Health Encyclopedia

Serum phosphorus

Definition

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

Alternative Names

Phosphorus - serum; HPO4^-2, PO4^-3; Inorganic phosphate

How the test is performed

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.

How to prepare for the test

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

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 check the blood level of phosphorus, particularly if you have a disorder known to cause abnormal phosphorus levels.

Normal Values

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.

What abnormal results mean

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:

What the risks are

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)

Special considerations

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

References

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

Review Date:3/18/2008
Reviewed By:Elizabeth H. Holt, MD, PhD, Assistant Professor of Medicine, Section of Endocrinology and Metabolism, Yale University. Review provided by VeriMed Healthcare Network.

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