Optimization of a sandwich sol particle immunoassay for human chorionic gonadotrophin

Colloidal gold particles coated with rabbit anti-HCG (Au-(anti-HCG)) were used in a spectrophotometric 'sandwich' SPIA for HCG. The effects of several reagent properties and assay variables were investigated in order to optimize the assay. Best results were obtained with 55 nm particles. Optimum assay conditions involved a sample incubation period of 2 h followed by aspiration of the sample and two washings. A suitable Au-(anti-HCG) concentration corresponded to a value of A1cm540nm = 2.0. The steepest dose-response curves were obtained with an Au-(anti-HCG) incubation period of 16 h at 37 degrees C. The test procedure was suitable for preparing dose-response curves for HCG in buffer, urine and serum. The practical measuring range for HCG dissolved in these media was 30-250 IU/l. The within-run coefficient of variation varied between approx. 4 and 12%. The detection limit for HCG (in buffer) was about 1 IU/l.