Calcium

What is being analysed?
Calcium is one of the body’s most important electrolytes: 99% of calcium is contained in the bones, and the rest is found inside cells and in the interstitial fluids with which the plasma is in balance. About half of the calcium in the circulation is in the free or ionised form, which is the biologically active part; the rest circulates bound to plasma proteins and is biologically inactive.
The blood calcium test usually measures total calcium, which includes both the ionised and the protein-bound portion; in particular cases ionised calcium may be requested. The urine test is a measure of how much calcium the kidneys excrete.

How is the sample collected for testing?
The sample is taken from a vein in the arm; for the urine test, a 24-hour urine collection is required

How is it used?
When is it requested?
What does the test mean?
Is there anything else I should know?

How is it used?
Calcium testing does not give direct information on the amount of calcium in the bones, but on the amount circulating in the blood; this information is useful in screening, diagnosing and monitoring many diseases of the bones, endocrine system, nervous system and kidneys, and many cancers.

As is often the case with laboratory tests, the result of this test must be assessed together with other tests. The level of calcium in the blood is regulated by a feedback mechanism involving parathyroid hormone (PTH), vitamin D, phosphorus and magnesium: conditions that disturb this mechanism can cause changes in blood calcium. Measuring PTH and phosphorus together with calcium can therefore help the doctor identify the cause of hyper- or hypocalcaemia.

Measuring free or ionised calcium is important during surgery, especially if a blood transfusion is needed, or when there are large changes in total protein. Particularly large fluctuations in ionised calcium can cause important clinical signs such as tetany, muscle spasms, arrhythmias and heart failure, mental confusion, lethargy and kidney failure.

When is it requested?
Calcium may be requested as a screening test during a regular medical check-up, as it is typically included in routine tests; it can be used as a diagnostic test in the case of:

• kidney stones
• bone diseases
• neurological and psychiatric disorders
• hypercalcaemia (weakness, loss of appetite, nausea, vomiting, constipation, abdominal pain)
• hypocalcaemia (muscle spasms, tingling in the fingers, tetany in the most severe cases).

The doctor may also request a calcium test if the patient has kidney failure, thyroid disease, gastrointestinal disease or cancer.

In some cases calcium may be measured periodically to monitor treatment and the course of the disease; this happens with some forms of cancer (for example breast, lung, bowel and multiple myeloma), in kidney failure or after a kidney transplant.

What does the test mean?
Calcium is usually requested together with albumin (the most abundant protein in plasma), since a significant part of the circulating calcium binds to it: the blood calcium value should always be assessed together with the albumin concentration (if albumin falls, calcium falls too).
The most common causes of hypercalcaemia are:

• hyperparathyroidism
• cancer or bone metastases
• sarcoidosis
• tuberculosis
• excessive vitamin D intake
• kidney transplant
• high total protein

Ionised calcium rises in all these conditions except high total protein, in which ionised calcium stays within the reference range.

The most common causes of hypocalcaemia are:

• hypoparathyroidism
• thyroidectomy
• magnesium deficiency
• excess phosphorus
• active vitamin D deficiency
• chronic kidney failure
• alkalosis
• low total protein

Ionised calcium falls in all these conditions except low total protein.

Is there anything else I should know?
The concentration of calcium in the blood is kept within a narrow range mainly by the action of PTH and vitamin D. PTH is released into the circulation by the parathyroid glands, located behind the thyroid, in response to hypocalcaemia; PTH stimulates the release of calcium from the bones and its reabsorption by the kidneys, so that overall it raises blood calcium; PTH also stimulates the activation of vitamin D in the kidneys. Vitamin D in turn stimulates the absorption of calcium from the gut and reduces its loss through the kidneys.
In newborn babies, especially premature and underweight babies, hypocalcaemia is common in the first days of life; neonatal hypocalcaemia is caused by the immaturity of the parathyroid glands, usually causes no symptoms and resolves on its own.
Measuring calcium in the blood or urine is not an index of the amount of calcium in the bones, which is instead assessed with an imaging test called bone densitometry (“DEXA” or “MOC”).
Some diuretics (for example thiazides) can cause hypercalcaemia.

1) Should I be concerned if my doctor only requests total calcium and not free or ionised calcium?
2) Which foods contain the most calcium?
3) Can my blood calcium result change if I eat calcium-fortified foods?
4) Can this test be done at home?

1) Should I be concerned if my doctor only requests total calcium and not free or ionised calcium?
No. Total calcium is enough for most situations, whether for diagnosis, screening or monitoring. Ionised calcium is measured during surgery or for urgent requests.

2) Which foods contain the most calcium?
The main sources of calcium are dairy products; calcium is also found, in smaller amounts, in eggs, green leafy vegetables, pulses and cereals. Many products on the market (for example fruit juices and cereals) are fortified with calcium.

3) Can my blood calcium result change if I eat calcium-fortified foods?
In general, eating calcium-fortified foods does not change blood calcium.

4) Can this test be done at home?
No. Calcium must be measured in a laboratory by qualified staff using appropriate equipment.