CONCURRENT LABEL METHOD WITH IN-111 AND CR-51 ALLOWS ACCURATE EVALUATION OF PLATELET VIABILITY OF STORED PLATELET CONCENTRATES

The precision and reproducibility of In-111 and Cr-51 platelet radiola bel 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 in vestigate the effects of PC storage age, different methods of red cell (RBC) and white cell (WBC) contamination correction, and label elutio n correction on the results of In-111 and Cr-51 kinetic studies.In-111 and Cr-51 platelet survival curves from the same PC, even when uncorr ected for elution and RBC contamination, exhibited excellent correlati on, irrespective of the age of the concentrate and its viability. Howe ver, slightly higher, but statistically significant, post-infusion per cent recoveries with Cr-51 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 Cr-51 than for In-111. Wit h determination of RBC/WBC activity by centrifugation/density separati on, RBC/WBC fractions from the injectate were found to contain 12.6 +/ - 3.8% v 7.1 +/- 3-6% of total Cr-51 and In-111 activity, respectively , in 20 studies. In addition, there was a significantly higher In-111 activity in plasma immediately post-infusion than with Cr-51, 5.2 +/- 1.3% v 2.8 +/- 1-6%, respectively, suggesting more label elution or ca rryover. After correction for the activity of RBC/WBC and for elution or carryover, essentially identical Cr-51/In-111 platelet survival cur ves were found. In 31 stored PC studies, the absolute average differen ce between Cr-51 and In-111 per cent recoveries was only 4 +/- 3% in a group of donors whose platelet recoveries ranged from 10% to 80%. Sim ilarly, the average difference between Cr-51 and In-111 survival was o nly 8 +/- 4 h within a range of survivals from 40 to 220 h. In conclus ion, after correction for elution and contaminating RBC/WBC binding, t hese studies show that Cr-51 and In-111 may be used interchangeably fo r labelling of stored PC, and that small differences between test and control platelets could be reliably detected using concurrent labellin g with simultaneous infusion.