A blood calcium test is ordered to screen for, diagnose, and monitor a range of conditions relating to the bones, heart, nerves, kidneys, and teeth. The test may also be ordered if a person has symptoms of a parathyroid disorder, malabsorption, or an overactive thyroid.

A total calcium level is often measured as part of a routine health screening. It is included in the comprehensive metabolic panel (CMP) and the basic metabolic panel (BMP), groups of tests that are performed together to diagnose or monitor a variety of conditions.

When an abnormal total calcium result is obtained, it is viewed as an indicator of an underlying problem. To help diagnose the underlying problem, additional tests are often done to measure ionized calcium, urine calcium, phosphorous, magnesium, vitamin D, parathyroid hormone (PTH) and PTH-related peptide (PTHrP). PTH and vitamin D are responsible for maintaining calcium concentrations in the blood within a narrow range of values.

If the calcium is abnormal, measuring calcium and PTH together can help determine whether the parathyroid glands are functioning normally. Measuring urine calcium can help determine whether the kidneys are excreting the proper amount of calcium, and testing for vitamin D, phosphorus, and/or magnesium can help determine whether other deficiencies or excesses exist. Frequently, the balance among these different substances (and the changes in them) is just as important as the concentrations.

Calcium can be used as a diagnostic test if a person has symptoms that suggest:

• Kidney stones
• Bone disease
• Neurologic disorders

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 present in the blood since the balance between free and bound is usually stable and predictable. However, in some people, the balance between bound and free calcium is disturbed and total calcium is not a good reflection of calcium status. In these circumstances, the measurement of ionized calcium may be necessary. Some conditions where ionized calcium should be the test of choice include: critically ill patients, those who are receiving blood transfusions or intravenous fluids, patients undergoing major surgery, and people with blood protein abnormalities like low albumin.

Large fluctuations in ionized 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 those who are critically ill, it can be extremely important to monitor the ionized calcium level in order to be able to treat and prevent serious complications.

A blood calcium test is often ordered when a person undergoes a general medical examination. It is typically included in the comprehensive metabolic panel (CMP) and the basic metabolic panel (BMP), two sets of tests that may be used during an initial evaluation or as part of a routine health screening.

Many people do not have symptoms of high or low calcium until their levels are very out of range. A health practitioner may order a calcium test when someone has:

• Kidney disease, because low calcium is especially common in those with kidney failure
• Symptoms of high calcium such as fatigue, weakness, loss of appetite, nausea, vomiting, constipation, abdominal pain, urinary frequency, and increased thirst
• Symptoms of very low calcium such as abdominal cramps, muscle cramps, or tingling fingers
• Other diseases that have been associated with abnormal blood calcium such as thyroid disease, parathyroid disorder, malabsorption, cancer, or malnutrition

An ionized calcium test may be ordered when someone has 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 ionized calcium. However, when calcium levels fall slowly, many people have no symptoms at all.

Calcium monitoring may be necessary when a person has certain kinds of cancer (particularly breast, lung, head and neck, kidney, or multiple myeloma), has kidney disease, or has had a kidney transplant. Monitoring may also be necessary when someone is being treated for abnormal calcium levels to evaluate the effectiveness of treatments such as calcium or vitamin D supplements.

A urine calcium test may be ordered when someone has symptoms of kidney stones, such as a sharp pain in the person's side or back around the kidneys, pain that may progress to lower in the abdomen, and/or blood in the urine.

Blood calcium levels do not indicate levels of bone calcium but rather how much calcium is circulating in the blood.

Calcium absorption, use, and excretion are regulated and stabilized by a feedback loop involving PTH and vitamin D. Conditions and diseases that disrupt calcium regulation can cause inappropriate acute or chronic elevations or decreases in calcium and lead to symptoms of hypercalcemia or hypocalcemia.

In most cases, total calcium is measured because the test is more easily performed than the ionized 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 of 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 ionized calcium test.

Normal calcium
A normal total or ionized calcium result together with other normal laboratory results generally means that a person's calcium metabolism is normal and blood levels are being appropriately regulated.

High total calcium (hypercalcemia)

Two of the more common causes of hypercalcemia are:

• Hyperparathyroidism, an increase in parathyroid gland function: this condition is usually caused by a benign tumor of the parathyroid gland. This form of hypercalcemia is usually mild and can be present for many years before being noticed.
• Cancer: cancer can cause hypercalcemia when it spreads to the bones and causes the release of calcium from the bone into the blood or when a cancer produces a hormone similar to PTH, resulting in increased calcium levels.

Some other causes of hypercalcemia include:

• Hyperthyroidism
• Sarcoidosis
• Tuberculosis
• Prolonged immobilization
• Excess vitamin D intake
• Thiazide diuretics
• Kidney transplant
• HIV/AIDS

Low total calcium (hypocalcemia)

The most common cause of low total calcium is:

• Low blood protein levels, especially a low level of albumin, which can result from liver disease or malnutrition, both of which may result from alcoholism or other illnesses. Low albumin is also very common in people who are acutely ill.With low albumin, only the bound calcium is low. Ionized calcium remains normal, and calcium metabolism is being regulated appropriately.

Some other causes of hypocalcemia include:

• Underactive parathyroid gland (hypoparathyroidism)
• Inherited resistance to the effects of parathyroid hormone
• Extreme deficiency in dietary calcium
• Decreased levels of vitamin D
• Magnesium deficiency
• Increased levels of phosphorus
• Acute inflammation of the pancreas (pancreatitis)
• Renal failure

Urinary calcium levels may be affected by the same conditions and diseases that affect blood levels (listed above). A high level of calcium in the urine (hypercalciuria) may lead to the formation of crystals or calculi (stones) in the kidneys. About 75% of kidney stones contain calcium.

Newborns, especially premature and low birthweight infants, often are monitored during the first few days of life for neonatal hypocalcemia using the test for ionized calcium. 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 supplemental calcium, given orally or intravenously.

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 used for this purpose.

Taking thiazide diuretic drugs is the most common drug-induced reason for a high calcium level. Taking lithium or tamoxifen may also increase a person's calcium level.