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 Range
- Ionized calcium concentration normally ranges from 1.1 to 1.3 mmol/L in adults, as established by multiple European trauma guidelines and clinical practice standards 1
- When expressed in conventional units, this corresponds to 4.6 to 5.4 mg/dL 1
- Some older literature reports a slightly narrower range of 4.65 to 5.28 mg/dL (1.16 to 1.32 mmol/L), though the 1.1-1.3 mmol/L range is more widely accepted in current practice 2
- Research data from U.S. medical centers report mean critical low limits of 0.82 ± 0.14 mmol/L (3.29 ± 0.56 mg/dL) and high limits of 1.55 ± 0.19 mmol/L (6.21 ± 0.76 mg/dL), though these represent pathological thresholds rather than normal ranges 3
Physiologic Context
- Ionized calcium represents approximately 45-48% of total calcium in extracellular plasma 1
- The remaining calcium is bound to proteins (40%) or complexed with anions like phosphate, citrate, lactate, and bicarbonate (12-15%) 2, 1
- Total serum calcium normally ranges from 8.6 to 10.3 mg/dL (2.15 to 2.57 mmol/L) in adults, though this varies by laboratory method 2
Critical pH Dependency
- Ionized calcium levels are highly pH-dependent - each 0.1 unit increase in pH decreases ionized calcium concentration by approximately 0.05 mmol/L 1, 4
- 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 acid-base disturbances can significantly alter the ionized fraction without changing total calcium 1
Clinical Significance of Thresholds
- Ionized calcium <0.9 mmol/L requires immediate treatment, particularly in massive transfusion, post-surgical, or critically ill settings 1, 5
- Ionized calcium <0.8 mmol/L is particularly concerning due to dysrhythmia risk and requires urgent correction 5
- Ionized calcium is essential for fibrin polymerization, platelet function, cardiac contractility, systemic vascular resistance, and activation of coagulation factors II, VII, IX, and X 1
Common Pitfalls
- Do not rely solely on total calcium measurements when protein levels are abnormal, as correction formulas have significant limitations and ionized calcium measurement is superior for detecting subtle abnormalities 2, 1
- Standard coagulation tests may appear normal despite significant hypocalcemia-induced coagulopathy because laboratory samples are citrated then recalcified before analysis 1, 5
- Reproducibility of ionized calcium measurement is generally worse than total calcium, and the technique is more time-consuming and expensive, but it remains the gold standard when precise assessment is needed 2