BACKGROUND: Extracellular potassium concentration [K+e] increases with duration of red blood cell storage. Sometimes red blood cells (RBC) are washed before transfusion to infants to reduce [K+e] of these components. AABB standards permit storage of washed RBCs at 4°C for 24 hours. The [K+e] of washed RBCs during storage is not known. Experiments were performed to provide those data.

STUDY DESIGN AND METHODS: One day after outdating, 26 RBC units were washed without irradiation or before or after irradiation (25 Gy), and [K+e] was measured for 24 hours. [K+e] was measured also immediately before transfusion of 29 nonoutdated irradiated and washed RBC units.

RESULTS: After washing, [K+e] increased in a time-dependent fashion. [K+e] increased more rapidly in preparations of irradiated than nonirradiated RBCs. [K+e] was less after washing after irradiation (1.6 ± 0.3, 2.4 ± 0.3, 3.0 ± 0.3, 3.6 ± 0.3. 4.2 ± 0.4, 5.3 ± 0.5, 8.6 ± 1.0, and 14.3 ± 1.3 mEq/L at 0, 1, 2, 3, 4, 6, 12, and 24 hr; mean ± SD) than washing before irradiation (p < 0.001). The increase in [K+e] during the first 6 hours after washing after irradiation was linear (0.61 ± 0.08 mEq K+/L/hr). The probability of a unit of RBCs having a [K+e] greater than 5 mEq per L is 0.0 to 0.2 percent 3 hours after washing irradiated RBCs and 0.0 to 1.1 percent 6 hours after washing nonirradiated RBCs.

CONCLUSIONS: [K+e] increases after washing irradiated and nonirradiated packed RBCs. After irradiation and washing, the [K+e] for the initial 6 hours can be predicted from the [K+e] immediately after washing. There is a low probability that a unit of RBCs would have a [K+e] greater than 5 mEq per L during 6 hours of storage at 4°C after washing if the cells are not irradiated and for 3 hours if the cells are irradiated.