Which of the following is not a suitable medium for suspensions of red blood cells?

• A. Serum
• B. Reagent grade water
• C. Saline
• D. Albumin

Answer: (B)

Red blood cell suspensions must be kept in a medium that is isotonic to preserve cell integrity and allow proper interactions during testing. Pure water is hypotonic to cells, so RBCs would take in water, swell, and lyse, making the suspension unusable for interpreting results. Saline provides the necessary isotonic environment to keep cells intact. Albumin is used to promote agglutination by reducing the zeta potential and allowing antibodies to interact more effectively with the cell surface. Serum, although protein-rich, contains varying antibodies and other factors that can interfere with reactions, so it is not used as a standard suspending medium for routine RBC suspensions. Hence, water is not suitable as a suspending medium.