Comparison of poly- and monoclonal antibodies as labels in a two-site immunochemiluminometric assay for intact parathyroid hormone.

We have evaluated the potential of monoclonal antibodies in the development of a non-isotopic immunometric assay for intact human parathyroid hormone (PTH). The assay has been designed to utilise a chemiluminescent acridinium ester labelled anti-aminoterminal (anti-N) antibody and a solid-phase anti-carboxyterminal antibody in order to measure specifically the intact hormone. In this system the characteristics of the labelled antibody proved crucial to the performance of the assay. A low affinity monoclonal reagent yielded insufficient analytical sensitivity, while a higher affinity monoclonal reagent cross-reacted poorly with the intact molecule relative to the amino terminal PTH fragment to which it was raised. Neither antibody could therefore match the performance of an affinity-purified polyclonal anti-N PTH reagent. These results highlight the problems to be addressed in the selection of suitable reagents for immunometric assay development when specificity and sensitivity are crucial requirements.