What is a phosphate in blood test?

A phosphate in blood test measures the amount of phosphate in a sample of your blood. Phosphate contains the mineral phosphorus. So, a phosphate test is sometimes called a phosphorus test.

Phosphate is a type of electrolyte. Electrolytes are electrically charged minerals. They help control the amount of fluid and the balance of acids and bases (pH balance) in your body. Your body also needs phosphates for many other important processes, such as:

• Building strong bones and teeth. Most of the phosphate in your body is found in your bones where it is combined with the mineral calcium.
• Making energy.
• Helping your nerves and muscles work properly.

The phosphorus in phosphate comes from the foods you eat, including nuts and seeds, dairy products, dried beans, meats, poultry, and eggs. Your body tightly controls the amount of phosphate in your blood mainly through your:

• Kidneys. They filter extra phosphate from your blood and get rid of it through urine (pee). If your phosphate level is low, your kidneys limit the amount lost through urine.
• Intestines. They control how much phosphorous you absorb from the foods you eat.

The amount of phosphate in your blood is also linked to your levels of:

• Calcium. When blood calcium levels increase, phosphate levels decrease. And when calcium levels decrease, phosphate levels increase.
• Vitamin D. It helps your body use phosphate.
• Parathyroid hormone (PTH). This hormone is made by parathyroid glands in your neck. It helps balance phosphate and calcium levels in your blood.

Abnormal phosphate levels may be a sign of problems with the systems in your body that control phosphate levels. To help understand the cause of abnormal levels, the test is often done with blood tests that measure calcium, vitamin D, and PTH.

Other names: phosphorus test, P, PO4, phosphorus-serum, phosphate, inorganic phosphorus

What is it used for?

A phosphate in blood test is often used with other tests to help diagnose and/or monitor:

• Kidney disease, especially chronic kidney disease. High phosphate levels are a common sign that the kidneys aren't working well to get rid of extra phosphate in the blood.
• Bone disorders. High phosphate levels can pull calcium out of bones and weaken them over time.
• Parathyroid disorders. Parathyroid hormones control the balance of phosphate and calcium in the blood. A phosphate test can help show how well the parathyroid glands are working.

A phosphate test may also be used to monitor people who:

• Have diabetes that isn't well controlled
• Have signs of an acid-base imbalance (having too much or too little acid in your blood)

Why do I need a phosphate in blood test?

Phosphate levels that are either high or mildly low usually don't cause any symptoms on their own. But you may need a phosphate test with other tests if you:

• Have had an abnormal result on a calcium blood test. A calcium test may be part of a routine exam. Abnormal levels are often linked to abnormal phosphate levels.
• Have symptoms of low calcium levels. High levels of phosphate can lower calcium levels in your blood, which can cause symptoms, such as:
  • Nails that break easily, and/or hair that becomes coarse
  • Rash and/or itchy skin
  • Muscle cramps
  • Bone pain
  • Seizures
  • Irregular heartbeat (arrhythmia)
• Have symptoms of severely low phosphate levels. This condition can become life-threatening and needs immediate treatment. Symptoms may include:
  • Muscle pain and/or weakness
  • Bone pain
  • Changes in your mental condition, such as feeling confused or very irritable
  • Seizures
  • Coma

You may also need to have your phosphate levels checked if you have or may have a condition that can cause abnormal phosphate levels, such as:

• Kidney disease
• Malnutrition
• Malabsorption syndrome
• A parathyroid gland disorder
• Using certain medicines

What happens during a phosphate in blood test?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

Your health care provider will tell you how to prepare for your test. You may need to fast (not eat or drink) for several hours before you give your blood sample.

Certain medicines, vitamins and supplements can affect the accuracy of your test results. So tell your provider about everything you take, including over-the-counter medicines. But don't stop taking any prescription medicines unless your provider tells you to.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

The results of your phosphate test may say "phosphorus levels" or "phosphate levels." These terms mean the same thing. The meaning of your test results depends on your symptoms, medical history, and the results of other tests.

In general:

A higher than normal phosphate level is called hyperphosphatemia. High levels are linked to many conditions, including:

• Late stages of chronic kidney disease and kidney failure. If damaged kidneys can't filter out extra phosphate, it can build up in your blood. If you have a condition that affects your kidneys, eating too much phosphorus-rich food may also lead to high phosphate levels.
• Hypoparathyroidism. With this condition, your parathyroid glands don't make enough parathyroid hormone (PTH). When PTH is low, calcium levels are low and phosphate levels are high.
• Acidosis. This means having too much acid in your blood from a pH imbalance. It may be caused by a lung disorder or other health conditions, including kidney disease.
• Long-term use of certain medicines. These include steroids, laxatives, and enemas that contain phosphate.

A lower than normal phosphate level is called hypophosphatemia. Mildly low levels often aren't a health problem. But many conditions that cause low levels may need treatment, including:

• Hyperparathyroidism. With this condition, your parathyroid glands make too much parathyroid hormone, which leads to high levels of blood calcium and low levels of phosphate.
• Vitamin D deficiency. A lack of this vitamin can lead to osteomalacia, a condition that causes soft bones. In children, this condition is called rickets.
• Getting too little phosphorus from food. This is uncommon in the U.S. But some people are more likely to have trouble getting enough phosphorus, including:
  • Premature babies
  • People who have certain rare genetic disorders, severe malnutrition, alcohol use disorder, or problems absorbing nutrients
• Long-term use of certain medicines. These include antacids that contain aluminum hydroxide or calcium carbonate and certain prescription diuretics ("water pills").

If your phosphate levels are not normal, it doesn't always mean you have a medical condition that needs treatment. Also, children often have higher phosphate levels because their bones are still growing. If you have questions about your results, talk with your provider.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a phosphate in blood test?

Your provider may order a phosphate in urine test instead of or with a phosphate in blood test.