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

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to a needle. The elastic band is removed from your arm. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area 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 check the blood level of phosphorus, particularly if you have a disorder known to cause abnormal phosphorus levels.

Normal values range from 2.4 - 4.1 mg/dl.

Note: mg/dl = milligrams per deciliter

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

Higher than normal levels may indicate:

• Bone metastasis
• Hypocalcemia
• Hypoparathyroidism
• Increased dietary or intravenous (IV) intake of phosphate
• Liver disease
• Renal failure
• Sarcoidosis

Lower than normal levels may indicate:

• Diabetic ketoacidosis
• Hypercalcemia
• Hyperinsulinism
• Hyperparathyroidism
• Too little dietary intake of phosphate or vitamin D, resulting in rickets (childhood) or osteomalacia (adult)

Other conditions under which the test may be performed:

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

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.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

The following can affect phosphorous levels:

• Antacids
• Enemas containing sodium phosphate
• Excess vitamin D supplements
• Glucose through a vein (intravenous)
• Laxatives containing sodium phosphate
• Methicillin

Yu SLA. Disorders of magnesium and phosphorous. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, PA: Saunders Elsevier; 2007: chap 120.