Hyperphosphatemia after massive transfusion of citrated blood is explained by which mechanism?

• A. Increased calcium citrate complexation
• B. Inability to excrete phosphate
• C. Massive cellular lysis releasing phosphate
• D. Crush injury

Answer: (A)

When a patient receives a massive transfusion of citrated blood, a large amount of citrate is infused along with the blood products. Citrate acts as an anticoagulant by binding calcium to form calcium-citrate complexes. This chelation lowers the amount of free (ionized) calcium in the blood, but it also shifts how calcium and phosphate interact. With calcium tied up as calcium-citrate, less calcium is available to form calcium-phosphate complexes, so phosphate remains more in the soluble serum pool, contributing to hyperphosphatemia. The key idea is that the infused citrate causes extensive calcium binding, i.e., increased calcium citrate complexation, which is the mechanism that best explains the rise in serum phosphate in this setting.