Linear relationship between lymphocyte counts in peripheral blood and buffy coat collected during extracorporeal photopheresis

BACKGROUND: Extracorporeal photopheresis (ECP) is commonly used to treat patients with graft-versus-host disease (GVHD) and lung transplant rejection (LTR) in our institution. The quantitative relationship between the number of white blood cells treated during ECP and the cell count in peripheral blood is unclear. STUDY DESIGN AND METHODS: Patients with GVHD and LTR receiving ECP with either UVAR XTS or CELLEX (Therakos) were prospectively recruited for this study. A complete cell count with differential was performed on preprocedural peripheral blood and samples from the collected buffy coats. Correlation analysis and linear regression were performed between cell counts in peripheral blood and buffy coat. Collection efficiency was compared between UVAR XTS and CELLEX. RESULTS: In all 52 patients, lymphocyte counts in buffy coat and peripheral blood showed strong correlation (r values were 0.85 and 0.983 for UVAR XTS and CELLEX, respectively; p < 0.001) with slopes of 2 and 5.1 for UVAR XTS and CELLEX, respectively (p < 0.001). The quantitative relationship remained robust in patients stratified by diagnoses. Monocytes also showed consistent correlation and linearity, but not neutrophils or combined white blood cells, red blood cells, or platelets. CELLEX enriched approximately twice as many lymphocytes and monocytes than UVAR XTS per procedure (p < 0.001). CONCLUSION: The preprocedural peripheral lymphocyte count can predict the number of lymphocytes within the buffy coat collected during ECP, which may justify the use of peripheral lymphocyte count as a surrogate for the cell dose treated per procedure. Peripheral monocyte counts may serve as an alternative. CELLEX is more efficient in collecting lymphocytes and monocytes than UVAR XTS under conditions tested.