Stability of ionized calcium measurements at concentrations less than 0.3 mmol/L by point-of-care blood gas instruments: application for postfilter calcium quality control in patients with citrate anticoagulation during extracorporeal circulation.

Background Measurements of ionized calcium (Ca(2+)) at concentrations less than 0.3 mmol/L are required for postfilter control in patients who receive extracorporeal circulation with sodium citrate anticoagulation. This study evaluates the stability of the Ca(2+) measurements at such concentrations. Methods The stability of the Ca(2+) measurements was tested by measuring daily the external standard Qualicheck concentration 3 s7950, Radiometer (0.22-0.25 mmol/L) by blood gas instruments ABL800 and ABL90, Radiometer. Two different Ca(2+) membrane lots were tested for the usual membrane lifetime of 12 weeks at ABL800 instruments. For the ABL90 instrument, the sensor cassette (with Ca(2+) membrane and electrode) was replaced after four weeks as required. Results We observed over 40% Ca(2+) increase within the usual 12 weeks lifetime of the Ca(2+) membrane at the ABL800 instruments. Measurements of Ca(2+) at concentrations less than 0.3 mmol/L were within acceptable limits for both ABL800 and ABL90 instruments when Ca(2+) membrane and sensor cassette were replaced after four weeks. Conclusions For ABL800 instruments, it is necessary to use an extra quality control (<0.3 mmol/L) in addition to the usual quality controls to monitor Ca(2+) measurements below 0.3 mmol/L. The acceptable stability of the Ca(2+) measurements can be achieved by the Ca(2+) membrane and sensor cassette replacement after four weeks. If the usual 12 weeks of Ca(2+) membrane lifetime is maintained, it may result in a clinically significant overestimation of Ca(2+) by ABL800 instruments.