Proper Method for Collecting Ionized Calcium
Collect ionized calcium using a heparinized syringe filled completely to capacity, analyze immediately or within 40 minutes, maintain anaerobic conditions throughout collection and handling, and avoid excess heparin which artificially lowers results.
Collection Technique
Syringe Selection and Anticoagulation
- Use heparinized syringes with a measured, standardized quantity of heparin rather than unmeasured amounts, as excess heparin artificially lowers ionized calcium concentrations 1
- Fill syringes completely to capacity - half-filled syringes can produce clinically significant decreases in ionized calcium measurements (up to 0.028 mmol/L lower) 2
- Balanced heparin or lithium heparin whole blood are both acceptable specimen types, though balanced heparin shows slight decline after 40 minutes while lithium heparin remains stable 3
Sample Handling Requirements
- Maintain strict anaerobic conditions during collection and handling to prevent CO₂ loss, which affects pH and subsequently ionized calcium levels 4, 3
- Analyze samples immediately or within 40 minutes of collection for optimal accuracy 3
- For lithium heparin specimens, there is no significant change over 40 minutes, whereas balanced heparin specimens show a small but significant decrease (-0.030 mmol/L) 3
Critical Pitfalls to Avoid
pH-Related Errors
- Prevent CO₂ loss from the sample, as ionized calcium varies inversely with pH - a 0.1 unit increase in pH decreases ionized calcium by approximately 0.05 mmol/L 5, 4
- Do not use pH-adjusted ionized calcium values unless both calcium and pH measurements are rigorously standardized, as reference intervals for pH-adjusted values can be dramatically skewed 3
Specimen Type Errors
- Avoid plasma specimens - the reference interval for plasma is dramatically lower than whole blood and produces unreliable results 3
- Serum can be used and has similar reference intervals to whole blood, though it may be affected by sample processing and clotting 2
- Do not use heparinized whole blood for comparison with serum values at normal blood pH, as heparinized whole blood ionized calcium is significantly lower due to calcium-heparin complex formation 4
Special Considerations for Critical Care Settings
Monitoring During Citrate Anticoagulation
- During continuous renal replacement therapy with citrate anticoagulation, perform frequent measurements of post-filter and serum ionized calcium to appropriately titrate citrate and calcium replacement solutions 6
- Monitor ionized calcium levels every 4-6 hours initially until stable, then twice daily in critically ill patients requiring calcium management 7