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Diagnostic Tests

Blood Test: Phosphorus

What Is a Phosphorus Test and Why Is It Done?

A phosphorus test looks at levels of phosphorus in a person’s blood. This mineral helps the body’s muscles, nerves, bones, and teeth work well. Phosphorus also helps the body process energy.

A phosphorus test can help alert doctors to some possible health problems:

  • kidney problems (whether the kidneys are excreting or retaining too much phosphorus)
  • gastrointestinal and nutritional conditions (problems with intestinal absorption or malnutrition)
  • calcium and bone problems


If your doctor wants you to get a phosphorus blood test, you won’t need to do anything special beforehand to get ready. However, certain medicines or supplements — like some antacids, diuretics, and laxatives — might alter the test results. So tell your doctor if you’re taking any over-the-counter medicines or herbal supplements.

It can help to wear a T shirt or other short-sleeve top on the day of the test to make things faster and easier for the technician who will be drawing the blood.

The Procedure

A health professional will usually draw the blood from a vein in your arm — most often on the inside of the elbow, but sometimes on the back of the hand. The technician cleans the skin surface with antiseptic and ties an elastic band (tourniquet) around the upper arm so the veins swell with blood and are easy to see.

Next, it’s time for the needle. It should feel like a quick pinprick. Occasionally, it can be hard to find a vein so a nurse, doctor, or technician may need to try more than once. That’s not the norm, though — most people’s veins are easy to find.

It’s best to try to relax and stay still during the procedure since tensing muscles can make it harder and more painful to draw blood. And if you don’t want to watch the needle being inserted or see the blood collecting, you don’t have to. Look the other way and maybe relax by focusing on saying the alphabet backwards, doing some breathing exercises, thinking of a place that makes you happy, or listening to your favorite music.

The technician will draw the blood so it collects in a vial or syringe. Collecting blood will only take a few minutes. Once the technician has enough blood, he or she removes the needle and covers the area with cotton or a bandage to stop the bleeding. After the test, you may notice some bruising — that’s normal and it should go away in a few days. Don’t be afraid to ask the technician if you have any questions about the blood draw.


A blood test is a safe procedure and there are no real risks. Some people may feel faint or lightheaded during a blood test. And, while nobody really loves needles, a few teens have a strong fear of them. If that’s you, talk to your doctor since there are ways to make the procedure easier for you.


The blood sample will be processed by a machine. It usually only takes a few hours or a day or so for your doctor to get the results. If the test shows problems with your phosphorus levels, your doctor may want to do other tests to find out what the cause is and how to treat it.

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