Ionized calcium is calcium that is freely flowing in your blood and not attached to proteins. It is also called free calcium.

All cells need calcium in order to work. Calcium helps build strong bones and teeth. It is important for heart function, and helps with muscle contraction, nerve signaling, and blood clotting.

This article discusses the test used to measure the amount of ionized calcium in blood.

See also: Serum calcium

Free calcium; Ionized calcium

Usually the blood is drawn from the wrist. However, it may also be drawn from the inside of the elbow, groin, or other artery. A heartbeat (pulse) is felt by pressing on the area above an artery. The doctor will usually check to make sure blood is flowing into the hand from the main arteries in the forearm (radial and ulnar arteries).

The area is cleaned with antiseptic and a needle is inserted. A small amount of anesthetic may be injected or applied before the needle is inserted. The blood will easily flow into the specially prepared (heparinized) syringe.

The needle is removed after there is enough of a blood sample. Pressure is applied to the puncture site for 5 - 10 minutes to stop the bleeding. You will be checked during this time to make sure the bleeding stops.

You should not eat or drink for at least 6 hours before the test. Your doctor may tell you to temporarily stop taking any drugs that can affect the test results. Calcium salts, hydralazine, lithium, thiazide diuretics, and thyroxine can increase your level of ionized calcium.

Never stop taking any medicine without first talking to your doctor.

Your doctor may order this test if you have signs of kidney or parathyroid disease. The test may also be done to monitor persons who have already been diagnosed with such diseases.

Usually, blood tests measure your total calcium level, which looks at both ionized calcium and calcium attached to proteins. You may need to have a separate ionized calcium test if you have factors that increase or decrease total calcium levels, such as abnormal blood levels albumin or immunoglobulins.

Normal values may vary slightly from laboratory to laboratory.

• Children: 4.4 - 6.0 milligrams per deciliter (mg/dL)
• Adults: 4.4 - 5.3 mg/dL

Greater-than-normal levels may be due to:

• Hyperparathyroidism
• Immobilization
• Decreased urinary calcium levels of unknown cause (idiopathic hypocalciuria)
• Metastatic bone tumor
• Milk-alkali syndrome
• Multiple myeloma
• Paget's disease
• Sarcoidosis
• Too much vitamin D
• Certain types of tumors
• Use of thiazide diuretics

Lower-than-normal levels may be due to:

• Hypoparathyroidism
• Malabsorption
• Osteomalacia
• Pancreatitis
• Renal failure
• Rickets
• Vitamin D deficiency

Fukagawa M, Kurokawa K, Papadakis MA. Fluid & electrolyte disorders. In: McPhee SJ, Papadakis MA, Tierney LM Jr. Current Medical Diagnosis and Treatment 2007. New York, NY: McGraw Hill; 2007.

Wysolmerski JJ, Insogna KL. The parathyroid glands, hypercalcemia, and hypocalcemia. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 266.