Summary

This test measures the level of calcium in your blood. Calcium is one of the most important minerals in your body and has many essential functions. About half of the calcium in your blood is referred to as 'free' and is the active form. The remaining half is 'bound'. There are two tests to measure blood calcium. The total calcium test measures both free and bound forms of calcium. The ionised calcium test measures only the free, active form. Both tests are used in diagnosing and monitoring many health problems including general health screening, kidney disease, parathyroid disorders, bone health, heart problems, cancer, and the effects of medications.

Why get tested?

Calcium is one of the most important minerals in your body. It is used in building and repairing bones and teeth. It also plays important roles in the electrical activity and pumping function of the heart, it is essential in regulating blood clotting and it transmits nerve impulses and regulates muscle contraction.

Calcium levels are regulated and stabilised by a feedback loop involving parathyroid hormone (PTH) which is produced by the parathyroid gland, located just behind your thyroid gland at the base of your throat, and Vitamin D levels. For more on this see ‘Any more to know?’ below.

There are many conditions and diseases that disrupt calcium regulation and these  can lead to higher or lower than normal levels and  cause symptoms of hypercalcaemia (too much calcium) or hypocalcaemia (too little calcium).

Two blood tests

There are several different forms of calcium in your blood. These include ionised calcium, also called free calcium, and calcium that is mainly bound to the protein albumin.  About half of the calcium in your blood is referred to as 'free' and is active. The remaining half is 'bound'.

There are two tests to measure blood calcium. The total calcium test measures both the free and bound forms. The ionised calcium test measures only the free, active form.

The total calcium test is the test most often ordered to evaluate calcium status. In most cases it is a good reflection of the amount of free calcium involved in metabolism since the balance between free and bound is usually stable and predictable. Many laboratories report corrected calcium or `Ca (corr)' which is just total calcium adjusted to compensate for abnormally high or low levels of albumin in the blood which can cause the total calcium level to appear falsely high or low.

However, in a few people, the balance between bound and free calcium is disturbed and total calcium or even corrected calcium is not a good reflection of calcium status. In those circumstances, measurement of ionised calcium is necessary.

Urine tests

Some calcium is lost from your body every day, filtered from the blood by the kidneys and passed into the urine. The amount of calcium that goes into your urine is regulated by the body to keep the calcium in the blood within a narrow range. Measurement of the amount of calcium in the urine is used to work out how much calcium is being removed by the kidneys.

Reasons to measure calcium

Calcium tests are used to diagnose and monitor many different health problems relating to the bones, heart, nerves, and kidneys.

• General health screening: A total calcium level is often measured as part of health screening. It is often asked for together with plasma phosphate, magnesium, and albumin. When there is an abnormal total calcium result it is viewed as an indicator that there is some kind of underlying problem. To help diagnose the underlying problem additional tests are often done to measure ionised calcium, urine calcium, phosphate, magnesium, vitamin D (including regular vitamin D and the active form, calcitriol or 1,25 dihydroxyvitamin D), and parathyroid hormone (PTH). PTH and vitamin D levels are responsible for maintaining calcium concentrations in the blood within a narrow range of values.
• Kidneys: Measuring urine calcium can help show whether the kidneys are getting rid of the right amount of calcium and testing the blood for vitamin D, phosphate, and magnesium can help decide whether other deficiencies or excesses exist. The balance between these different substances and the changes in them can be just as important as the concentrations.
• Bones: Blood and urine calcium measurements cannot tell how much calcium is in the bones. A test similar to an X-ray called a bone density or 'Dexa' scan is needed for this.
• Parathyroid gland: The parathyroid gland is responsible for regulating the levels of calcium in the body. Measuring calcium and PTH together can help show whether the parathyroid gland is working normally. 
• Heart: Large fluctuations in ionised calcium can cause the heart to slow down or to beat too fast, and can cause muscles to go into spasm, cause confusion or even coma. If someone is critically ill  it is extremely important to know the ionised calcium level to be able to intervene and prevent serious complications.
• Cancer: You may need calcium monitoring if you have certain types of cancer - particularly breast, lung, head and neck, kidney, and multiple myeloma.
• Kidney disease: Calcium levels are monitored in people with kidney disease or have had a kidney transplant. Monitoring may also be needed if you are being treated for abnormal calcium levels to evaluate the effectiveness of treatments such as calcium or vitamin D supplements.
• Critically ill patients: Some conditions where ionised calcium should be the test of choice include people who are critically ill and are receiving transfusions or intravenous (IV) fluids, people undergoing major surgery, and those people with blood protein abnormalities such as low albumin.
• Medications: Taking thiazide fluid tablets is the most common drug-induced reason for a high calcium level.

Having the test

Sample

Blood and/or urine

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.  

Total calcium

In most cases total calcium is measured because the test is more easily performed than the ionised calcium test and requires no special handling of the blood sample. Total calcium is usually a good reflection of free calcium since the free and bound forms are typically each about half of the total. However, because about half the calcium in blood is bound to protein, total calcium test results can be affected by high or low levels of protein, especially albumin. In such cases it is more useful to measure free calcium directly using an ionised calcium test. 
 

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.

Total Calcium Reference Intervals

The reference intervals for this test are common reference intervals which means that all laboratories in Australia should be using this same target range.

Adult

2.10 - 2.60 mmol/L

Paediatrics

0 day to <1 week           1.85 - 2.80 mmol/L

1 week - <26 weeks      2.20 - 2.80 mmol/L

26 weeks - <2 years      2.20 - 2.70 mmol/L

2 years - <18 years       2.20 - 2.65 mmol/L

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?

Any more to know?

There are at least three hormones involved in the regulation of the level of calcium in the blood: parathyroid hormone (PTH), calcitonin and calcitriol.  

PTH is produced by a group of small glands in the neck near the thyroid gland, stimulated by a decrease in 'free' calcium. PTH causes the release of calcium from bone and decreases calcium losses from the kidneys, so that calcium levels rise. PTH also stimulates production of vitamin D by the kidneys.

Vitamin D, in turn, increases calcium absorption from  the intestine, but decreases calcium lost from the kidneys in urine. It also decreases the production of PTH. Overall, as Vitamin D levels rise, PTH falls. In healthy people, these two hormones keep blood calcium at normal levels, even though maintaining that balance in the blood may cause calcium to be released from bones.

Newborns, especially premature and low birthweight infants 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.

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