Patient Test Information

Phosphorus

Also known as:

P; PO4; Phosphate

Formal name:

Inorganic Phosphate

Related tests:

Calcium; Electrolytes; Vitamin D; PTH; Magnesium

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.

How is it used?

Phosphorus tests are most often ordered along with other tests, such as those for calcium, parathyroid hormone (PTH), and/or vitamin D, to help diagnose and/or monitor treatment of various conditions that cause calcium and phosphorus imbalances.

While phosphorus tests are most commonly performed on blood samples, phosphorus is sometimes measured in urine samples to monitor its elimination by the kidneys.

When is it ordered?

Since mildly abnormal phosphorus levels usually cause no symptoms, phosphorus testing is typically performed in follow up to an abnormal calcium test and/or when symptoms of abnormal calcium such as fatigue, muscle weakness, cramping, or bone problems are present.

Phosphorus testing may also be ordered along with other tests when symptoms suggest kidney and gastrointestinal disorders.

When conditions causing abnormal phosphorus and/or calcium levels are found, testing for both may be ordered at regular intervals to monitor treatment effectiveness.

When someone has diabetes or signs of an acid-base imbalance, a health practitioner may sometimes monitor phosphorus levels.

What does the test result mean?

Low levels of phosphorus (hypophosphatemia) in the blood may be due to or associated with:

  • Hypercalcemia, especially due to hyperparathyroidism
  • Overuse of diuretics
  • malnutrition
  • Alcoholism
  • Severe burns
  • Diabetic ketoacidosis (after treatment)
  • Hypothyroidism
  • Hypokalemia
  • Chronic antacid use 
  • Rickets and osteomalacia (due to vitamin D deficiencies)

Higher than normal levels of phosphorus (hyperphosphatemia) in the blood may be due to or associated with:

  • Kidney failure
  • Liver disease
  • Hypoparathyroidism
  • Diabetic ketoacidosis (when first seen)
  • Increased dietary intake (phosphate supplementation)

Is there anything else I should know?

Abnormally high levels of phosphorus can lead to organ damage due to calcification, deposits of calcium phosphate in the tissues. This is rare, however, and it is more common that high phosphorus levels lead to cardiovascular disease or osteoporosis.

Phosphate levels are normally higher in children than in adults because their bones are actively growing. Low phosphate levels in children can inhibit bone growth and high levels may be an indication of a condition that disrupts the body's balance of minerals.

Soft drinks and pre-packaged food items are high in phosphorus content, which some nutritionists believe contributes to over-consumption of phosphorus.

Blood and urine levels of phosphorus may be affected by the use of enemas and laxatives containing sodium phosphate, excess dietary vitamin D supplements, and by intravenous glucose administration.

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.

  1. If there are no symptoms, how will I know if I have an abnormal phosphorus level?

    Abnormal phosphorus levels are usually detected because of the relationship with and effect on calcium levels. Calcium is routinely tested as part of the comprehensive metabolic panel (CMP) and basic metabolic panel (BMP), tests that are frequently ordered as part of a health exam. If you have an abnormal calcium level, your health care provider usually will check your phosphorus level.

  2. Can vegetarians meet their phosphorus needs without resorting to meat or dairy?

    Yes, but only about 50% of the phosphorus in plant sources such as beans, lentils, grains, peanuts and almonds is available to the body because we lack the enzymes to process it. An exception to this is yeast breads because yeast provides the necessary enzyme.