Accuracy of commercial blood gas analyzers for monitoring ionized calcium at low concentrations

Background: Variable ionized calcium measurements in post filter blood samples during continuous renal replacement therapy (renal dialysis) using regional citrate anticoagulation (RCA) have been reported using commercial blood gas analyzers, resulting in analyzer-dependent differences in decisions regarding adjustment of citrate dose.

Methods: We evaluated accuracy for measurement of iCa at low concentrations by 4 commercial blood gas analyzers using primary reference solutions formulated down to 0.15mmol/l iCa.

Results: Of the 4 analyzers tested, GEM Premier 4000 demonstrates acceptable accuracy for iCa measurement with a median deviation of -6.7% (-0.01mmol/l) at 0.15mmol/l, while other analyzers tested show increasingly positive biases from +40% (+0.06mmol/l) to +60% (+0.09mmol/l) relative to target. These relative differences are consistent with discordant results reported for measurement of iCa in blood during RCA. Interference from sodium with measured iCa and carryover from system rinse solution to sample are likely contributors to variability.

Conclusions: We conclude the GEM Premier 4000 shows acceptable accuracy for measuring iCa at low concentrations required to control citrate dose during RCA. The method presented here may be used to test accuracy of any blood gas analyzer prior to use in clinical applications requiring measurement of iCa at low concentrations.