The elution of 99-Tcm from red cells and its effect on red-cell volume measurement

Technetium-labelled erythrocytes provide a satisfactory agent for the measurement of red-cell volume with the advantages that there is a low radiation dose and repeated measurements can be readily carried out. A method of labelling red cells using (99)Tc(m)O(4) (-) with a small amount of stannous chloride has been evaluated in 20 patients, and compared with measurements of red-cell volume with (51)Cr-labelled erythrocytes carried out simultaneously in the same subjects. No significant differences between the results by either method occurred if the period of observation was limited to 20 minutes after the injection of labelled cells. Thereafter, elution of Tc from the cells made the determination of red-cell volume with (99)Tc(m) less reliable. Accordingly, the use of (99)Tc(m) as the label for red-cell volume measurements will result in errors if mixing of the erythrocyte pool is delayed, unless an elution correction factor is applied.