The risk of metastatic calcification increases when the product of calcium x phosphorus exceeds which value?

• A. 25
• B. 75
• C. 95
• D. 55

Answer: (D)

Calcium-phosphate product in body fluids helps predict when calcium phosphate will start to precipitate in soft tissues. When the product of serum calcium (in mg/dL) and serum phosphorus (in mg/dL) exceeds about 55 mg^2/dL^2, the solubility limit is reached and calcium phosphate can deposit in tissues such as blood vessels, lungs, and kidneys. Keeping this product at or below roughly 55 reduces the risk of metastatic calcification. Clinically, this means managing calcium and phosphate levels together—using phosphate binders, limiting calcium infusions, and adjusting nutritional or renal replacement therapy to stay near or below that threshold. Values markedly above 55 (e.g., 75 or 95) indicate higher risk, while well below 55 is safer; the commonly cited threshold is 55 mg^2/dL^2, which is why that value is the correct reference.