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Test ID: CAI Calcium, Ionized, Serum

Reporting Name

Calcium, Ionized, S

Useful For

Assessing calcium states during liver transplantation surgery, cardiopulmonary bypass, or any procedure requiring rapid transfusion of whole blood in neonates and critically ill patients


Second-order test in the evaluation of patients with abnormal calcium values

Clinical Information

Ionized calcium, which accounts for 50% to 55% of total calcium, is the physiologically active form of calcium.


Low ionized calcium values are often seen in kidney disease, critically ill patients, or patients receiving rapid transfusion of citrated whole blood or blood products.


Increased serum ionized calcium concentrations may be seen with primary hyperparathyroidism, ectopic parathyroid hormone-producing tumors, excess intake of vitamin D, or various malignancies.


Nomograms have been used to calculate ionized calcium from total calcium, albumin, and pH values. However, calculated ionized calcium results have proven to be unsatisfactory. A Mayo study of 114 patients found significant differences between ionized and total calcium in 26% of patients.


Serum ionized calcium concentrations 50% below normal will result in severely reduced cardiac stroke work. With moderate to severe hypocalcemia, left ventricular function may be profoundly depressed.


Ionized calcium values are higher in children and young adults.


Ionized calcium result has been adjusted to pH 7.40 to account for changes in specimen pH that may occur during transport. Ionized calcium concentration increases approximately 0.2 mg/dL per 0.1 pH unit decrease.

Report Available

Same day/1 day

Day(s) Performed

Monday through Sunday

Clinical Reference

Rifai N, Horwath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018

Method Name

Ion-Selective Electrode (ISE)

Specimen Type

Serum SST

Specimen Required

Container/Tube: Serum gel or serum gel microtainer

Specimen Volume: Full tube

Collection Instructions:

1. Allow blood to clot for 30 minutes.

2. Serum gel tube/microtainer must be centrifuged within 1 hour of collection. Centrifuge with stopper in place for 7 minutes at 3000 rpm to ensure that the gel barrier separates the serum and cells.

3. Keep specimen anaerobic; do not aliquot.

Specimen Minimum Volume

1.75 mL in a 3.5 mL (50% full) in serum gel tube or 1 full serum gel microtainer

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum SST Refrigerated 7 days SERUM GEL TUBE

Reference Values


≤13 days old: Not established

14 days-<1 year: 5.21-5.99 mg/dL

1-<2 years: 5.04-5.84 mg/dL

2-<3 years: 4.87-5.67 mg/dL

3-23 years: 4.83-5.52 mg/dL

24-97 years: 4.57-5.43 mg/dL

≥98 years: Not established



≤13 days old: Not established

14 days-97 years old: 7.35-7.48

≥98 years old: Not established


For SI unit Reference Values, see

Test Classification

This test has been modified from the manufacturer's instructions. Its performance characteristics were determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the US Food and Drug Administration.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
CAI Calcium, Ionized, S 57333-7


Result ID Test Result Name Result LOINC Value
CAIS Calcium, Ionized, S 57333-7
PHCC pH 2753-2


If not ordering electronically, complete, print, and send a Renal Diagnostics Test Request (T830) with the specimen. 

Mayo Clinic Laboratories | Endocrinology Catalog Additional Information: