Biological activity of calcitonin and its assessment

Radioimmunoassay has been the method of choice for the routine determination of calcitonin (CT) in biological fluids, mainly due to its precision, sensitivity, and ability to handle large numbers of samples. However, spuriously high results are often obtained due to undefined cross-reactivities of polyclonal antisera to precursors, polymers, metabolites, or fragments of CT, or to other physiological and pathological variants of the hormone. In routine practice, radioimmunoassays of CT are being replaced by immunometric two-site 'sandwich' assays utilizing monoclonal antibodies against defined antigenic epitopes. Hormone activity is, however, accurately quantifiable by biological assays based on the exquisite hormonal sensitivity of isolated osteoclasts. The assays are highly specific and remarkably sensitive. Their use in routine laboratories is nevertheless limited only to specific situations.