Critical Deviations of Ionized Calcium Measurements When Using Blood Gas Analyzers to Monitor Citrate Dialysis

During the course of acute kidney injury (AKI) patients may require renal replacement therapy (RRT). The preferred therapeutic measure for such patients is continuous RRT (CRRT). Anticoagulation is required to prevent clotting of the extracorporeal circuit. The actual KDIGO guidelines recommend citrate as the first line anticoagulant.

Citrate dose infused into the extracorporeal circuit should achieve an extracorporeal calcium concentration of 0.2 - 0.3 mmol/L. Here, we evaluated two blood gas analysers for their ability of covering the calcium concentration range needed for CRRT (Radiometer ABL 835; Instrumentation Laboratory GEM 4000). Measurements of iCa from 0.2 to 3.0 mmol/L were performed in aqueous 0.9% NaCl solutions with and without human serum albumin (HAS) and also in patient samples.

Using the GEM analyser, differences of measured results to target values were low throughout the whole concentration range. Using the ABL system, the difference increased with lower target values and exceeded up to 60% at 0.2 mmol/L. The results were reproduced in patient samples.

Measuring Ca2+ concentrations could result in an overdosing or underdosing of citrate when using an analytical method which is different to the instrument used initially to achieve the recommended concentrations. If measurement of the new method results in lower Ca2+ concentration and, therefore, reduced anticoagulation by citrate infusion this could lead to more clotting events. Overestimation of the calcium concentration by the new method in the extracorporeal circuit would result in an increased citrate dose delivered to the patient, leading to in vivo hypocalcemia and a pronouncement of citrate induced acid base derangements. Therefore, to monitor Ca2+ concentrations in CRRT during citrate anticoagulation, specific target values for each individual instrument must be established.