Serum Phosphorus Test

Written by Gretchen Holm | Published on June 1, 2012
Medically Reviewed by George Krucik, MD

What is a Serum Phosphorous Test?

Phosphorus is an important part of several of your body’s processes. It helps with bone growth, energy storage, and nerve and muscle production. Many foods, especially meats and dairy products, contain phosphorus, so it’s usually easy to get enough of this mineral in your diet.

Most of your body’s phosphorus is contained in your bones and teeth. However, some is in your blood. Your doctor can assess your blood phosphorus levels using a serum phosphorus test.

Hyperphosphatemia is when you have too much phosphorus in your blood. Hypophosphatemia is the opposite: having too little phosphorus. Various conditions, including liver disease and vitamin D deficiency, can cause your blood phosphorus level to become too high or too low.

A serum phosphorus test can be used to determine whether you have high or low phosphorus levels, but it cannot help your doctor diagnose the cause of your condition. The doctor will need to perform more tests to determine what is causing your abnormal serum phosphorus test results.

Why Doctors Perform a Serum Phosphorus Test

Your doctor may order a serum phosphorus test if he or she suspects that your phosphorus level is too low or too high. Either extreme can lead to health problems.

Symptoms that may indicate your phosphorus level is too low include:

  • anxiety or irritability
  • bone issues, such as: pain, fragility, and poor development (in children)
  • breathing irregularity
  • fatigue
  • loss of appetite
  • muscle weakness
  • weight gain or loss

If the level of phosphorus in your blood is too high, you may have deposits of phosphorus (combined with calcium) in your muscles. This is rare and only occurs in people with severe calcium absorption or kidney problems. More commonly, excess phosphorus leads to cardiovascular disease or osteoporosis (weakening of your bones).

Your doctor may also order a serum phosphorus test if you received abnormal results from a blood calcium test. Your body needs to maintain a delicate balance between levels of calcium and phosphorus, so an abnormal result on a calcium test may indicate that your phosphorus levels are also atypical.

Preparing for the Serum Phosphorus Test

Many medications can affect your phosphorus levels, including antacids and vitamin D supplements (when taken in excess) and intravenous glucose. Medications that contain sodium phosphate can also affect your phosphorus levels. Be sure to tell your doctor about any medications you’re taking. He or she may instruct you to temporarily stop using medications that could interfere with your test results.

The Serum Phosphorus Test

You don’t typically need to fast before this test. Your doctor will let you know if he or she wants you to fast for any reason.

The test involves a simple blood draw. Your doctor or nurse will use a small needle collect a sample of blood from a vein in your arm or hand. The sample will be sent to a laboratory for analysis.

Normal Serum Phosphorus Test Results

Serum phosphorus is measured in milligrams of phosphorus per deciliter of blood (mg/dL). A normal range is generally 2.4 to 4.1 mg/dL.

The normal range varies slightly depending on your age. It’s natural for children to have higher phosphorus levels because they need more of this mineral to help their bones develop.

Abnormal Serum Phosphorus Test Results

High Levels

If your kidney function is impaired, excess phosphorus will likely build up in your bloodstream. Avoiding high-phosphorus foods, such as milk, nuts, beans, and liver can help you lower your phosphorus levels.

In other cases, high phosphorus levels may be caused by:

  • certain medications, such as laxatives that contain phosphates
  • dietary problems, such as consuming too much phosphate and/or vitamin D
  • diabetic ketoacidosis (when your body runs out of insulin and begins to burn fatty acids instead)
  • hypocalcemia (low serum calcium levels)
  • hypoparathyroidism (impaired thyroid gland function, leading to low levels of thyroid hormone)
  • liver disease

Low Levels

Low phosphorus levels may be caused by a range of nutritional problems and medical conditions, including:

  • chronic use of antacids
  • lack of vitamin D
  • not getting enough phosphorus in your diet
  • malnutrition
  • alcoholism
  • hypercalcemia (high serum calcium levels)
  • hyperparathyroidism (an overactive thyroid gland, leading to high levels of thyroid hormone)
  • severe burns

Risks of a Taking a Serum Phosphorus Test

  • As with any blood test, there is a slight risk of bruising, bleeding, or infection at the puncture site. You may also feel lightheaded after having blood drawn. In rare cases, your vein may become swollen after blood is drawn. This is known as “phlebitis.” Applying a warm compress to the site several times a day can ease the swelling.
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