Blood mixing warning sign. Computer screen warning against mixing unmatched donor blood types. This case concerns CMV-negative blood that has not been infected by the cytomegalovirus (CMV). Mixing this blood with CMV-positive blood may infect a patient who receives this blood. The CMV is present in a large proportion of the population, but normally without symptoms. High-risk groups include unborn babies and patients with suppressed or compromised immune systems. Donor blood is separated into its components for more efficient use of the blood. They are often mixed back together to produce the optimum mix of blood components. This blood is not yet ready for clinical use.