Concurrent label method with 111In and 51Cr allows accurate evaluation of platelet viability of stored platelet concentrates.

The precision and reproducibility of 111In and 51Cr platelet radiolabel agents for in vivo kinetic studies of stored platelet concentrates (PC) were investigated. The objective was to develop a precise method with concurrent labelling of two platelet populations using different isotopes, which would allow identification of small differences in in vivo platelet quality. Identical labelling procedures were used to investigate the effects of PC storage age, different methods of red cell (RBC) and white cell (WBC) contamination correction, and label elution correction on the results of 111In and 51Cr kinetic studies. 111In and 51Cr platelet survival curves from the same PC, even when uncorrected for elution and RBC contamination, exhibited excellent correlation, irrespective of the age of the concentrate and its viability. However, slightly higher, but statistically significant, post-infusion per cent recoveries with 51Cr labelled platelets were found. Two factors were identified as the cause for this difference. There was a higher affinity of contaminating RBC/WBC in PC for 51Cr than for 111In. With determination of RBC/WBC activity by centrifugation/density separation, RBC/WBC fractions from the injectate were found to contain 12.6 +/- 3.8% v 7.1 +/- 3.6% of total 51Cr and 111In activity, respectively, in 20 studies. In addition, there was a significantly higher 111In activity in plasma immediately post-infusion than with 51Cr, 5.2 +/- 1.3% v 2.8 +/- 1.6%, respectively, suggesting more label elution or carryover. After correction for the activity of RBC/WBC and for elution or carryover, essentially identical 51Cr/111In platelet survival curves were found. In 31 stored PC studies, the absolute average difference between 51Cr and 111In per cent recoveries was only 4 +/- 3% in a group of donors whose platelet recoveries ranged from 10% to 80%. Similarly, the average difference between 51Cr and 111In survival was only 8 +/- 4 h within a range of survivals from 40 to 220 h. In conclusion, after correction for elution and contaminating RBC/WBC binding, these studies show that 51Cr and 111In may be used interchangeably for labelling of stored PC, and that small differences between test and control platelets could be reliably detected using concurrent labelling with simultaneous infusion.