Endocrine Abstracts (2002) 3 P143

Choice of calcitonin assay

M Busbridge, A Donaldson, PA Hill & RS Chapman

Clinical Chemistry, Hammersmith Hospital, London, UK.

Calcitonin assays remain a key requirement for the diagnosis and monitoring of medullary thyroid carcinoma (MTC). Until recently calcitonin assays were mainly competitive radioimmunoassays (RIA) but there are now more specific immunometric assays (IMA) available. Cost and staffing pressures are emcouraging routine clinical chemistry laboratories to perform easier kit methodologies rather than refer samples to specialist centres. However, endocrine tumours are known to secrete a variety of hormonal forms and it remains a subject of debate whether the use of highly specific IMA is appropriate.

In this study samples from known MTC patients with high circulating calcitonin were subject to HPLC separation and assay by an 'in house' RIA and a commercial chemiluminescent IMA (Nicholls Advantage). The study demonstrated variability between patients in the immunoreactive forms which are separated by HPLC and, although both assays recognised intact human calcitonin, other molecular forms occurred to a variable extent according to the two methods. In one patient with advanced metastatic disease a high molecular weight form, probably dimeric calcitonin, was shown to be at a higher concentration than intact calcitonin by the IMA but the reverse was shown for the RIA.

This study demonstrates that MTC's may produce a variety of forms of immunoreactive calcitonin and that there is patient to patient variability. The immunoassays either RIA or IMA detect these forms to different extents suggesting that extreme care is required in selection of an assay and interpretation of results to ensure detection of MTC.

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