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Serum Phosphorus Test

What Is a Serum Phosphorous Test?

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

Your bones and teeth contain most of your body’s phosphorus. However, some phosphorous is in your blood. Your doctor can assess your blood phosphorus level 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 determine whether you have high or low phosphorus levels, but it can’t help your doctor diagnose the cause of your condition. Your doctor will need to perform more tests to determine what’s causing abnormal serum phosphorus test results.

Why Do I Need a Serum Phosphorus Test?

Your doctor may order a serum phosphorus test if they suspect 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:

  • changes in your mental state (e.g. anxiety, irritability, or confusion)
  • bone issues, such as pain, fragility, and poor development (in children)
  • irregular breathing
  • 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. An abnormal result on a calcium test may indicate that your phosphorus levels are also atypical.

What Are the Risks Associated with a Serum Phosphorus Test?

As with any blood test, there’s 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 swell after blood is drawn. This is known as phlebitis. Applying a warm compress to the site several times a day can ease the swelling.

How Do I Prepare for a Serum Phosphorus Test?

Many medications can affect your phosphorus levels, including antacids, 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. They may instruct you to temporarily stop using medications that could interfere with your test results.

What Is the Procedure for a Serum Phosphorus Test?

You don’t typically need to fast before this test. Your doctor will let you know if they want you to fast for any reason.

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

What Do the Results Mean?

Normal Results

Serum phosphorus is measured in milligrams of phosphorus per deciliter of blood (mg/dL). A normal range (in adults) is generally 2.5 to 4.5 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 Results

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

Besides reduced kidney function, high phosphorus levels may be due to:

  • certain medications, such as laxatives that contain phosphates
  • dietary problems, such as consuming too much phosphate 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 parathyroid gland function, leading to low levels of parathyroid hormone)
  • liver disease

Low phosphorus levels may be due to 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 (overactive parathyroid gland or glands, leading to high levels of parathyroid hormone)
  • severe burns

Your doctor will analyze your results and discuss them with you. Be sure to ask your doctor any questions you have about your results.

Content licensed from:

Written by: Gretchen Holm
Medically reviewed on: Nov 24, 2015: Steven Kim, MD

This feature is for informational purposes only and should not be used to replace the care and information received from your health care provider. Please consult a health care professional with any health concerns you may have.
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