The stratus automated immunofluorometric assay system: a modification for measuring urinary human choriogonadotropin.

The fluorometric enzyme immunoassays that can be run on the automated Stratus analyzer (American Dade, Miami, FL) are based on monoclonal antibodies coated to glass-fiber paper tabs. The available assays are specifically designed for quantification of analytes in plasma and serum, but not in urine. For example, eight urine samples with hCG concentrations <2.5 mt.units/L according to the Tandem-E immunoenzymometric assay for urinary intact hCG (Hybritech Europe SA, Liege, Belgium) had apparent hCG concentrations ranging between 9.3 and 16 mt. unitsfL when measured with the Stratus. To further investigate this observation, we processed 35 urine samples for intact hCG with the Stratus (y) and compared these with results with the Tandem-E (x). Regression analysis between the two “sandwich”-type assays revealed the following equation: y = 1.45x + 9.20 mt.unita/L, S = 7.0, = 55 mt. unitsiL, r = 0.987 (Figure 1A). Apart from possible differences in specificities between the two hCG assays compared, we suspected a matrix effect with the Stratus. To test this, we treated the glass-fiber paper tabs of the hCG assay with 50 L of Stratus sample diluent (i.e., processed human serum included in the kit), then assayed 38 urine samples for hCG content with these pretreated tabs on the Stratus [Stratus(+)]. Regression analysis of these results yielded the following: Stratus(+)