Also known as:Total Calcium; Ionized Calcium
Related tests:Phosphorus; Vitamin D Tests; PTH; Magnesium; Albumin; Comprehensive Metabolic Panel; Basic Metabolic Panel; Kidney Stone Analysis; Kidney Stone Risk Panel
Why Get Tested?
To screen for, diagnose, and monitor a range of conditions
When to Get Tested?
As part of a routine metabolic panel; when you have symptoms of a disorder, or known presence of one, affecting your kidneys, bones, thyroid, parathyroid, or nerves or when symptoms of significantly increased or decreased calcium concentrations are present; when someone is critically ill, to monitor ionized calcium levels; when someone has certain types cancer; when someone is being treated for abnormal calcium levels, to evaluate the effectiveness of treatment
A blood sample drawn from a vein in your arm; sometimes a random or a timed urine collection such as a 24-hour urine sample
Test Preparation Needed?
Current practices do not require fasting. You may be instructed to stop taking certain medications that can affect the test results, such as lithium, antacids, diuretics, and vitamin D supplements, among others.
How is it used?
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.
When is it ordered?
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.
What does the test result mean?
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.
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:
- Prolonged immobilization
- Excess vitamin D intake
- Thiazide diuretics
- Kidney transplant
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.
Is there anything else I should know?
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.
What is being tested?
Calcium is the most abundant and one of the most important minerals in the body. It is essential for cell signaling and the proper functioning of muscles, nerves, and the heart. Calcium is needed for blood clotting and is crucial for the formation, density, and maintenance of bones. This test measures the amount of calcium in the blood or urine.
About 99% of calcium is found complexed in the bones, while the remaining 1% circulates in the blood. Calcium levels are tightly controlled; if there is too little absorbed or ingested, or if there is excess loss through the kidney or gut, calcium is taken from bone to maintain blood concentrations. Roughly half of the calcium in the blood is "free" and is metabolically active. The remaining half is "bound" to albumin, with a smaller amount complexed to anions, such as phosphate, and these bound and complexed forms are metabolically inactive.
There are two tests to measure blood calcium. The total calcium test measures both the free and bound forms. The ionized calcium test measures only the free, metabolically active form.
Some calcium is lost from the body every day, filtered from the blood by the kidneys and excreted into the urine. Measurement of the amount of calcium in the urine is used to determine how much calcium the kidneys are eliminating.
How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm. If a urine collection is required, a 24-hour urine sample or a timed collection of a shorter duration is obtained. Sometimes a random urine collection may be used, although a timed collection is preferred.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Current practices do not require fasting. You may be instructed to stop taking certain medications, such as lithium, antacids, diurectics, and vitamin D supplements, among others, to ensure the most accurate test results.
- Should I be concerned if my doctor only orders a total calcium test and not free or ionized calcium?
No. Measuring total calcium is usually sufficient for screening purposes. The total calcium test is more easily performed than the ionized calcium test since it requires no special handling of the blood sample.
The ionized calcium test requires specialized handling and expedited delivery of the blood sample to the laboratory for prompt analysis. The necessary handling and delivery is difficult to achieve outside of a hospital setting.
In most cases, total calcium is a good substitute for free calcium since the free and bound forms are usually each about half of the total.
The direct measurement of ionized calcium is particularly important during surgery as well as in severely ill patients, when changes in total calcium do not reliably tell how abnormal the ionized calcium level is.
- What foods are high in calcium?
Dairy products are the main source of calcium, but lesser amounts are found in eggs, green leafy vegetables, broccoli, legumes, nuts, and whole grains. Many fruit juices are now fortified with calcium.
- If I consume foods fortified with calcium, would it change my laboratory results?
In general, consuming fortified foods will not directly affect your calcium test results. Talk to your health care provider about your calcium requirements to help determine whether you should be taking calcium and/or vitamin supplements.
- Can I perform this test at home?
No. While there are hand-held instruments available, these are intended for use in a hospital or medical office setting and must be operated by trained personnel.
- My doctor ordered a test for albumin-corrected calcium. What is it?
Since about half of calcium in the blood is bound to albumin, an abnormally high or low level of albumin may affect the interpretation of calcium results and "free" or "ionized calcium" must be measured. (See "What is being tested?") Sometimes, however, laboratories do not have the resources to measure free, biologically active calcium directly. In these cases, some laboratories calculate an albumin-corrected calcium or adjusted calcium with a formula that uses the results of total calcium and albumin tests. This is sometimes used, for example, with patients who have liver disease or kidney failure.
However, some studies have questioned the usefulness of this practice. There is some uncertainty whether corrected calcium results provide a better evaluation of the amount of free calcium present in the blood compared to a standard total calcium test.
© 2017 American Association for Clinical Chemistry, republished from Lab Tests Online.*
Descriptions of clinical laboratory tests were originally prepared for use on Lab Tests Online, an award-winning patient education website on clinical laboratory testing. Lab Tests Online is produced by the American Association for Clinical Chemistry (AACC), a global scientific and medical professional organization dedicated to clinical laboratory science and its application to healthcare. The Lab Tests Online website is developed in collaboration with other laboratory professional societies and is funded in part through corporate sponsorships.