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

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.

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.

For an 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.

The health care provider may advise you to stop taking drugs that may affect the test.

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.

This test is performed to evaluate the blood level of phosphorus, particularly when the person has a disorder known to cause abnormal phosphorus levels.

Most of the body's phosphorus is combined with calcium in the bones, 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 those on a normal diet unless the person has a malabsorption syndrome (inadequate absorption of nutrients in the intestinal tract).

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 conversion of 25-hydroxy vitamin D to 1,25-dihydroxy vitamin D in the kidneys

Normal values range from 2.4 to 4.1 mg/dl. Normal value ranges may vary slightly among different laboratories.

Note: mg/dl = milligrams per deciliter

Higher-than-normal levels may indicate:

• Bone metastasis
• Hypocalcemia
• Hypoparathyroidism
• Increased dietary or IV intake of PO4
• Liver disease
• Renal failure
• Sarcoidosis

Lower-than-normal levels may indicate:

• Diabetic ketoacidosis
• Hypercalcemia
• Hyperinsulinism
• Hyperparathyroidism
• Inadequate dietary intake of PO4 or vitamin D resulting in rickets (childhood) or osteomalacia (adult)

Additional conditions under which the test may be performed:

• Multiple endocrine neoplasia (MEN) II
• Secondary hyperparathyroidism
• Tertiary hyperparathyroidism

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Antacids can bind PO4 and decrease absorption.

Nonpharmacological factors that can affect PO4 measurements include: enemas containing sodium phosphate, excess vitamin D supplements, and intravenous glucose administration (because PO4 enters cells along with glucose).

Drugs that can increase PO4 measurements include: laxatives containing Na2HPO4 (sodium phosphate), methicillin, and excess vitamin D or 1,25-dihydroxy vitamin D.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.