Serum phosphorus
Alternative names:
HPO4-2; inorganic phosphate; phosphorus - serum; PO4
How the test is performed:
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill 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 tourniquet 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. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.
How to prepare for the test:
Fast for 6 hours before the test.
The health care provider may advise you to withhold drugs that may affect the test (see "special considerations").
Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
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 in the blood and other soft tissues and body fluids as phosphate (PO4) ions. Dietary phosphorus is efficiently absorbed, so low PO4 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 and, to a lesser extent, calcitonin. 1,25-dihydroxy vitamin D increases absorption of calcium and phosphate in the intestines.
PTH 1) increases calcium and PO4 release from bone; 2) decreases loss of calcium and increases loss of PO4 in the urine; and 3) increases activation of 25-hydroxy vitamin D to 1,25-dihydroxy vitamin D in the kidneys.
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