Serum Phosphorus

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Why is the Test Performed?

This test is performed to evaluate the blood level of phosphorus, particularly in the presence of disorders known to cause abnormal phosphorus levels.

Most of the body's phosphorus is combined with calcium within the skeleton, but about 15% exists in the blood and other soft tissues and body fluids as phosphate (PO4) ions. Dietary phosphorus is efficiently absorbed, so low PO4 caused by dietary deficiency is unlikely in the absence of a malabsorption syndrome (inadequate absorption of nutrients in the intestinal tract) for individuals on a normal diet.

PO4 levels are controlled by PTH, 1,25-dihydroxy Vitamin D and, to a lesser extent, calcitonin. The 1,25-dihydroxy Vitamin D increases absorption of calcium and phosphate in the intestines. PTH 1) increases calcium and PO4 release from bone; 2) decreases loss of calcium and increases loss of PO4 in the urine; and 3) increases activation of 25-hydroxy Vitamin D to 1,25-dihydroxy Vitamin D in the kidneys.

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