What is a Phosphate Blood Test?

A phosphate test measures the amount of phosphate in your blood. If that sounds too simple, stay tuned for a slight curveball.

Phosphate is what you get when you combine the mineral phosphorus with oxygen. When you eat certain foods -- such as beans, nuts, cereal, milk, eggs, beef, chicken, and fish -- phosphorous enters your intestines. When it meets up with oxygen, it becomes phosphate.

Most phosphates team up with calcium to build and repair your bones and teeth. Some support your muscles and nerves.

Here's where things get tricky: When it comes to testing, phosphorus is sometimes called phosphate and vice versa. Don’t let all those “ph-ph” words confuse you.

Why Do I Need a Test?

Though you absorb phosphate in the intestines, it's filtered and removed through your kidneys.

Abnormal phosphate levels -- with symptoms such as bone problems, fatigue and weakness -- may point to a kidney disorder. Your doctor may also want you to get a phosphate test if she suspects:

Since phosphate works so closely with calcium, doctors also use the test to determine whether your body is getting and using calcium the right way. Issues related to calcium can be a sign of low vitamin D, or they may include:

  • Bone problems
  • Feeling very tired
  • Muscle weakness
  • Cramping

How Should I Prepare for It?

Your doctor will give you instructions; follow them carefully.

You may be asked not to eat anything from midnight before the test until after the test is over.

Make sure your doctor knows about everything you're taking, including prescription and over-the-counter medicine, herbal supplements, vitamins, and recreational or illegal drugs.

What Happens During the Test?

After cleaning the skin on part of your arm, a technician will insert a needle into one of your veins. He may wrap an elastic band around the upper part of your arm to make that vein easier to find.

Once enough blood goes into a tube, the technician will take off the band, pull out the needle, and stop the bleeding with a cotton ball or bandage. He’ll label the tube of blood, and it will be sent to a lab.

With any blood test, you may feel:

  • Pain when the needle is inserted
  • Dizziness
  • Soreness or bruising
  • Bleeding

Continued

What Can Affect My Test Results?

If you've had dialysis, it may affect your phosphate levels, as can some medicines. Some things you eat and drink might have an effect, too, so check with your doctor about whether avoid these before the test:

  • Beans (many different kinds)
  • Beer
  • Cheese
  • Chocolate
  • Cola
  • Fish

What Do My Test Results Mean?

When the results are too high or too low, it's a sign that something's not right.

If your phosphate levels are low, it may be a sign of:

High phosphate levels may indicate:

What Other Tests Might I Have?

Your doctor may give you other tests along with the phosphate blood test to help narrow down her diagnosis. These can include tests for:

  • Calcium
  • Vitamin D
  • PTH (hormone produced by the parathyroid gland)
WebMD Medical Reference Reviewed by Laura J. Martin, MD on January 22, 2017

Sources

SOURCES:

Lab Tests Online UK: "Phosphate."

Lab Tests Online: "Phosphorus."

KidsHealth: "Blood Test: Ferritin."

University of Rochester Medical Center: "Phosphorus."

Mayo Clinic: “Hyperparathyroidism.”

World Health Organization: “Cardiovascular diseases.” 

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