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Also known as: P; PO4; Phosphate
Formal name: Inorganic Phosphate

At a Glance

Why Get Tested?

To evaluate the level of phosphorus in your blood and to aid in the diagnosis of conditions known to cause abnormally high or low levels of phosphorus

When to Get Tested?

In follow up to an abnormal calcium level; when you have a kidney disorder or uncontrolled diabetes; when you are taking calcium or phosphate supplements

Sample Required?

A blood sample drawn from a vein in your arm or sometimes a timed urine collection (usually a 24-hour sample)

Test Preparation Needed?

Overnight fasting may be required for a blood sample; follow any instructions that you are given.

The Test Sample

What is being tested?

Phosphorus is a mineral that combines with other substances to form organic and inorganic phosphate compounds. The terms phosphorus and phosphate are often used interchangeably when talking about testing, but it is the amount of inorganic phosphate in the blood that is measured with a serum phosphorus/phosphate test.

Phosphates are vital for energy production, muscle and nerve function, and bone growth. They also play an important role as a buffer, helping to maintain the body's acid-base balance.

Phosphorus comes into the body through the diet. It is found in many foods and is readily absorbed by the intestines. About 70-80% of the body's phosphates combine with calcium to help form bones and teeth, another 10% are found in muscle, and about 1% is in nerve tissue. The rest is found within cells throughout the body, where they are mainly used to store energy.

Normally, only about 1% of total body phosphates are present in the blood. 

A wide variety of foods, such as beans, peas and nuts, cereals, dairy products, eggs, beef, chicken, and fish, contain significant amounts of phosphorus. The body maintains phosphorus/phosphate levels in the blood by regulating how much it absorbs from the intestines and how much it excretes via the kidneys. Phosphate levels are also affected by the interaction of parathyroid hormone (PTH), calcium, and vitamin D.

Phosphorus deficiencies (hypophosphatemia) may be seen with malnutrition, malabsorption, acid-base imbalances, hypercalcemia, and with disorders that affect kidney function. Phosphorus excesses (hyperphosphatemia) may be seen with increased intake of the mineral, hypocalcemia, and with kidney dysfunction.

Someone with a mild to moderate phosphorus deficiency often does not have any symptoms. With a severe phosphorus deficiency, symptoms may include muscle weakness and confusion. An extreme excess of phosphorus may cause symptoms that are similar to those seen with low calcium, including muscle cramps, confusion, and even seizures.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm. If a timed urine sample is required, you will be asked to save all of your urine over a set time period (usually 24 hours).

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

Overnight fasting may be required prior to collecting a blood sample; follow any instructions that you are given.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Holm, Gretchen. (Published June 1, 2012.) Serum Phosphorus Test. Available online at through Accessed October 2013.

Blood Test: Phosphorus. (Updated March 2011.) Available online at through Accessed October 2013.

Phosphorus - blood. MedlinePlus Medical Encyclopedia. Available online at Accessed December 2013.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

Cohen, D. [Updated] (2002 February 15, Updated). Kidney diet - dialysis patients. MEDLINEplus [On-line information]. Availableonline at

Zangwill, M. [Updated] (2001 February 02). Phosphorus in diet. MEDLINEplus [On-line information]. Available online at

Knochel, J. [Reviewed] (2001 January 10). Phosphorus. The Linus Pauling Institute [On-line information]. Available online at

Merck. Phosphate Metabolism. The Merck Manual of Diagnosis and Therapy [On-line information]. Available online at through

Spengler, R. (2000, November 16, Updated). Phosphate in Blood. WebMDHealth [On-line information]. Available online at through

Spengler, R. (2000, November 16, Updated). Phosphate in Urine. WebMDHealth [On-line information]. Available online at through

Dugdale, D. (Updated 2009 November 15). Serum Phosphorus. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed March 2010.

Patterson, L. and DeBlieux, P. (Updated 2009 December 3). Hyperphosphatemia. eMedicine [On-line information]. Available online at through Accessed March 2010.

Moore, D. and Rosh, A. (Updated 2009 September 22). Hypophosphatemia. eMedicine [On-line information]. Available online at through Accessed March 2010.

Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 722-723.

Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 852-855.