What is a phosphate in blood test?

A phosphate in blood test measures the amount of phosphate in your blood. Phosphate is an electrically charged particle that contains the mineral phosphorus. Phosphorus works together with the mineral calcium to build strong bones and teeth.

Normally, the kidneys filter and remove excess phosphate from the blood. If phosphate levels in your blood are too high or too low, it can be a sign of kidney disease or other serious disorder.

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

What is it used for?

A phosphate in blood test may be used to:

  • Diagnose and monitor kidney disease and bone disorders
  • Diagnose parathyroid disorders. Parathyroid glands are small glands located in the neck. They make hormones that control the amount of calcium in the blood. If the gland makes too much or too little of these hormones, it can cause serious health problems.

A phosphate in blood test is sometimes ordered along with tests of calcium and other minerals.

Why do I need a phosphate in blood test?

You may need this test if you have symptoms of kidney disease or a parathyroid disorder. These include:

  • Fatigue
  • Muscle cramps
  • Bone pain

But many people with these disorders don’t have symptoms. So your provider may order a phosphate test if he or she thinks you may have kidney disease based on your health history and the results of calcium tests. Calcium and phosphate work together, so problems with calcium levels can mean problems with phosphate levels as well. Calcium testing is often part of a routine checkup.

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?

Certain medicines and supplements can affect phosphate levels. Tell your health care provider about any prescription and over-the-counter medicines you are taking. Your provider will let you know if you need to stop taking them for a few days before your test.

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 terms phosphate and phosphorus can mean the same thing in test results. So your results may show phosphorus levels rather than phosphate levels.

If your test shows you have high phosphate/phosphorus levels, it may mean you have:

  • Kidney disease
  • Hypoparathyroidism, a condition in which your parathyroid gland doesn’t make enough parathyroid hormone
  • Too much vitamin D in your body
  • Too much phosphate in your diet
  • Diabetic ketoacidosis, a life-threatening complication of diabetes

If your test shows you have low phosphate/phosphorus levels, it may mean you have:

  • Hyperparathyroidism, a condition in which your parathyroid gland produces too much parathyroid hormone
  • Malnutrition
  • Alcoholism
  • Osteomalacia, condition that causes bones to become soft and deformed. It’s caused by a vitamin D deficiency. When this condition happens in children, it’s known as rickets.

If your phosphate/phosphorus levels are not normal, it doesn’t necessarily mean you have a medical condition needing treatment. Other factors, such as your diet, can affect your results. Also, children often have higher phosphate levels because their bones are still growing. If you have questions about your results, talk to your health care provider.