Development of a highly sensitive enzyme immunoassay for human calcitonin using solid phase coupled with multiple antibodies.

We have developed a highly sensitive chemiluminescent enzyme immunoassay for human calcitonin using three different mouse monoclonal antibodies that recognize the N-terminal, C-terminal and central portions of a human calcitonin molecule. The assay signal in a two-step sandwich enzyme immunoassay for human calcitonin using a solid phase coupled with a mixture of monoclonal antibodies. CT08 and OCT1, was 3.7-fold higher than when using either or both solid phases coupled with CT08 or OCT1, respectively. This enhancement is the result of improved avidity of immobilized antibodies and greater stability of the complex of immobilized antibodies and calcitonin in the first reaction, which resulted in greater reactivity of the immunocomplex with alkaline phosphatase-conjugate in the second reaction. The present assay showed a linear response up to 2.5 micrograms/L of human calcitonin and a high specificity for human calcitonin, but not for rat calcitonin, human calcitonin gene-related peptide and rat calcitonin gene-related peptide. The detection limit of human calcitonin was estimated to be 0.29 ng/L at zero (assay blank) + 3 SD. Interbatch coefficients of variation ranged from 2.2-26.7%.