The phosphate urine test measures the amount of phosphate in a sample of urine collected over 24 hours (24-hour urine test). Phosphate is a charged particle (ion) that contains the mineral phosphorus. The body needs phosphorus to build and repair bones and teeth, help nerves function, and make muscles contract. Most (about 85%) of the phosphorus contained in phosphate is found in bones. The rest of it is stored in tissues throughout the body.

The kidneys help control the amount of phosphate in the body. Extra phosphate is filtered by the kidneys and passes out of the body in the urine. If there is not enough phosphate, less is found in the urine. A high level of phosphate in the urine may be caused by a kidney problem. High levels also may be caused by eating a meal high in phosphorus or by an overactive parathyroid gland.

Why It Is Done

A test to measure phosphate in urine may be done to:

• Help diagnose kidney problems that affect phosphate levels.
• Help find the cause of kidney stones.

How To Prepare

Calcium and phosphate levels are often measured at the same time.

Many medicines can change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results may indicate. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?) .

How It Is Done

Urine phosphate is usually measured in a sample taken from all the urine produced in a 24-hour period.

To collect your urine for 24 hours:

• You start collecting your urine in the morning. When you first get up, empty your bladder but do not save this urine. Write down the time that you urinated to mark the beginning of your 24-hour collection period.
• For the next 24 hours, collect all your urine. Your doctor's office or lab will usually provide you with a large container that holds about 1gal. The container has a small amount of preservative in it. Urinate into a small, clean container and then pour the urine into the large container. Do not touch the inside of the container with your fingers.
• Keep the large container in the refrigerator for the 24 hours.
• Empty your bladder for the final time at or just before the end of the 24-hour period. Add this urine to the large container and record the time.
• Do not get toilet paper, pubic hair, stool (feces), menstrual blood, or other foreign matter in the urine sample.