At what calcium-phosphorus product does metastatic calcification occur?

• A. 30
• B. 55
• C. 40
• D. 70

Answer: (B)

Metastatic calcification happens when calcium and phosphate in the blood combine to exceed the solubility limit and precipitate in soft tissues. The value most clinicians use as a practical threshold is about 55 mg^2/dL^2 for the calcium-phosphorus product. When Ca x P stays at or below this level, the risk of widespread deposition is lower; when it rises above this threshold, calcium phosphate can start to deposit in tissues such as the lungs, kidneys, blood vessels, and gastric mucosa. This is particularly a concern in patients with chronic kidney disease or severe hyperphosphatemia, where phosphate levels are high and calcium may be elevated from therapy or comorbid conditions. Keeping the calcium-phosphorus product under roughly 55 helps minimize the chance of metastatic calcification, which is why that threshold is used as a practical target in nutrition support and renal/metabolic care.