Radioactive Sodium Chromate and the Measurement of Red Cell and Plasma Volume in Man

Hexavalent chromium, in the form of sodium chromate, has been shown to label red cells,¹ in contrast to the trivalent ion, which, as chromic chloride, becomes bound to plasma proteins.² These two forms of the radioactive isotope Cr⁵¹ have been used to measure, respectively, red cell³ and plasma volumes.⁴ Recently the labelling of erythrocytes for clinical use has been simplified by the introduction of the ascorbic acid technique.⁵ In this method the labelled red cells are injected into the patient with their plasma, which contains the radiochromium not taken up by the erythrocytes. This excess is treated with ascorbic acid and thus prevented from further penetrating the red cell membrane. Although this technique was introduced originally for its advantages in the labelling of red cells for red cell volume or survival determinations, it was also hoped that the reduced CrO₄ present in the plasma might act similarly to CrCl₃ and