Received on August 21, 2008
Accepted on December 22, 2008

Evidence-based Medicine and Test Utilization

Reducing Routine Ionized Calcium Measurement

¹ Department of Laboratory Medicine, University of Washington, Seattle, Washington

^* To whom correspondence should be addressed. E-mail: gbaird{at}u.washington.edu.

BACKGROUND: Ionized calcium (iCa) is measured frequently in hospitalized patients, and hypocalcemia is frequently found, seemingly supporting the practice.

METHODS: We retrieved the results of 58 040 iCa tests and records of intravenous (IV) and oral calcium supplementation from laboratory and hospital information systems and evaluated them for frequency of testing, frequency of hypocalcemia, and effects of calcium supplementation.

RESULTS: Serial and daily iCa testing was common and responsible for a substantial fraction of all iCa tests ordered. Half of all patients tested had iCa values below the reference interval. IV, but not oral, calcium therapy increased mean iCa concentrations, but the effect of calcium administration was small compared with the spontaneous increase in iCa that occurred in similar patients who received no calcium treatment. A retrospective analysis suggested that a low total calcium (tCa) concentration (<2.00 mmol/L, <8 mg/dL) could identify most patients with low iCa (<1.0 mmol/L). Introduction of a reflexive strategy reduced iCa testing by 72%–76% and reduced IV calcium gluconate therapy by 45%–81%.

CONCLUSIONS: Testing for iCa and IV calcium supplementation were significantly reduced with a reflexive calcium testing strategy that provided iCa testing only to patients with low tCa. Adverse clinical outcomes possibly associated with hypocalcemia did not increase.