The Binding of Thiocyanate to Albumin

Since the report of Crandall and Anderson (1) in 1934, the distribution of thiocyanate ion in the body has been used as a measure of extracellular fluid volume. All such work has been based on the procedure of administering a known amount of a thiocyanate, and after an interval, measuring the concentration of thiocyanate in the serum. The amount of thiocyanate injected, or more accurately, the thiocyanate remaining in the body at this time, is divided by the serum concentration, and the resulting number is termed the "thiocyanate space." The assumptions usually made, implicitly or explicitly, in this procedure are that thiocyanate is not metabolized and does not penetrate cells except red cells, that equilibrium exists between serum and extravascular thiocyanate, and that the thiocyanate concentration is essentially the same in the extravascular fluid as it is in the serum. The work reported in this paper' shows that the latter assumption is wrong and that the error resulting from its use is great. Methods of determining serum thiocyanate measure total thiocyanate since they depend on calibration curves made from known amounts of thiocyanate in serum (2) or can be shown to recover about 100%o of thiocyanate added to serum (3). If any thiocyanate is bound to a non-diffusible component of blood, such as protein, then the bound portion will not be free to diffuse out of the vascular compartment. Accordingly, the