Is very fresh blood needed in preparing leukocyte-poor red blood cells by inverted centrifugation?

It is recommended that units selected for inverted centrifugal preparation of leukocyte-poor red blood cells be less than 24 hours old. The reason for this is unclear and the recommendation poses practical problems. We evaluated the effectiveness of inverted centrifugation in reducing the WBC:RBC ratio in 31 units stored up to six days at 4 C. Leukocyte removal was at least as effective with six-day stored units as with less than one day old, averaging 77 per cent reduction with 76 per cent recovery of red blood cells. No reduction in leukocytes occurred with six days storage alone. It appears to be unnecessary to restrict leukocyte removal by inverted centrifugation to units less than 24 hours old.