Sodium Chromate 51

The chromium in this agent is present as the dianionic chromate ion in which form it appears to bind to the red blood cell in two steps, initially by a rapid but reversible attachment to the cell membrane followed by a slower nearly irreversible binding to intracellular hemoglobin and reduction to the anionic state. It has been suggested that the slow rate of uptake is dependent on the rate at which chromate can penetrate the cell membrane. Binding is maintained until the red blood cells are sequestered by the spleen or until elution of the chromium occurs into the plasma. The chromium is then readily excreted mainly in the urine. Once liberated by elution or erythrocyte senescence, chromium 51 is not available for relabeling of red cells.

In normal individuals the erythrocyte survival half-time T ½  as measured by the chromium 51 "random labeling" technique, generally ranges between 25 and 35 days. This apparent short survival time, when compared to the 120 day life span of the red blood cells, is due to the elution of chromium from the cells and to cell damage that probably occurs during the process of withdrawing them from the body and labeling. Subnormal T ½ may be indicative of blood toss, sequestration of red blood cells by the  spleen, or shortened cell viability, as occurs In hemolytic anemia