DETECTION OF RICIN BY COLORIMETRIC AND CHEMILUMINESCENCE ELISA

A highly sensitive and specific ELISA was developed to detect ricin in biological fluids. The assay utilizes an affinity-purified goat polyc lonal antibody to adsorb ricin from solution. The same antibody (bioti nylated) is then used to form a sandwich, and avidin-linked alkaline p hosphatase allows color development and measurement of optical density at 405 nm. Our routine assay uses a standard curve over the range of 0-10 ng/ml ricin, with accurate quantitation below 1 ng/ml (100 pg/wel l) in assay buffer as well as in a 1:10 dilution of human urine or 1:5 0 dilution of human serum spiked with ricin. Ricin measured in spiked samples demonstrated accuracy typically within 5% of the expected valu e in all matrices. The coefficient of variation ranged from 3-10% at 1 0 ng/ml to 8-25% at 2.5 ng/ml. Two variations on the routine assay wer e also investigated. First, lengthened incubation times and additional time for color development allowed accurate quantitation in serum dil utions as low as 1:2. Second, increased concentrations of biotinylated antibody and avidin-linked enzyme from 1:250 to 1:70 enhanced the sen sitivity of the assay 10-fold, achieving a detection limit of at least 100 pg/ml (10 pg/well). The assay was also configured to a format bas ed upon chemiluminescence, which allowed quantitation in the 0.1-1 ng/ ml range, but was subject to slightly greater variability than the col orimetric assay.