Ionized calcium

Alternative names
Free calcium;

Ionized calcium is a test that measures the amount of free calcium (Ca++, Ca +2) in the blood. (See also serum calcium.) Ionized or free calcium is the metabolically-active portion of calcium and is not bound to proteins in the blood.

How the test is performed
Blood is drawn from a vein (venipuncture) or capillary. The laboratory centrifuges the blood to separate the cells from the serum. The ionized calcium test is done on serum.

How to prepare for the test

Fast for at least 6 hours before the test. Your medical provider may instruct you to withhold drugs that can affect the test.

Drugs that can increase ionized calcium measurements include calcium salts (found in nutritional supplements or antacids), hydralazine, lithium, thiazide diuretics, and thyroxine.

Why the test is performed

Serum calcium is usually measured to screen for or monitor diseases of the bone or calcium regulation disorders (that is, diseases of the parathyroid gland or kidneys). All cells require calcium for numerous functions. Calcium is especially important in the structure of bones and teeth. Calcium is vital for muscle contraction, heart function, transmission of nerve impulses, and blood clotting.

About half of the calcium in the serum is loosely associated with proteins. The other half (which is the metabolically active portion) is called ionized calcium. The usual methods for measuring calcium measure the total calcium level (bound + free). Ionized calcium is measured when other factors complicate the interpretation of the normal serum calcium test.

If the levels of binding proteins are increased or decreased (for example, in the presence of abnormal amounts of albumin or immunoglobulins), the amount of serum calcium will appear to be increased or decreased, because it is the free calcium that is regulated hormonally by the body. In these circumstances, ionized calcium is a more reliable measure of calcium levels.

Normal Values

  • Children: 4.4 to 6.0 mg/dl  
  • Adult: 4.4 to 5.3 mg/dl

    Note: mg/dl = milligrams per deciliter

Normal Values may vary slightly from laboratory to laboratory.

What abnormal results mean
Greater-than-normal levels may indicate:

  • Hyperparathyroidism  
  • Metastatic bone tumor  
  • Milk-alkali syndrome  
  • Multiple myeloma  
  • Paget’s disease  
  • Sarcoidosis  
  • tumors producing a PTH-like substance  
  • Vitamin D intoxication

Lower-than-normal levels may indicate:

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

Special considerations
An excess ingestion of milk or Vitamin D as a dietary supplement can increase calcium levels.

Johns Hopkins patient information

Last revised: December 4, 2012
by Amalia K. Gagarina, M.S., R.D.

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