What Is a Urine Phosphate Test?

When you think about strong bones, calcium usually comes to mind first. But another mineral, phosphorous, is just as important. Your muscles and nerves need it, too. And it helps your body turn food into energy.

In your body, it’s found in the form of phosphates. They’re made when phosphorous combines with something else, like oxygen.

Your kidneys filter extra phosphates from your blood, and they go out of your body in your urine. If something’s wrong with your kidneys, your pee might have too many phosphates in it.

A urine phosphate test measures how much phosphate is in your pee over a 24-hour period. Your doctor might also call it a phosphorous test.

When Would I Need One?

Your doctor might recommend a urine phosphate test if she thinks you might have an issue with your kidneys or you get kidney stones often. In some cases, the test can offer some clues about why they keep coming back.

Calcium and phosphate levels are closely tied, so you may also have this test if other tests show your calcium levels aren’t quite right.

How Do I Get Ready for It?

You don’t need to do anything special ahead of time. Just be sure to tell your doctor about any medicines, herbs, vitamins, or supplements you take. They could affect your results. For example, laxatives with sodium phosphate or lots of vitamin D can skew your numbers.

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How Is It Done?

Your doctor gives you a container to collect your urine over a 24-hour period. It may have a substance in it that helps keep the urine stable until it’s tested. Your doctor may ask you to keep the container in the refrigerator when you’re not using it.

Here’s what you’ll do:

  • When you first get up in the morning, pee into the toilet, not the container. Then write down what time it is.
  • After that, pee into the container each time you go for the rest of the day and night.
  • The next morning, get up at the same time as the day before and pee into the container one last time.

It may help to set an alarm the morning of the second day to make sure you stick to the 24-hour window.

Results are typically ready within a day, but it could take longer, depending on the lab your doctor uses.

What Do the Results Mean?

In general, higher levels than normal could mean a number of things, including:

And several things can cause lower-than-normal phosphate levels, like:

  • Not enough phosphorous or vitamin D in your diet
  • Taking insulin
  • Too much potassium in your diet
  • Using antacids for a long time

But your age, diet, and gender can affect your phosphate levels, as can pregnancy, exercise, and even the time of year. And different labs have different ways of doing the test. It’s best to talk to your doctor about exactly what your results mean.

WebMD Medical Reference Reviewed by Minesh Khatri, MD on December 26, 2017

Sources

SOURCES:

Lab Test Online: “Phosphorous,” “Kidney Stone Analysis.”

National Health Service: “Vitamins and Minerals.”

National Health Service, Homerton University Hospital: “Urine Phosphate.”

Medscape: “Phosphate (Phosphorous).”

National Kidney Foundation: “Phosphorous and Your CKD Diet.”

Indian Journal of Endocrinology and Metabolism: “Renal phosphate handling: Physiology.”

University of Rochester Medical Center: “Kidney Stone (Urine).”

Mayo Medical Laboratories: “Phosphate, 24 Hour, Urine.”

Quest Diagnostics: “Phosphorous, 24 HR Urine.”

LabCorp: “Phosphorous, 24-Hour Urine.”

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