What is being tested?
Calcium is one of the most important minerals in the body. It is essential for the proper functioning of muscles, nerves and the heart and is required in blood clotting and in formation of bones. About 99% of it is found in the bones, while the remainder circulates in the blood. Roughly half of the calcium is referred to as 'free' and is active. The remaining half is 'bound' calcium. It is attached to albumin and other compounds and is metabolically inactive.
There are two tests to measure plasma calcium. The total calcium test measures both the free and bound forms. The ionised calcium test measures only the free, metabolically active form.
Some calcium is lost from your body every day, filtered from the blood by the kidneys and excreted into the urine. The amount of calcium excreted 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 determine how much calcium is being eliminated by the kidneys.
How is it used?
Plasma calcium is tested to screen for, diagnosis, and monitor a range of conditions relating to the bones, heart, nerves, kidneys and teeth. Plasma calcium levels do not directly tell how much calcium is in the bones, but rather, how much calcium is circulating in the blood.
A total calcium level is often measured as part of health screening. It is sometimes included in the E/LFT panel of tests but is often asked for together with plasma phosphate and albumin. When an abnormal total calcium result is obtained it is viewed as an indicator of 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 and parathyroid hormone (PTH). PTH and vitamin D are responsible for maintaining calcium concentrations in the blood within a narrow range of values.
Measuring calcium and PTH together can help determine whether the parathyroid gland is functioning normally. Measuring urine calcium can help determine whether the kidneys are excreting the proper amount of calcium, and testing for vitamin D, phosphate, and/or magnesium can help determine whether other deficiencies or excesses exist. Frequently the balance among these different substances, and the changes in them, are just as important as the concentrations.
Calcium can be used as a diagnostic test if you go to your doctor with symptoms that suggest:
The total calcium test is the test most frequently 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 patients, 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 conditions where ionised calcium should be the test of choice include: critically ill patients who are receiving transfusions or IV fluids, patients undergoing major surgery, and patients with blood protein abnormalities like low albumin.
Large fluctuations in ionised calcium can cause the heart to slow down or to beat too rapidly, can cause muscles to go into spasm (tetany), and can cause confusion or even coma. In critically ill patients, it is extremely important to know the ionised calcium level to be able to intervene and prevent serious complications.
When is it requested?
Calcium is often ordered as a screening test when you undergo a general medical examination. It is sometimes included in the E/LFT panel of tests.
Your doctor also may order a calcium test when you have:
Your doctor may order an ionised calcium test when you have numbness around the mouth and in the hands and feet and muscle spasms in the same areas. These can be symptoms of low levels of ionised calcium. However, when calcium levels fall slowly, many people have no symptoms at all.
You may need calcium monitoring when you have certain kinds of cancer (particularly breast, lung, head and neck, kidney, and multiple myeloma), have kidney disease, or have had a kidney transplant. Monitoring may also be necessary when you are being treated for abnormal calcium levels to evaluate the effectiveness of treatments such as calcium or vitamin D supplements.
What does the result mean?
Calcium absorption, use, and excretion are regulated and stabilised by a feedback loop involving PTH and Vitamin D. Conditions and diseases that disrupt calcium regulation can cause inappropriate acute or chronic increases or decreases in calcium and lead to symptoms of hypercalcaemia (excessive) or hypocalcaemia (deficient).
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. In such cases it is more useful to measure free calcium directly using an ionised calcium test.
Normal calcium
A normal calcium or ionized calcium result together with other normal laboratory results generally mean that your calcium metabolism is normal and blood levels are appropriately regulated
High total calcium - hypercalcaemia
Two of the more common causes of hypercalcaemia are:
Some other causes of hypercalcaemia include:
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.
Some other causes of hypocalcaemia include:
Reference intervals
Adult
2.10 - 2.60 mmol/L
2.10 - 2.60 mmol/L (corrected for albumin)
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
The reference intervals shown above are known as a harmonised reference interval. This means that eventually all laboratories in Australia will eventually use this same interval so wherever your sample is tested, the reference interval should be the one shown above. Laboratories are in the process of adopting these harmonised intervals so it is possible that the intervals shown on the report of your results for this test may be slightly different until this change is fully adopted.
Is there anything else I should know?
Two hormones control blood calcium within a small range of values. Parathyroid hormone (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 in the intestine, but decreases calcium lost from the kidneys in urine. It also decreases the production of PTH. Overall, as Vitamin D levels rise, calcium levels rise and 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.
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 purpose.
Taking thiazide diuretic drugs (fluid tablets or water tablets) is the most common drug-induced reason for a high calcium level.
Common questions
Dairy products are the main source of calcium, but lesser amounts are found in eggs, green leafy vegetables, broccoli, beans and pulses, nuts, and whole grains.
In general, consuming fortified foods will not directly affect your calcium test results. Talk to your doctor about your calcium requirements. Your doctor can help you determine whether you should be taking calcium and/or vitamin supplements.
No. While there are hand-held instruments available, these are intended for use in a hospital or GP surgery setting and must be operated by trained personnel.
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