Clinical validation – the final step in translational medicine! From CD34 enumeration to probability estimation of autograft quality

Flow cytometric enumeration of CD34+ hematopoietic stem and progenitor cells (HPC) is widely used for evaluation of graft adequacy of peripheral blood stem cell grafts, and is also useful to plan the apheresis session necessary to obtain these grafts. A state-of-the-art method to enumerate CD34+ cells has been developed that makes use of a multiparameter definition of HPC based on their light scatter characteristics, dim expression of CD45+ and utilities fluorescent counting beads. This approach allows the determination of the absolute CD34+ cell count directly from the cytometer. This method can be extended with a viability stain and additional markers for further immunophenotypic characterisation of CD34+ cells, and has been successfully implemented in multicenter trials (barnett et al., 1998:. The lower threshold of a safe HPC graft is still poorly defined). In this context it is important to recall that collection, processing, cryopreservation and thawing all influence the viability of the re-infused HPC. Using such a standardised assay, it should be possible to define more accurately the lower threshold of a safe HPC graft in terms of short and long-term hematopoietic reconstitution.