A rapid quantitative capillary tube enzyme immunoassay for human chorionic gonadotropin in urine

A quantitative capillary tube enzyme immunoassay (CTEIA) method for the determination of human urinary chorionic gonadotropin (hCG) has been developed. The method utilizes an antibody-coated capillary tube through which the test fluid is passed and a urease-labelled second antibody in an immunometric format. Any hCG in the test solution is ‘captured’ by the immobilized antibody which is hybridoma derived and specific for the β-subunit of hCG. The second hCG-specific antibody, conjugated to the enzyme urease, is used to detect the captured hCG on the internal surface of the capillary tube. The amount of urease bound to the surface is determined by the introduction of a substrate solution containing urea and the pH indicator bromothymol blue. The rate of colour change, from yellow to blue, caused by the release of ammonia from urea by urease, is determined in a spectrophotometer using a cell holder adapted to accommodate capillary tubes. The initial rate of absorbance change is directly proportional to the concentration of hCG in the sample in the range 0–100 mIU/ml. The test can detect concentrations of hCG as low as 10 mIU/ml in a total elapsed time of 5 min.