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
A test to measure phosphate in blood may be done to:
- Check phosphate levels if you have kidney disease or bone disease.
- Help find problems with certain glands, such as the parathyroid glands.
- Find a reason for abnormal vitamin D levels.
How To Prepare
Many medicines can change the results of this test. Be sure to tell your doctor about all the non-prescription and prescription medicines you take, including vitamin D supplements.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form .
How It Is Done
The health professional taking a sample of your blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
- Clean the needle site with alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Put a gauze pad or cotton ball over the needle site as the needle is removed.
- Put pressure on the site and then put on a bandage.
In a newborn baby, the blood sample is usually taken from the heel (heel stick).
For a heel stick blood sample, several drops of blood are collected from the heel of your baby. The skin of the heel is first cleaned with alcohol and then punctured with a small sterile lancet. Several drops of blood are collected in a small tube. When enough blood has been collected, a gauze pad or cotton ball is placed over the puncture site. Pressure is maintained on the puncture site briefly, and then a small bandage is usually put on.
How It Feels
You may feel nothing at all from the needle puncture, or you may feel a brief sting or pinch as the needle goes through the skin. Some people feel a stinging pain while the needle is in the vein. But many people do not feel any pain (or have only minor discomfort) once the needle is positioned in the vein. The amount of pain you feel depends on the skill of the health professional drawing your blood, the condition of your veins, and your sensitivity to pain.
A brief pain, like a sting or a pinch, is usually felt when the lancet punctures the skin. Your baby may feel a little discomfort with the skin puncture.
There is very little risk of complications from having blood drawn from a vein. You may develop a small bruise at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the needle is withdrawn.
In rare cases, the vein may become inflamed after the blood sample is taken. This condition is called phlebitis and is usually treated by applying a warm compress several times daily.
There is very little risk of a serious problem developing from a heel stick. A small bruise may develop at the puncture site.
Continued bleeding can be a problem for babies with bleeding disorders. There is a possibility that a bleeding problem may be discovered while collecting the blood for this test.
A phosphate test measures the amount of phosphate in a blood sample. Phosphate is a charged particle (ion) that contains the mineral phosphorus. Phosphate levels are usually higher in children than in adults because of the active bone growth occurring in children.
Results are usually available in 1 to 2 hours.
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
High phosphate levels may be caused by:
- Some tumours such as lymphoma.
- Kidney disease, underactive parathyroid glands (hypoparathyroidism), acromegaly, healing fractures, untreated diabetic ketoacidosis, or certain bone diseases.
- Too much vitamin D in the body.
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 tumours.
What Affects the Test
Results from a blood phosphate test may be affected by:
- Using some medicines that can decrease phosphate levels, such as acetazolamide and epinephrine. A large infusion of sugar (glucose) that causes insulin levels to increase can also decrease phosphate levels.
What To Think About
- Results of a test to measure phosphate in blood are not useful on their own. Other electrolytes (such as calcium, chloride, magnesium, potassium, and sodium) may also be measured.
- Other blood tests, such as a blood urea nitrogen (BUN) test or a creatinine test, can also be used to check kidney function.
- Children with low phosphate levels may grow more slowly than other children.
- Low phosphate levels may occur in people who have type 2 diabetes or when a person who has diabetic ketoacidosis is being treated with insulin.
Current as of: July 29, 2019
Author: Healthwise Staff
E. Gregory Thompson MD - Internal Medicine
Donald Sproule MDCM, CCFP - Family Medicine
Adam Husney MD - Family Medicine
Kathleen Romito MD - Family Medicine
Alan C. Dalkin MD - Endocrinology
Current as of: July 29, 2019