Serum phosphorus

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Blood test

Overview

Risks

Results

Serum phosphorus

Definition:

The serum phosphorus test measures the amount of phosphate in the blood.

Alternative Names:
Phosphorus - serum; PO4; Inorganic phosphate; HPO4-2
How the test is performed:

Adult or child:

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

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A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

Infant or young child:

The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to the puncture site if there is any bleeding.

How to prepare for the test:

The health care provider may advise you to withhold drugs that may affect the test (see "special considerations").

Infants and children:

The preparation you can provide for this test depends on your child's age, previous experiences, and level of trust. For general information regarding how you can prepare your child, see the following topics:

Infant test or procedure preparation (birth to 1 year)
Toddler test or procedure preparation (1 to 3 years)
Preschooler test or procedure preparation (3 to 6 years)
Schoolage test or procedure preparation (6 to 12 years)
Adolescent test or procedure preparation (12 to 18 years)

How the test will feel:

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is 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 -- as phosphate (PO4) ions -- in the blood and other soft tissues and body fluids. Dietary phosphorus is efficiently absorbed, so a low PO4 level 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. The 1,25-dihydroxy vitamin D increases absorption of calcium and phosphate in the intestines. PTH:

Increases calcium and PO4 release from bone
Decreases loss of calcium and increases loss of PO4 in the urine
Increases activation of 25-hydroxy vitamin D to 1,25-dihydroxy vitamin D in the kidneys