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.
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.
Having the test
Blood and/or urine
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.
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.
|Normal calcium||If other laboratory results are normal this generally means that your calcium metabolism is normal and blood levels are appropriately regulated.|
|High total calcium - hypercalcaemia|
Hyperparathyroidism caused by an increase in parathyroid gland function. This is usually due to a benign tumour of the parathyroid gland. This form of hypercalcamia is usually mild and can be present for many years before being noticed.
Cancer can cause hypercalcaemia when it spreads to the bones, which releases calcium into the blood, or when a cancer produces a hormone similar to PTH, resulting in increased calcium levels.
Other less common causes:
|Low total calcium - hypocalcaemia|
Low blood protein levels, especially a low level of albumin, is the most common cause of low total calcium. In this condition, only the bound calcium is low. Ionised calcium remains normal and calcium metabolism is being regulated appropriately.
Other causes of hypocalcaemia include:
Your results will be compared to reference intervals (sometimes called a normal range).
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.
2.10 - 2.60 mmol/L
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:
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.
Pathology Tests Explained (PTEx) is a not-for profit group managed by a consortium of Australasian medical and scientific organisations.
With up-to-date, evidence-based information about pathology tests it is a leading trusted sources for consumers.
Information is prepared and reviewed by practising pathologists and scientists and is entirely free of any commercial influence.