Phosphate is usually measured as which form?

• A. Organic phosphate
• B. Inorganic phosphate
• C. Phospholipid phosphate
• D. Phosphoric acid

Answer: (B)

Phosphate measured in clinical samples is the inorganic form. In the body, phosphate exists both as inorganic ions (orthophosphate, the form detected by standard assays) and as organic phosphate bound in molecules such as nucleotides, phospholipids, and proteins. The common colorimetric methods for serum phosphate react specifically with the inorganic phosphate to form a colored complex, giving a readout of the inorganic phosphate concentration. Organic phosphate would not produce the same signal unless it were first broken down to release orthophosphate. Therefore, the test results reflect inorganic phosphate, which is the clinically relevant pool for assessing phosphate status. Phosphoric acid and phospholipid phosphate constitute parts of the bound organic forms and are not what these routine assays quantify.