Summary

• Calcium is one of the most important minerals in your 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.
• There are two main types of calcium in your blood.
• One circulates in your blood and is called free or ionised calcium and the other is bound mostly to the protein albumin in your blood and is called bound calcium.
• The total calcium test measures both free and bound forms of calcium.
• The ionised calcium test measures only the free calcium.
• The total calcium test is used in general health screening.
• Both total and ionised calcium tests are used to diagnose and monitor kidney disease, parathyroid disorders, bone health, cardiovascular issues, certain cancers and the effects of various medications.

What is calcium

Calcium is an electrolyte. Electrolytes are minerals that have an electric charge when dissolved in water – or blood. In your body, they help regulate many systems including heart rhythms, nerves and muscles. They are also involved in helping maintain the amount of fluid and the acid-base balance in your body.

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 have any one of a wide range of medical conditions.

• Too much calcium and you are said to have hypercalcaemia.
• Too little calcium, and you have hypocalcaemia.

How you get calcium, phosphate and magnesium from your diet

Intestines absorb calcium, phosphate and magnesium from your diet. If you have low levels of any of these it may be because you are not getting enough in your diet or you have vomiting or diarrhoea. Conditions in your gut which make your intestines ability to absorb these electrolytes such as coeliac disease or inflammatory bowel disease (Crohn’s disease and ulcerative colitis) may also affect the levels of these electrolytes.

How excess calcium, phosphate and magnesium are removed from your body

Kidneys filter excess calcium, phosphate and magnesium from your blood and remove them through your urine. If your levels are low, your kidneys respond by reducing the amount being removed from the blood. Abnormal levels are seen in disorders that cause too little or too much to be removed by the kidneys.

Calcium levels are controlled through a feedback system

Your body works to keep calcium at just the right balance. Levels are kept in balance through a feedback system under the control of your parathyroid glands, which are tiny glands at the back of your thyroid.

• When circulating levels of free calcium are low in your blood and your body needs more, the parathyroid glands sense this and make parathyroid hormone (PTH). Hormones are the body’s chemicals messengers. They are made by various glands and travel through the bloodstream to regulate body functions.
• When PTH is released into your blood, it triggers your kidneys to reduce the amount of calcium being filtered from your blood and removed through your urine. At the same time PTH signals to cells in your bones to release calcium into your blood. As a result of both actions, blood calcium levels rise.
• PTH also stimulates your kidneys to make calcitriol, the activated form of vitamin D. This increases the amount of calcium being absorbed from food in your intestine.

Two blood tests

There are several different forms of calcium in your blood and two are measured. These are free calcium which circulates in your blood and is also called ionised calcium, and bound calcium which is calcium that is bound mainly to the protein albumin in your blood. Albumin is the most common protein in your blood and transports many substances including hormones, minerals and waste products around the body.

You have roughly equal amounts of free calcium circulating in your blood and bound calcium attached to proteins.

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.

• 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 present in roughly equal amounts.
  However, high or low levels of protein in the blood can cause the results of this test to appear falsely high or low. In these cases, the laboratory will report a corrected calcium or “Ca (corr)’ result which is a total calcium result which takes into account high or low levels of albumin in the blood. Your doctor may also recommend having an ionised calcium test to measure free calcium directly.
• Ionised calcium: In a few people, 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 differences in blood pH or high PTH levels.

Urine tests

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

Calcium and phosphate testing

Calcium works together with another mineral phosphate. Both are needed for bone strength.

Calcium and phosphate tests are often ordered together. This is because calcium and phosphate have a tight relationship, particularly in conditions affecting bone building, parathyroid function and kidney function.

Calcium and phosphate work together in an inverse relationship. In other words, when calcium levels rise, phosphate levels fall, and when calcium levels fall, phosphate levels rise. This is because phosphate binds to the free type of calcium, lowering the amount in your blood.

When you have blood tests for calcium and phosphate, the higher the phosphate level, the lower the level of free calcium.

When your kidneys are not working properly, the level of phosphate in your blood rises. When phosphate is high in your blood it binds to calcium and causes a build-up of calcium in blood vessels and other parts of the body. This can lead to weak bones and heart disease. If you have abnormal calcium levels, your doctor usually will check your phosphate and alkaline phosphatase (ALP) levels. ALP is an enzyme that is found in high levels in the blood when there is a bone disorder.

Calcium and phosphate are assessed together in cases of hyperparathyroidism, chronic kidney disease and osteoporosis. Calcium is also often ordered along with magnesium, albumin and vitamin D.

Magnesium

Magnesium is needed by your body to properly absorb calcium from your diet and for your body to use calcium effectively. It helps convert vitamin D into its active form, calcitriol, which is essential for calcium absorption.

It is also needed for the release and action of PTH. When magnesium levels are low, not enough PTH is produced, leading to lower calcium levels. A low magnesium level can, over time, cause persistently low calcium levels. This means that measuring magnesium can be helpful in diagnosing problems with calcium.

Why get tested?

General health screen

A calcium test is sometimes included in a general health screen. If you have an abnormal calcium result, you may be asked to have more tests to find the cause. These tests may measurer:

• ionised calcium
• urine calcium
• phosphate
• magnesium
• vitamin D
• calcitonin
• parathyroid hormone (PTH)
• thyroid hormones

Kidney disease

If you have kidney disease or if you have had a kidney transplant, measuring calcium in your urine can help show whether your kidneys are removing the right amount of calcium.

Parathyroid gland disorders

Measuring total calcium and PTH together can help show whether your parathyroid gland is working normally.

Cancer

You may need calcium monitoring if you have certain types of cancer - particularly breast, lung, head and neck, kidney and multiple myeloma (a type of blood cancer).

Heart problems

Large changes in ionised calcium can cause the heart to slow down or to beat too fast and can cause muscles to go into spasm. If someone is critically ill, it is extremely important to know their ionised calcium level to be able to intervene and prevent serious complications.

Gastrointestinal disorders

Calcium testing can be helpful in checking for conditions that affect you’ your gut (also called the gastrointestinal tract), interfering with the absorption of calcium as well as phosphate and magnesium. These are conditions as coeliac disease or inflammatory bowel disease (Crohn’s disease and ulcerative colitis).

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.

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.

More to know

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.
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