Normal Value of Ionized Calcium
The normal range for ionized calcium in adults is 1.1–1.3 mmol/L (or 4.6–5.4 mg/dL). 1
Standard Reference Values
- Ionized calcium concentration normally ranges from 1.1 to 1.3 mmol/L in adults, as established by multiple European trauma guidelines 1
- When expressed in conventional units, this corresponds to 4.6 to 5.4 mg/dL 1
- Some sources cite a slightly narrower range of 1.16 to 1.32 mmol/L (4.65 to 5.28 mg/dL) for adults 2
- The most recent guidelines consistently support the 1.1–1.3 mmol/L range as the standard reference 1
Physiological Context
- Ionized calcium represents approximately 45-48% of total calcium in extracellular plasma 1
- The remainder is bound to proteins (40%) or complexed with anions like phosphate, citrate, and bicarbonate (12-15%) 2, 1
- This free calcium fraction is the physiologically active form essential for cardiac contractility, coagulation, and neuromuscular function 1
Critical pH Dependency
Each 0.1 unit increase in pH decreases ionized calcium concentration by approximately 0.05 mmol/L. 1
- This occurs because hydrogen ions displace calcium from albumin binding sites, while alkalosis enhances calcium binding to albumin 2, 1
- You must account for pH changes when interpreting ionized calcium levels, as the measured value is highly pH-dependent 1
- In patients with acid-base disturbances, actual ionized calcium (not pH-corrected) is the preferred measurement 1
Clinical Thresholds
- Ionized calcium <0.9 mmol/L requires immediate treatment, particularly in massive transfusion, post-surgical, or critically ill settings 1, 3
- Levels <0.8 mmol/L are particularly concerning due to increased dysrhythmia risk 3
- Critical low limits at most medical centers average 0.82 ± 0.14 mmol/L 4
- Critical high limits average 1.55 ± 0.19 mmol/L at medical centers 4
Common Pitfalls
- Do not rely solely on total calcium measurements when protein levels are abnormal, as correction formulas have significant limitations 1
- Laboratory coagulation tests may appear normal despite hypocalcemia because samples are citrated then recalcified before analysis, masking the true impact 1, 3
- Even mild hypocalcemia impairs the coagulation cascade and platelet function, so borderline values should not be ignored in critically ill patients 1, 3