Phosphate in Urine

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.

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.

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?).

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 1 gal (4 L). 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.

There is no pain while collecting a 24-hour urine sample.

There is no chance for problems while collecting a 24-hour urine sample.

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.

Normal

Normal values may vary from lab to lab. Results are usually available in 1 to 2 days.

Many conditions can change phosphate levels. Your doctor will talk with you about any abnormal results that may be related to your symptoms and medical history.

High values

High phosphate levels may be caused by:

• Kidney diseases.
• An overactive parathyroid gland (hyperparathyroidism).
• Not getting enough vitamin D.
• Taking too much vitamin D.
• Some bone diseases (such as osteomalacia).

Low values

Low urine phosphate levels may be caused by:

• An underactive parathyroid gland (hypoparathyroidism).
• Kidney or liver diseases.

Reasons you may not be able to have the test or why the results may not be helpful include:

• Taking certain medicines, such as diltiazem (such as Cardizem), aspirin, corticosteroids, diuretics, or parathyroid hormone.
• Taking a lot of antacids.
• Not collecting all of your urine within the 24-hour collection period.

• Phosphate may also be measured in the blood. For more information, see the medical test Phosphate in Blood.
• Results of a test to measure phosphate in urine are seldom useful on their own. They should always be interpreted along with the results of other tests.
• The loss of too much phosphate into the urine may cause conditions that damage the bones, such as rickets or osteomalacia.

Other Works Consulted

• Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.

• Handbook of Diagnostic Tests (2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.