Reduced-volume radioimmunoassays for parathyrin and calcitonin in serum, for use in pediatric and small-animal studies.

Radioimmunoassays for circulating parathyrin and calcitonin used in most laboratories require 0.6--0.7 ml of serum or plasma, or more, for assay of one hormone. Such volumes are often difficult to obtain safely or repeatedly from pediatric patients or in small-animal studies. We modified our existing procedures to markedly decrease sample and reagent requirements. All reagent volumes were proportionately reduced to one-fourth the usual (from 500 microliter final incubation volume to 125 microliter), and serum or plasma volumes from 200, 100, and 50 microliter to 50, 25, and 10 microliter. We used smaller (10.3 X 50 mm) tubes and slightly modified the separation with charcoal. Results, validated by studies in rats, showed the sensitivity to match that of the usual assays; inter- and intra-assay variance was less than 20%. Simultaneous regular- and reduced-volume assays of parathyrin in sera from 19 children gave almost identical results (r = 0.9987). Both hormones can be assayed in less than 400 microliter of serum.