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Caring for Your Child

Blood Test: Phosphorus

What It Is

A phosphorus blood test is done to assess phosphorus levels in the blood. Phosphorus, a mineral our bodies get mostly from food, helps:

  • form healthy bones and teeth
  • process energy in the body
  • supports muscle and nerve functioning

Why It’s Done

Doctors may order a phosphorus test to help diagnose or monitor certain conditions, such as:

  • kidney disorders (to see if the kidneys are excreting [releasing] or retaining [keeping] too much phosphorus)
  • gastrointestinal and nutritional disorders (to look for problems with intestinal absorption or malnutrition)
  • calcium and bone problems (because calcium and phosphorus work closely together in the body, and the levels of one can yield important information about the other)

Kids tend to have higher phosphorus levels than adults, mainly because their bones are still growing. When there’s not enough phosphorus, bone growth and other body functions may be affected. When there’s too much, it can be a sign of conditions that affect the balance of minerals in the body.


No special preparations are needed for this test. However, certain drugs — especially antacids, laxatives, and diuretics — might affect the test results, so tell your doctor about any medications your child is taking.

On the day of the test, having your child wear a T-shirt short-sleeved shirt can make things easier for the technician who will be drawing the blood.

The Procedure

A health professional will usually draw the blood from a vein. For an infant, the blood may be obtained by puncturing the heel with a small needle (lancet).

If the blood is being drawn from a vein, the skin surface is cleaned with antiseptic, and an elastic band (tourniquet) is placed around the upper arm to apply pressure and cause the veins to swell with blood. A needle is inserted into a vein (usually in the arm inside of the elbow or on the back of the hand) and blood is withdrawn and collected in a vial or syringe.

After the procedure, the elastic band is removed. Once the blood has been collected, the needle is removed and the area is covered with cotton or a bandage to stop the bleeding. Collecting blood for this test will only take a few minutes.



What to Expect

Either method (heel or vein withdrawal) of collecting a sample of blood is only temporarily uncomfortable and can feel like a quick pinprick. Afterward, there may be some mild bruising, which should go away in a few days.

Getting the Results

The blood sample will be processed by a machine. The results are commonly available after a few hours or the next day.

If phosphorus levels are found to be either elevated or deficient, further testing may be necessary to determine what’s causing the problem and how to treat it.


The phosphorus test is considered a safe procedure. However, as with many medical tests, some problems can occur with having blood drawn. These include:

  • fainting or feeling lightheaded
  • hematoma (blood accumulating under the skin causing a lump or bruise)
  • pain associated with multiple punctures to locate a vein

Helping Your Child

Having a blood test is relatively painless. Still, many kids are afraid of needles. Explaining the test in terms your child can understand might help ease some of the fear.

Allow your child to ask the technician any questions he or she might have. Tell your child to try to relax and stay still during the procedure, as tensing muscles and moving can make it harder and more painful to draw blood. It also may help if your child looks away when the needle is being inserted into the skin.

If You Have Questions

If you have questions about the phosphorus test, speak with your doctor. You can also talk to the technician before the procedure.

Reviewed by: Yamini Durani, MD
Date reviewed: July 2014