Calcium, Ionized
The ionised calcium test measures the free, unbound fraction of serum calcium—the only biologically active form—and is the definitive measure when total calcium is confounded by altered protein binding or acid–base disturbances. It is essential in critical illness, hypoalbuminaemia, perioperative monitoring, and acute symptomatic hypocalcaemia or hypercalcaemia, where it guides immediate…
1
Biomarkers
Blood (serum)
Sample
1–2 business days
Turnaround
No
Fasting
What is the iCa test?
The Ionised Calcium test measures the free, unbound fraction of serum calcium—the only form that is biologically active—and is ordered when total calcium is difficult to interpret or when acute, symptomatic disturbances are suspected. Because roughly half of total calcium circulates bound to albumin, changes in protein concentration or acid–base balance can shift total calcium without altering the ionised fraction, and ionised calcium alone reveals whether the physiologically active pool is truly abnormal. Practitioners rely on ionised calcium in the intensive care unit, during surgery, in patients with hypoalbuminaemia or critical illness, and whenever rapid, accurate calcium assessment is needed to guide resuscitation or intervention. It is also the preferred test for neonatal hypocalcaemia, suspected parathyroid or vitamin D disorders when total calcium is equivocal, and monitoring of patients receiving large-volume transfusion or citrate-based dialysis, where citrate binding can acutely lower ionised calcium without changing total levels.
What does the iCa test measure?
Ionised calcium represents the free, biologically active fraction of total serum calcium, accounting for roughly half the total pool. The remainder is bound to albumin and other proteins or complexed with anions such as citrate and phosphate. Ionised calcium participates directly in neuromuscular excitability, membrane stability, intracellular signalling, coagulation, and bone mineralisation. Its concentration is regulated within narrow limits by parathyroid hormone, calcitriol, and calcitonin, which together respond to moment-to-moment changes in free calcium to maintain physiological homeostasis.
Ionised calcium is the definitive measure of calcium status when protein binding is altered or acid–base disturbances are present. Total calcium can be misleading in hypoalbuminaemia, critical illness, and pH shifts, whereas ionised calcium reflects true availability and governs symptoms. It is the test of choice for diagnosing true hypocalcaemia and hypercalcaemia in the intensive care unit, operating theatre, and any setting where albumin or pH is abnormal, and it is essential for guiding acute intervention when neuromuscular or cardiac manifestations are present.
Why is the iCa test ordered?
- Investigation of suspected hypocalcaemia or hypercalcaemia in critically ill, post-surgical, or hospitalised patients, where albumin and pH shifts make total calcium unreliable
- Assessment of symptomatic hypocalcaemia—tetany, perioral or acral paraesthesiae, seizures, or prolonged QT interval—when rapid confirmation is needed
- Evaluation of neonatal or paediatric hypocalcaemia, especially in premature infants, infants of diabetic mothers, or following birth asphyxia
- Monitoring during massive transfusion, plasmapheresis, or citrate-based renal replacement therapy, where citrate chelation can acutely lower ionised calcium
- Work-up of primary hyperparathyroidism or suspected parathyroid dysfunction when total calcium is borderline or discordant with the clinical picture
- Intraoperative or perioperative monitoring during parathyroid or thyroid surgery, where acute changes in parathyroid hormone rapidly affect ionised calcium
- Assessment of acid–base disorders, where alkalosis raises and acidosis lowers protein binding, shifting total calcium independently of the ionised fraction
- Patients with chronic kidney disease, nephrotic syndrome, cirrhosis, or malnutrition, in whom hypoalbuminaemia confounds total calcium measurement
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
1–2 business days
Collection notes
Phlebotomy required at the processing lab's draw site (Quest, LabCorp, or Access Medical Laboratories, depending on routing). Strict pre-analytical handling is essential: collect in a syringe or tube free of liquid heparin (lithium-heparin validated for ionised calcium is acceptable), fill the tube completely to minimise air exposure, and transport anaerobically on ice if analysis will be delayed. Many point-of-care and central laboratory analysers measure ionised calcium on whole blood or serum; confirm the specimen type with the performing laboratory.
Specimen requirements
Minimum 1 mL whole blood or serum, collected anaerobically. Analyse within 30 minutes of collection, or transport on ice in a sealed syringe or tube. Avoid prolonged tourniquet application during venepuncture, which can transiently alter ionised calcium.
What can affect iCa results?
Alkalosis increases calcium binding to albumin, lowering ionised calcium even when total calcium is unchanged; acidosis decreases binding, raising the ionised fraction. Rapid correction of pH can precipitate symptomatic hypocalcaemia or hypercalcaemia.
Heparin binds calcium, falsely lowering ionised results; specimens must be collected in syringes free of liquid heparin, or in lithium-heparin tubes validated for ionised calcium measurement.
Loss of CO₂ from the specimen raises pH, increasing protein binding and spuriously lowering ionised calcium. Specimens must be analysed within 30 minutes or transported anaerobically on ice.
Ionised calcium is temperature-dependent; most analysers measure at 37°C and correct to the patient's body temperature. Hypothermia lowers, and hyperthermia raises, the ionised fraction.
Citrate in transfused blood products, during apheresis, or in dialysis binds ionised calcium, causing acute hypocalcaemia that is not reflected in total calcium. The clinical context must guide interpretation.
Acute infusion of calcium salts transiently raises ionised calcium; albumin infusion alters binding and may shift the ionised fraction. Timing of the draw relative to infusion affects the result.
What do iCa results mean?
Elevated
Hypercalcaemia — most commonly primary hyperparathyroidism or malignancy (humoral hypercalcaemia of malignancy, osteolytic metastases). Other causes include vitamin D intoxication, granulomatous disease, immobilisation, thiazide diuretics, lithium, and familial hypocalciuric hypercalcaemia. Severe hypercalcaemia presents with confusion, polyuria, renal impairment, and shortened QT interval; ionised calcium directs acute treatment.
Normal
A normal ionised calcium, particularly when total calcium is abnormal, indicates that the active fraction is preserved and that the discrepancy is due to protein binding or pH. It reassures that acute intervention is not required, though the underlying cause of the total calcium abnormality should still be addressed.
Reduced
Hypocalcaemia — causes include hypoparathyroidism (post-surgical, autoimmune, or genetic), vitamin D deficiency, chronic kidney disease with secondary hyperparathyroidism, hypomagnesaemia (which impairs PTH secretion and action), acute pancreatitis, citrate or phosphate binding, and hungry bone syndrome following parathyroidectomy. Symptoms include tetany, Chvostek and Trousseau signs, seizures, laryngospasm, and QT prolongation; ionised calcium is the definitive test and guides replacement.
Note: Ionised calcium must be collected and analysed under strict pre-analytical conditions — anaerobic handling, prompt analysis, and avoidance of heparin contamination are essential. Interpret alongside total calcium, albumin, magnesium, phosphate, PTH, and vitamin D, and correlate with the clinical setting, particularly acid–base status and recent infusions. A single ionised calcium does not diagnose the underlying disorder; it confirms whether the physiologically active pool is abnormal and guides immediate management.
Frequently asked questions
Total calcium measures all forms in serum — bound to albumin, complexed with anions, and free. Ionised calcium measures only the free, active fraction. When albumin or pH is abnormal, total calcium can be misleading, and ionised calcium is the definitive test.
How to access the iCa test
The iCa reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- Liamis G, et al. Hypocalcemia in patients with acute pancreatitis: pathophysiology and clinical implications. Pancreatology. (2014)
- Schafer AL, Shoback DM. Hypocalcemia: diagnosis and treatment. In: Endotext [Internet]. MDText.com. (2022)
- Bove-Fenderson E, Mannstadt M. Hypocalcemic disorders. Best Pract Res Clin Endocrinol Metab. (2018)
- Zaloga GP. Hypocalcemia in critically ill patients. Crit Care Med. (1992)
- Bushinsky DA, Monk RD. Electrolyte quintet: Calcium. Lancet. (1998)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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