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Test Talk: Calcium

Test requirements and diagnostic value for those who draw blood

by Dennis Ernst • June 09, 2020


blood test graphic

It's amazing how many metabolic processes depend on calcium, making it an essential test not only for assessing well being, but for disease diagnosis.

Only about one percent of the body's calcium circulates in the bloodstream. Half of that is free (ionized); the rest is bound to albumin or anions like phosphate. Vitamin D and parathyroid hormone (PTH) are responsible for maintaining blood calcium levels in a narrow range. That's why physicians often order Vitamin D and PTH levels on patients that have abnormal calcium results.  When the balance between bound and free calcium is disturbed, a total calcium result is not very helpful, in which case the physician orders an ionized calcium level. 

Besides being part of a routine metabolic panel for wellness screening, abnormal calcium levels are associated with conditions affecting the kidneys, bones, thyroid, parathyroid, heart and nerves. With so many systems depending on this mineral to function properly, it's no wonder it's so frequently ordered.

Conditions and diseases in which total calcium levels become abnormal include kidney stones, thyroid and parathyroid disorders, malnutrition, malabsorption, bone diseases, and neurological disorders. Conditions in which ionized calcium provides better diagnostic information include recently transfused and infused patients, critically ill patients, patients undergoing major surgery, and those with abnormal blood proteins or protein levels.

Calcium levels are lowest in the bloodstream between 2:00-4:00 a.m. and highest at 8:00 p.m.

Total calcium is typically tested on serum samples whereas ionized calcium can be drawn into heparinized tubes as well. Neither require the patient to be fasting. Fist-pumping elevates ionized calcium levels. Serum to be tested for calcium is stable at room temperature for 48 hours before centrifugation, but heparinized samples for ionized calcium have an uncentrifuged room temperature stability of only two hours. After centrifugation, total calcium is stable for seven days at room temperature and ten days if kept refrigerated. Some studies suggest a 3-week stability for total calcium when stored in a refrigerated environment.


  • LabTestsOnline. American Association for Clinical Chemistry. AACC. Accessed 6/5/2020.
  • CLSI. Procedures for the Handling and Processing of Blood Specimens for Common Laboratory Tests; Approved Guideline—Fourth Edition. GP44-A4. Clinical and Laboratory Standards Institute. Wayne, Pennsylvania 2010.
  • Wu A. Tietz Clinical Guide to Laboratory Tests---Fourth Edition. Elsevier. St. Louis, Missouri. 2006.
  •  World Health Organization. Use of Anticoagulants in Diagnostic Laboratory Investigations. WHO. Geneva, Switzerland. 2002.



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