Why must a unit of packed red blood cells be completed within 4 hours of starting the transfusion?

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The main concept is patient safety by limiting the time a transfused blood unit spends exposed to conditions that can compromise the product or patient. Once a unit of packed red blood cells starts infusing, it sits in the IV line at or toward body temperature, creating an environment where any bacteria present can multiply and the red cells become more fragile. Completing the transfusion within about 4 hours minimizes the chance of bacterial growth in the unit and reduces the risk of hemolysis from prolonged exposure and handling. This time limit helps keep the blood component in good condition as it delivers to the patient, rather than allowing deterioration that could lead to hemolysis or infection. The other choices don’t address these risks directly: dehydration, clotting factor preservation, and fever are not the primary reasons for the 4-hour window.