Test Overview

A phosphate test measures the amount of phosphate in a blood sample. Phosphate is a charged particle (ion) that contains the mineral phosphorus. The body needs phosphorus to build and repair bones and teeth, help nerves function, and make muscles contract. Most (about 85%) of the phosphorus contained in phosphate is found in bones. The rest of it is stored in tissues throughout the body.

The kidneys help control the amount of phosphate in the blood. Extra phosphate is filtered by the kidneys and passes out of the body in the urine. A high level of phosphate in the blood is usually caused by a kidney problem.

The amount of phosphate in the blood affects the level of calcium in the blood. Calcium and phosphate in the body react in opposite ways: as blood calcium levels rise, phosphate levels fall. A hormone called parathyroid hormone (PTH) regulates the levels of calcium and phosphorus in your blood. When the phosphorus level is measured, a vitamin D level, and sometimes a PTH level, is measured at the same time. Vitamin D is needed for your body to take in phosphate.

The relation between calcium and phosphate may be disrupted by some diseases or infections. For this reason, phosphate and calcium levels are usually measured at the same time.

Why It Is Done

This test may be done to check phosphate levels if you have kidney disease or bone disease. It helps find problems with certain glands, such as the parathyroid glands. The test is also used to find a cause for abnormal vitamin D levels.

How To Prepare

Your doctor may ask you not to eat or drink anything other than water for at least 8 hours before the test.

How It Is Done

Blood test

A health professional uses a needle to take a blood sample, usually from the arm.

Heel stick

A heel stick is used to get a blood sample from a baby. The baby's heel is poked, and several drops of blood are collected. A baby may have a tiny bruise where the heel was poked.

How long the test takes

This test will take a few minutes.

How It Feels

Blood test

When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.

Heel stick

A brief pain, like a sting or a pinch, is usually felt when the lancet punctures the skin. A baby may feel a little discomfort with the skin puncture.

Risks

Blood test

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Heel stick

There is very little risk of a problem from a heel stick. A baby may get a small bruise at the puncture site.

Results

Normal

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

Phosphate levels are usually higher in children than in adults. That's because of the active bone growth occurring in children.

High values

High phosphate levels may be caused by:

Low values

Low phosphate levels may be caused by:

• Hyperparathyroidism, certain bone diseases (such as osteomalacia), lack of vitamin D, or some kidney or liver diseases.
• Severe malnutrition or starvation.
• A condition such as sprue that prevents the intestines from absorbing nutrients properly.
• Alcohol use disorder.
• High calcium levels.
• Some types of tumors.