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Also known as: Total Calcium; Ionized Calcium
Formal name: Calcium

At a Glance

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

Sample Required?

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.

The Test Sample

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.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

(Updated Nov. 17, 2011.) Calcium – Blood Test. National Institutes of Health, Medline Plus. Available online at through Accessed October 2013.

(Updated Sept. 20, 2010.) Calcium (Ca) in Blood. WebMD. Available online at through Accessed October 2013.

Labriola L, Wallemacq P, Gulbis B, Jadoul M. The impact of the assay for measuring albumin on corrected (‘adjusted’) calcium concentrations. Nephrol. Dial. Transplant. (2009) 24 (6): 1834-1838. Available online at through Accessed February 2014.

Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. 4th edition, St. Louis: Elsevier Saunders; 2006, Pp 1896-1897.

Gregory C. Sephel PhD FACB MT(ASCP). Lab Tests Online adjunct board member. Director Clinical Pathology, VA TN Valley Healthcare System; Associate Professor Pathology, Microbiology, Immunology, Vanderbilt University School of Medicine.

Sources Used in Previous Reviews

Stephen E. Kahn, PhD, DABCC. Professor, Pathology, Cell Biology, Neurobiology and Anatomy; Associate Director, Clinical Laboratories; Section Chief, Chemistry, Toxicology and Near Patient Testing; Loyola University Medical Center, Maywood, IL.

Mary F. Burritt, PhD. Professor of Laboratory Medicine, Division of Clinical Biochemistry/Immunology, Mayo Clinic, Rochester, MN.

Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 222-225.

Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC. Winter, w. and Harris, N. Chapter 34: Calcium Biology and Disorders. Pp 387-397.

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Clinical Chemistry: Principles, Procedures, Correlations. Bishop M, Fody E, Schoeff L, eds. 5th ed. Philadelphia: Lippincott Williams & Wilkins; 2004.

Clinical Chemistry: Theory, Analysis, and Correlations. Kaplan L, Pesce A, Kazmierczak, eds. 4th ed. St. Louis: The C. V. Mosby Company; 2002.

A Manual of Laboratory & Diagnostic Tests (seventh edition). Fischbach F, Dunning M, editor. Philadelphia: Lippincott Williams & Wilkins, 2003.

Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis C, Ashwood E, Bruns D, eds. St. Louis: Elsevier Inc., 2006.

ACP Medicine: VI Diseases of Calcium Metabolism and Metabolic Bone Disease. Holt E, Inzucchi S. American College of Physicians, Aug 2005. (Online reference, accessed July 2007) Available online through

Dugdale, D. (Updated 2009 November 15). Calcium – blood test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed May 2010.

Mayo Clinic Staff (2009 May 29). Hypercalcemia. MayoClinic [On-line information]. Available online at through Accessed May 2010.

Hemphill, R. (Updated 2009 August 5). Hypercalcemia. eMedicine [On-line information]. Available online at through Accessed May 2010.

Beach, C. (Updated 2010 March 29). Hypocalcemia. eMedicine [On-line information]. Available online at through Accessed May 2010.

Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 222-225.

Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 198-207.

(October 2007) National Kidney and Urologic Disease Information Clearininghouse. Kidney stones in Adults. Available online at through Accessed Sept 2010.