An ionized calcium test measures the levels of a specific form of calcium in your blood. It’s performed by taking a small blood sample. You’ll typically need to fast before the test.

Calcium is an important mineral that your body uses in many ways. It increases the strength of your bones and teeth and helps your muscles and nerves function.

A serum calcium blood test measures the total calcium in your blood. There are several different forms of calcium in your blood. These include ionized calcium, calcium bound to other minerals called anions, and calcium bound to proteins like albumin. Ionized calcium, also known as free calcium, is the most active form.

A serum calcium test usually checks the total amount of calcium in your blood. This includes ionized calcium and calcium bound to proteins and anions. Your doctor may want to check your blood calcium levels if you have signs of kidney disease, certain kinds of cancers, or problems with your parathyroid gland.

Ionized calcium levels give more information about active, ionized calcium. It may be important to know your ionized calcium levels if you have abnormal levels of proteins, such as albumin, or immunoglobins in your blood. If the balance between bound calcium and free calcium isn’t normal, it’s important to find out why. Free calcium and bound calcium each typically make up half of your body’s total calcium. An imbalance can be a sign of a major health issue.

You may need to have your ionized calcium level checked if:

  • you’re receiving blood transfusions
  • you’re critically ill and on intravenous (IV) fluids
  • you’re having major surgery
  • you have abnormal levels of blood proteins

In these cases, it’s important to understand exactly how much free calcium you have available.

Low levels of free calcium can cause your heart rate to slow down or speed up, cause muscle spasms, and even result in a coma. Your doctor may order an ionized calcium test if you have any signs of numbness around your mouth or in your hands and feet, or if you have muscle spasms in the same areas. These are symptoms of low free calcium levels.

An ionized calcium test is harder to perform than a serum calcium test. It requires special handling of the blood sample, and it’s only done in certain cases.

You’ll need to fast for six hours before you have your blood drawn for an ionized calcium test. This means you shouldn’t eat or drink anything other than water during that time.

Discuss your current medications with your doctor. You may have to stop taking certain medications before the test, but only if your doctor tells you to do so. Examples of drugs that can affect your ionized calcium levels include:

  • calcium salts
  • hydralazine
  • lithium
  • thyroxine
  • thiazide diuretics

Don’t stop taking a medication without talking to your doctor about it first.

An ionized calcium test uses a small amount of your blood. A healthcare professional will get a blood sample by performing a venipuncture. They’ll clean a section of skin on your arm or hand, insert a needle into your vein through your skin, and then draw a small amount of blood into a test tube.

You may feel some moderate pain or a mild pinching sensation during the procedure. After your doctor removes the needle, you may feel a throbbing sensation. You’ll be instructed to apply pressure to the site where the needle entered your skin. Your arm will then be bandaged. You should avoid using that arm for heavy lifting for the rest of the day.

There are some very rare risks involved in taking a blood sample, including:

  • lightheadedness or fainting
  • hematoma, which occurs when blood accumulates under your skin
  • infection
  • excessive bleeding

Bleeding for a long period of time after the procedure may indicate a more serious bleeding condition.

Normal levels

Normal levels of ionized calcium are different in adults and children. In adults, a level of 4.64 to 5.28 milligrams per deciliter (mg/dL) is normal. In children, a normal ionized calcium level is 4.8 to 5.52 mg/dL.

Abnormal levels

If you have low levels of ionized calcium in your blood, it can indicate:

  • hypoparathyroidism, which is an underactive parathyroid gland
  • inherited resistance to parathyroid hormone
  • malabsorption of calcium
  • a vitamin D deficiency
  • osteomalacia or rickets, which is a softening of the bones (in many cases due to a vitamin D deficiency)
  • a magnesium deficiency
  • high phosphorus levels
  • acute pancreatitis, which is an inflammation of the pancreas
  • kidney failure
  • malnutrition
  • alcoholism

If you have a high level of ionized calcium in your blood, it can indicate:

  • hyperparathyroidism, which is an overactive parathyroid gland
  • a sedentary lifestyle or lack of mobility
  • milk-alkali syndrome, which is high levels of calcium in the body due to consuming too much milk, antacids, or calcium carbonate over time
  • multiple myeloma, which is cancer of the plasma cells (a type of white blood cell that produces antibodies)
  • Paget’s disease, which is a disorder that results in deformity due to abnormal bone destruction and growth
  • sarcoidosis, which is an inflammatory disease that affects the eyes, skin, and other organs
  • tuberculosis, which is a potentially life-threatening disease caused by the bacterium Mycobacterium tuberculosis
  • a kidney transplant
  • the use of thiazide diuretics
  • certain kinds of tumors
  • an overdose of vitamin D

Your doctor will discuss your results with you. They’ll also help determine your next steps if any are needed.