What Is a Urine Phosphate Test?

Medically Reviewed by Minesh Khatri, MD on March 17, 2024
Written by William Moore
3 min read

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. Spot tests can be performed, too. 

Your doctor might recommend a urine phosphate test if they think 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.

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.

Your doctor may give you either a spot urine test or a container to collect your urine over a 24-hour period, depending on the reason you are getting the test. 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 for the 24-hour test:

  • 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 on 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.

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

  • Too much phosphorous in your diet
  • An issue with your parathyroid gland
  • Kidney disease

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

  • Not enough phosphorous or vitamin D in your diet
  • Taking insulin
  • Using antacids for a long time
  • Kidney disease

But your age, diet, and gender can affect your phosphate levels, as can pregnancy, exercise, medications, 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.