Calcium
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- Also Known As:
- Total Calcium
- Ionized Calcium
- Formal Name:
- Calcium

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At a Glance
Why Get Tested?
To screen for, diagnose, and monitor a range of medical 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 of cancer; when someone is being treated for abnormal calcium levels, to evaluate the effectiveness of treatment
Sample Required?
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.
Test Preparation Needed?
Current practices do not require fasting. You may be instructed to stop taking certain medications, such as lithium, antacids, diuretics, and vitamin D supplements, among others, to ensure the most accurate test results.
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 and teeth. This test measures the amount of calcium in the blood or urine, which reflects the amount of total and ionized calcium in the body.
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 proteins, primarily albumin and to a lesser extend globulins, 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 and sweat. Measurement of the amount of calcium in the urine is used to determine how much calcium the kidneys are eliminating.
Common Questions
View Sources
Sources Used in Current Review
2017 review performed by William Nowatzke, PhD, DABCC, President, MeDx Consultants, LLC.
Thakker RV. The parathyroid glands, hypercalcemia, and hypocalcemia. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016.
Rifai N, Horvath AR, and Wittwer C. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Sixth Edition, ISBN: 978-0-323-35921-4, Elsevier, Inc. 2018.
D’Orazio P, [Guideline] Clinical and Laboratory Standards Institute. C31-A2 Ionized Calcium Determinations: Precollection Variables, Specimen Choice, Collection, and Handling; Approved Guideline – Second Edition. Vol.12, No.10:2001 ISBN Number: 1-56238-436-8.
D’Orazio P and Graham GA, [Guideline] Clinical and Laboratory Standards Institute. C39AE A Designated Comparison Method for the Measurement of Ionized Calcium in Serum, 1st Edition,: 2000 ISBN Number: 1-56238-398-1.
Calvi LM and Bushinsky DA, When Is It Appropriate to Order an Ionized Calcium? J Am Soc Nephrol 19: 1257–1260, 2008.
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.
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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 http://www.acpmedicine.com.
Dugdale, D. (Updated 2009 November 15). Calcium – blood test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003477.htm. Accessed May 2010.
Mayo Clinic Staff (2009 May 29). Hypercalcemia. MayoClinic [On-line information]. Available online at http://www.mayoclinic.com/print/hypercalcemia/DS00976/DSECTION=all&METHOD=print. Accessed May 2010.
Hemphill, R. (Updated 2009 August 5). Hypercalcemia. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/766373-overview. Accessed May 2010.
Beach, C. (Updated 2010 March 29). Hypocalcemia. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/767260-overview. 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 http://kidney.niddk.nih.gov/kudiseases/pubs/stonesadults/. Accessed Sept 2010.
(Updated Nov. 17, 2011.) Calcium – Blood Test. National Institutes of Health, Medline Plus. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003477.htm. Accessed October 2013.
(Updated Sept. 20, 2010.) Calcium (Ca) in Blood. WebMD. Available online at http://www.webmd.com/a-to-z-guides/calcium-ca-in-blood. 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 http://ndt.oxfordjournals.org/content/24/6/1834.long. 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). Testing.com adjunct board member. Director Clinical Pathology, VA TN Valley Healthcare System; Associate Professor Pathology, Microbiology, Immunology, Vanderbilt University School of Medicine.
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