Abstract: A method and an apparatus are provided for determining T-cell repertoire recovery after allo-HSCT or identifying patients at high risk of infection. Combination of 5?-RACE PCR with deep sequencing was used to quantify TCR diversity in 33 individuals using a single oligonucleotide pair. Analysis of duplicate blood samples revealed highly reproducible detection of expanded TCR clonotypes. After 6 months, recipients of cord blood grafts without anti-thymocyte globulin therapy approximated the TCR diversity of healthy subjects, whereas recipients of T-cell-depleted peripheral blood stem cell grafts had a 28-fold and 14-fold lower CD4+ and CD8+ T-cell diversity, respectively. After 12 months, these differences had leveled out for the CD4+, but not the CD8+ T-cell compartment.