Use of an Immersible Scintillation Counter in the Urinary Vitamin B 12 Excretion Test.

The urinary excretion test for vitamin B12 absorption devised by Schilling, Clatanoff, and Korst (1955) and Callender and Evans, in the same year, is probably the most commonly used method in the assessment of this function of the gut, and particularly in the diagnosis of pernicious anaemia. The reproducibility of the results obtained in this manner and the convenience of the procedure compared with the others employed, e.g. blood radioactivity after a given dose of radioactive cobalt-labelled B12 (Booth and Mollin, 1956), make the urinary excretion method the choice of many laboratories. The urine samples obtained containing labelled B12 may be counted in a 'ring' type Geiger-Mueller counter, or by activated crystal scintillation counters in well, annular, or end-on settings. In this study the EKCO type N691 general purpose scintillation detector, fitted with an N609, crystal was used; it is of a completely immersible design and appeared to be potentially suitable for the counting of large samples of radioactive liquids. Using the technique described, the value of this counter in the Schilling test was assessed, after an initial evaluation of the accuracy of the method. The apparatus was tested in view of its capability of producing adequately high counts in a short time. METHOD A standard solution of cobalt58-labelled vitamin B12 was prepared by dissolving the contents of an ampoule containing 1-0 psc. of this material (Radiobiological Laboratory, Amersham) in 1,000 ml. water. Dilutions of this standard solution were prepared and 10 ml. aliquots of each dilution counted in an annular type counter (EKCO N550A with an N533A crystal) and N530 scaler, as described by Berlyne, Liversedge, and Emery (1957), for 3,000 seconds and 100 seconds respectively for each dilution. At the same time 500 ml. samples of the same dilutions were prepared and counted using the type N691 detector in conjunction with a type N640 amplifier and N530 scaler for 100 seconds. The solutions were contained in 600 ml. Pyrex beakers shielded by 1 in. of lead brick on a cast iron base, and the detector immersed in the fluid to the level at which the crystal is fitted. A simple vertical mounting for the cylindrical detector was arranged to facilitate its removal, washing (in running tap water), and re-immersion. Background counts were performed using 500 ml. tap water in the same type of beaker. The results of these studies are shown in Table I.