Test Quick Guide

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. Still, the amount of inorganic phosphate in the blood is measured with a serum phosphorus/phosphate test.

Phosphorous tests are used to evaluate the mineral level in your blood and aid in diagnosing conditions known to cause abnormally high or low levels of phosphorus.

About the Test

Purpose of the test

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, it is sometimes measured in urine samples to monitor its elimination by the kidneys.

What does the test measure?

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.

We get the phosphorus we need through the foods we eat. It is found in many foods and is readily absorbed by the digestive tract. Most of the body’s phosphates combine with calcium to help form bones and teeth. Smaller amounts are found in muscle and nerve tissue. The rest is found within cells throughout the body, mainly used to store energy.

Only about 1% of total body phosphates are present in the blood. Various 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 PTH, calcium, and vitamin D.

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

Someone with mild to moderate phosphorus deficiency often has no symptoms. With a severe phosphorus deficiency, symptoms may include muscle weakness and confusion. An extreme excess of phosphorus may cause symptoms similar to those seen with low calcium, including muscle cramps, confusion, and even seizures.

When should I get this test?

Mildly abnormal phosphorus levels usually cause no symptoms. Therefore 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 care practitioner may sometimes monitor phosphorus levels.

Finding a Phosphorus Test

How can I get a phosphorus test?

Phosphorus testing uses a blood sample from a blood draw at your doctor’s office, a hospital, or a laboratory. Your doctor may order a phosphorus test as a follow-up to abnormal calcium levels if you have a kidney disorder or uncontrolled diabetes or are taking calcium or phosphate supplements.

You can also order a phosphorus test online without a doctor’s order.

Can I take the test at home?

While you can order a phosphorus test from home, you will need to visit a laboratory to have your blood sample taken.

How much does the test cost?

How much a phosphorus test costs will depend on several factors, including where your blood test sample is collected and your level of health insurance coverage. Your insurance company may pay for some or all of these costs if your doctor prescribes your phosphorus test. For the most definitive information about likely costs, talk with your doctor’s office and medical insurance company.

Taking a Phosphorus Test

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

Before the test

You may need to fast for 12 to 24 hours before having your sample taken. Be sure to follow any instructions that you are given.

During the test

During a phosphorus test, a blood sample will be taken from a vein in your arm. Typically, an elastic band will be tied around your upper arm to increase blood flow in your arm and make it easier to access the vein.

The lab technician will rub an antiseptic wipe over your skin near the vein before inserting the needle. A vial of blood will be withdrawn, and then the needle will be taken out.

The total blood draw usually lasts only a few minutes. There may be some pain during the procedure, and many people feel a brief sting when the needle is inserted.

After the test

After a phosphorus test, a cotton swab or bandage will be placed over the puncture site to stop any bleeding. You can return to most normal activities once the test is over. Slight pain or bruising can affect your arm but normally goes away quickly.

Phosphorus Test Results

Receiving test results

Results from a phosphorus test are usually available within a few business days after the lab receives your sample. You may receive your results via mail or an online health portal. Your doctor may also contact you to review your results and schedule a follow-up appointment.

Interpreting test results

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

  • Increased blood calcium (hypercalcemia), especially due to hyperparathyroidism
  • Overuse of diuretics
  • Malnutrition, malabsorption
  • Chronic alcoholism
  • Severe burns
  • Diabetic ketoacidosis (after treatment)
  • Hypothyroidism
  • Decreased blood potassium (hypokalemia)
  • Chronic antacid use
  • Rickets and osteomalacia (due to vitamin D deficiencies)
  • Hemodialysis

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

  • Kidney failure
  • Liver disease (such as cirrhosis)
  • Hypoparathyroidism
  • Diabetic ketoacidosis (when first seen)
  • Increased dietary intake (phosphate supplementation)
  • Tumor lysis syndrome (after cancer treatment)
  • Spread of cancer to bones
  • Vitamin D toxicity (too much vitamin D)

Asking questions can help you understand the meaning of your phosphorus test results. When you talk with your doctor, some of these questions may be helpful to review:

  • Was my phosphorus level low, normal, or high?
  • Were any other measurements taken along with phosphorus? If so, were they normal or abnormal?
  • If my phosphorus was too high or too low, what is the most likely cause?
  • Are there any follow-up tests that you recommend?
  • Should I have another phosphorus test?
  • Should I make any changes to my diet?

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