Phosphate in Blood

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. But this 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.

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.

Many medicines can change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take.

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 indicate. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

Blood test

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.

Heel stick

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 applied.

Blood test

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. However, 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.

Heel stick

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.

Blood test

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.

Continued bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell the health professional before your blood is drawn.

Heel stick

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 within 1 to 2 hours.

Normal

Normal values may vary from lab to lab.

High values

High phosphate levels may be caused by:

• Kidney disease, underactive parathyroid glands (hypoparathyroidism), acromegaly, rhabdomyolysis, healing fractures, untreated diabetic ketoacidosis, or certain bone diseases.
• Too much vitamin D in the body.
• A decrease in magnesium levels.
• Pregnancy.

Low values

Low phosphate levels may be caused by:

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

Results from a blood phosphate test may be affected by:

• Drinking alcohol before the test.
• Using some medicines that can increase phosphate levels, such as androgen hormones, anabolic steroids, and enemas that contain phosphate.
• Taking too much vitamin D.
• Using some medicines that can decrease phosphate levels, such as antacids, insulin, acetazolamide, and epinephrine. A large infusion of sugar (glucose) that causes insulin levels to increase can also decrease phosphate levels.
• Having a disease, such as lymphoma, that causes calcium levels to rise or fall.
• Having a rare disease, such as diabetes insipidus, that causes the kidneys to produce large amounts of urine.

• 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. For more information, see the medical tests Calcium in Blood, Calcium in Urine, Chloride, Magnesium, Potassium, and Sodium.
• Other blood tests, such as a blood urea nitrogen (BUN) test or a creatinine test, can also be used to check kidney function. For more information, see the medical tests Blood Urea Nitrogen and Creatinine and Creatinine Clearance.
• 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.

Other Works Consulted

• Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.

• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.

• Handbook of Diagnostic Tests (2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.

• Pagana KD, Pagana TJ (2002). Mosby’s Manual of Diagnostic and Laboratory Tests, 2nd ed. St. Louis: Mosby.