugates, prepared by adsorption of antibodies on colloidal gold particles, were used in a homogeneous sol particle immunoassay (SPIA) for human chorionic gonadotrophin (HCG). The technique is based on sol particle agglutination, resulting in colour reduction. Optimal results were obtained using buffered conjugates prepared from 50 nm particles. Addition of polyethylene glycol (PEG) 6 000 to the conjugates, up to 15 g/l, increased the agglutination rate considerably. The optimal PEG 6 000 concentration of the conjugate depended on the desired incubation time and measuring range. The influence of temperature on the agglutination was negligible in the temperature range between 4 and 45 degrees C. Higher conjugate concentrations resulted in steeper dose-response curves. However, the measuring range (between 62.5 and 2 000 IU/l HCG) and the detection limit (approx. 50 IU/l HCG) were about the same. The dose-response curves for HCG dissolved in buffer or in urine were almost identical and their reproducibility was satisfactory. In our experience, homogeneous SPIAs have a high practicability, are easy to automate and provide an interesting new tool for the measurement of a variety of analytes.