Alternative Names: Free calcium; Ionized calcium

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.

How the test will feel:

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 - 6.0 mg/dl
• Adult: 4.4 - 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

What the risks are:

Special considerations:

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