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Electrolytes
Calcium (ionised) (iCa)
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Ionised calcium is the physiologically active fraction, independent of albumin concentration, and is measured with an ion-selective electrode, usually on a blood gas analyser.
Reference interval
| Conventional (mg/dL) | SI (mmol/L) | |
|---|---|---|
| Adults | 4.6–5.3 mg/dL | 1.15–1.32 mmol/L |
Specimen: Whole blood, serum or heparinised plasma (anaerobic) · Conventional × 0.2495 = SI
Causes of high values
Hypercalcaemic states: primary hyperparathyroidism, malignancy, vitamin D excess and acidosis (which increases the ionised fraction).
Causes of low values
Hypoparathyroidism, vitamin D deficiency, chronic kidney disease, massive transfusion (citrate), pancreatitis, sepsis, hypomagnesaemia and alkalosis (which lowers the ionised fraction).
Pre-analytical and method notes
Ionised calcium is pH-dependent (about 0.05 mmol/L change per 0.1 pH unit, inversely); samples must be collected anaerobically and analysed promptly. Reference intervals vary by analyser and laboratory.
References
- Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. Elsevier — Tier 3 (textbook)
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