998 CLINICAL CHEMISTRY, Vol.19,No.9, 1973 Liquid-ScintillationCounting of [57Co]-Application to the Radioassay of Vitamin B12

Cobalt-57 can be measured in a liquid scintillation counting system. We describe the calibration of such a system for this gamma emitter. The counting pro-cedure has been used in the radioassay of vitamin B12. Results of assays for which counting of bound radioisotope was done in a gamma scintillation spec-trometer and in a liquid scintillation counter show that either counting system can be used for the measurement of [57Co]. Additional Keyphrases: counting of gamma emitters #{149} radio/abe/ed vitamin 8,2 #{149}diagnosis of the anemias The liquid scintillation counting of the gamma-emitting radioisotopes of iodine, [125!] and [131!], has been described (1-4). Although these radioisotopes can be readily detected in well counters with solid sodium iodide crystal detectors, the ability to count gamma emitters by liquid scintillation counting of-fers some advantages. The liquid scintillation counting of gamma emit-ters provides additional capability for a laboratory equipped with only a liquid scintillation counter. In the case of a laboratory contemplating an initial pur-chase of counting equipment, the acquisition of a liquid scintillation counter brings into the laboratory a single system for the measurement of a large num-ber of radioisotopes, both beta and gamma emitters. In the radioassay of vitamin B12, the tracer sub-stance is vitamin B12, cyanocobalamin, labeled with the-y-emitting isotope of cobalt, [57Co]. Knowledge of the vitamin B12 concentration in a serum or plas-ma has been shown to be essential in the diagnosis of a variety of anemic conditions (5). The full signifi-cance of a vitamin B12 deficiency is at times ob-tained only by a concomitant assessment of the folic acid concentration in a patient’s serum (6). In ra