Electrolytes
Ionised calcium
Ionised calcium is the free, biologically active fraction of calcium in blood, roughly half of the total calcium and not bound to albumin or other anions. Point-of-care blood gas analysers measure it directly with an ion-selective electrode, giving a faster and more physiologically relevant result than total calcium, particularly when albumin or acid-base status is abnormal.
Why it is measured
It guides the management of acutely unwell patients, including critical care, massive transfusion of citrated blood products, regional citrate anticoagulation during renal replacement therapy, and the assessment of suspected hypocalcaemia or hypercalcaemia. Because it is the active form, it can reveal abnormalities that an albumin-adjusted total calcium may miss.
| Typical range | Indicative adult interval approximately 1.15 to 1.33 mmol/L (about 4.6 to 5.3 mg/dL). Ranges vary by analyser and method, and results are pH dependent, so each service should confirm the range supplied with its own device and customisation profile. |
|---|---|
| Sample | Whole blood: arterial, venous or capillary, handled anaerobically. Collect into a balanced (low) lithium heparin syringe; high or unbalanced heparin binds calcium and falsely lowers the result. Analyse promptly to limit pH drift, which alters the bound fraction. |
| Turnaround | Typically about 1 to 3 minutes once the sample is loaded onto a point-of-care blood gas analyser, reported alongside the other blood gas and electrolyte parameters. |
Point of care devices that report it
- Abbott i-STAT (CG8+ cartridge)
- Radiometer ABL90 FLEX
- Werfen GEM Premier 5000
- Nova Biomedical Stat Profile Prime
- Siemens Healthineers RAPIDPoint 500
- Roche cobas b 221
Questions, answered
How does ionised calcium differ from total calcium?
Total calcium measures all the calcium in the sample, including the portion bound to albumin and to anions such as bicarbonate and citrate. Ionised calcium measures only the free, physiologically active fraction directly with an ion-selective electrode. When albumin is low or acid-base status is disturbed, albumin-adjusted total calcium formulae can be unreliable, which is why direct ionised calcium is often preferred in acute and critical care settings.
Why must the sample be analysed quickly and handled anaerobically?
Calcium binding to albumin is pH dependent. If a sample is exposed to air or stands too long, loss of carbon dioxide raises the pH, more calcium binds to protein, and the measured ionised calcium falls. Anaerobic collection and prompt analysis on the point-of-care device keep the result representative of the patient's true state. This is general operational guidance, not interpretation of any individual result.
Does the type of heparin in the syringe matter?
Yes. Standard or excess heparin can chelate calcium and produce a falsely low ionised calcium. Manufacturers supply balanced or calcium-titrated (low) heparin tubes and syringes designed to minimise this effect, and these should be used for ionised calcium sampling. Follow the device and consumable instructions for the correct collection device.