WBC reduction in cryopreserved RBC units

BACKGROUND: WBC reduction of blood components by filtration is widely practiced to decrease the incidence of alloimmunization. Freezing RBCs reduces the WBC load but is insufficient to achieve the currently recommended US limit of 5 × 10⁶ cells per unit. STUDY DESIGN AND METHODS: Blood units were WBC reduced by filtration or by buffy-coat (BC) removal and then frozen in the presence of a high-glycerol concentration. The count of residual WBCs was determined by flow cytometry after deglycerolization. RESULTS: Without WBC reduction, the total number of WBCs present after freezing and thawing was 11.5 ± 9.2 × 10⁶ WBCs per unit (n = 18). Particulate residues from monocytes and neutrophils that were detected in the remaining cell populations were positive for CD66b, CD3, CD14, and CD41. Removal of 40 mL of BC at the time of blood collection lowered the number of WBCs after freezing and deglycerolization to 1.9 ± 1.20 × 10⁶ per unit (n = 11). Similar results were obtained when only 20 mL of BC was removed using a modified blood-bag design. Unfiltered RBC units that were stored for 15 days at 4°C after BC removal contained fewer than 5 × 10⁶ WBCs after deglycerolization. Units WBC reduced by filtration before freezing had no detectable WBCs after thawing and washing (n = 14) and did not contain particulate residues. Filtration after deglycerolization was effective in reducing the WBC count below 10⁶, although some debris was still present. CONCLUSION: RBC freezing alone will not reduce residual counts to recommended levels. However, initial removal of BC can provide an economical alternative to WBC filtration for cryopreserved units. Units that were not WBC reduced before freezing can be filtered after deglycerolization when needed.