Summary

• There are two main types of calcium in your blood.
• One is termed ‘bound’ and is stuck to the normal proteins in your blood and the other is termed ‘free’.
• The total calcium test measures both free and bound forms of calcium. 
• The ionised calcium test measures only the free calcium.
• Usually, the total calcium test is enough for a diagnosis but under certain conditions the ionised or “free” calcium needs to be measured separately.

Testing calcium - two blood tests

Calcium is one of the most important minerals in the body. Although best known for its role in forming bones and teeth, it is used by every cell in your body and helps with nerve signalling, muscle contractions and keeping your heart pumping. It is also needed for blood clotting.  

Only about one percent of calcium circulates in your blood and the rest is stored in your bones and teeth. If there is too much or too little calcium in your blood, it may be a sign that you could have any one of a wide range of medical conditions.

The two main forms of calcium in your blood are free calcium, also called ionised calcium, and bound calcium that is mainly attached to proteins in your blood, especially albumin. Albumin is made by your liver and helps keep fluid from leaking out of your blood vessels into other tissues.  

Free or ionised calcium is the active form of calcium, and this is the type that is used by your body to function properly. You have roughly equal amounts of free calcium circulating in your blood and bound calcium attached to proteins. Both are tested to help in diagnosing and monitoring many health problems.

When is ionised calcium tested?

The total calcium test measures both free and bound forms of calcium. In most cases, total calcium is measured because the test is more easily performed than the ionised calcium test and the blood sample you give needs no special handling. Ionised calcium testing is technically more difficult to manage. Your blood sample needs to be collected in a special air-tight tube and must be transported to the laboratory within about half an hour or else kept on ice.

Total calcium is usually a good measure of free calcium since the amounts of free and bound forms are typically the same. However, because about half the calcium in blood is attached to protein, total calcium test results can be affected by high or low levels of albumin and other blood proteins. This can cause the total calcium level result to appear falsely high or low.

Sometimes ionised calcium needs to be measured directly in a separate test because there can be changes in ionised calcium, but not total calcium. These changes are caused by variations in blood pH or high PTH levels (see next section). 

How blood calcium levels are controlled

• Your body tries to keep your calcium levels at just the right level. It does this through a feedback system that involves parathyroid hormone (PTH) and calcitriol, the active form of vitamin D.  
• Your parathyroid glands are tiny glands at the back of your thyroid. When levels of free calcium are low, they sense this in the blood and make PTH.
• The PTH is released into the blood, and it triggers your kidneys to slow the amount of calcium being removed from your blood and passed into your urine. At the same time the PTH signals to cells in your bones to release into calcium into your blood. As a result, blood calcium levels rise.  
• PTH also stimulates your kidneys to make calcitriol which is the activated form of vitamin D. This increases the amount of calcium being absorbed from food in your intestine.
• As calcitriol levels rise, and with calcium levels going back to normal PTH falls. In healthy people, PTH and vitamin D keep blood calcium at stable levels.   

Why get tested?

Ionised calcium is usually ordered if the results of a total calcium test are not what they are expected to be. If you have a low or high total calcium result it is viewed as sign of a possible underlying problem.  

Ionised calcium testing is also used if you have a condition that affects your body's ability to balance the amounts of free and bound calcium in your blood. If you have conditions like kidney disease, parathyroid disorders, or certain cancers, your body might struggle to balance free and bound calcium.

Measuring ionised calcium assesses calcium levels more accurately, which is especially important in critical situations like liver transplantation, cardiopulmonary bypass (a type of heart surgery), or in critically ill patients. It provides a clearer picture of a person’s calcium status and helps guide treatment. Multiple myeloma can cause blood protein abnormalities, and this can lead to high levels of total calcium and normal ionised calcium.

Ionised calcium is also ordered if you are seriously ill and receiving blood transfusions or IV fluids or undergoing major surgery. This is so that your medical team can intervene and prevent possible serious complications. Large fluctuations in ionised calcium can cause the heart to slow down or to beat too quickly, send muscles into spasm, and can cause confusion or even coma.

Further testing

If your total calcium results are abnormal more tests are often done as well as ionised calcium. Urine calcium, phosphate, magnesium, vitamin D and parathyroid hormone (PTH) can be measured.

Measuring calcium and PTH together can help decide whether your parathyroid gland is working normally.  Measuring urine calcium can help decide whether your kidneys are removing the right amount of calcium for your body to function properly. Vitamin D, phosphate, and/or magnesium are often tested together. The balance between these different substances and the changes in them can often be just as important as the individual levels themselves. For more on calcium testing see Calcium.

Having the test

Sample

Blood.

Any preparation?

None.

Your results

Reading your test report

Your results will be presented along with those of your other tests on the same form. You will see separate columns or lines for each of these tests.

Reference intervals

Your results will be compared to reference intervals (sometimes called a normal range).

• Reference intervals are the range of results expected in healthy people.
• When compared against them your results may be flagged high or low if they sit outside this range.
• Many reference intervals vary between labs so only those that are standardised or common across most laboratories are given on this website.
• If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation. Your results need to be interpreted by your doctor.

Reference intervals for ionised calcium

The normal range for ionised calcium is between 1.16 mmol/L and 1.30mmol/L.

Any more to know?

Newborns, especially premature and low birthweight babies and those born to a diabetic mother, are often monitored during the first few days of life for neonatal hypocalcaemia. This can occur because of an immature parathyroid gland and doesn't always cause symptoms. The condition may resolve itself or may require treatment with calcium given orally or by injection.

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record.

You and your healthcare provider can now access your results whenever and wherever needed. Get further trustworthy health information and advice from healthdirect.