Patient Education - Endocrine Encyclopedia
Endocrine Surgery Encyclopedia
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 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.
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 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:
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
What abnormal results mean:
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
What the risks are:
- 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
Special considerations:
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.
Review Date: 3/13/2006
Reviewed By: Robert Hurd, M.D., Department of Biology, College of Arts and Sciences, Xavier
University, Cincinnati, OH. Review provided by VeriMed Healthcare Network.
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