Labeled antibody-dye conjugate mixed with sample specimen and binds to the free drug present forming an antibody-antigen complex. This complex competes with immobilized antigen conjugate in the test zone preventing the formation of a red color band when the drug concentration in the specimen is above 500ng/ml. unbound dye conjugates bind to the reagent in the negative control zone and produces a red color band, demonstrating that the reagents and device are functioning correctly