Abstract
Continuous renal replacement therapy (CRRT) is a standard of care for critically ill patients according to the guidelines for Kidney Disease Improving Global Outcomes Clinical Practice. Within the CRRT dialysis instrument, blood requires anticoagulation with citrate with a goal to have ionized calcium (iCa) <0.4 mmol/L. In our hospital system, nephrologists who manage the therapy rotate through these hospitals, although in each of the hospitals, instruments that measure iCa are different. The objective of this study was to demonstrate the differences in measurement by these instruments.
Method
Twelve patient samples were drawn in triplicate and compared in the three different analyzers: RAPIDPoint, Nova Biomedical, and GEM4000 analyzers. One of the analyzers uses whole blood while the other two measure iCa in plasma. In one of the facilities, samples were spun, decapped, sent to the automated line outlet, and tested after having been exposed to air, and thus an experiment in another facility decapped the specimens for an hour and retested.
Results
Postfilter iCa measurements ranged from 0.18 to 0.64 mmol/L; however, range was different for each of the instruments: The Nova measurements averaged 0.55 mmol/L (range 0.42-0.64), the Rapid Point 0.36 (0.28-0.45), and the GEM 0.30 (0.18-0.39). The average difference between results of the Nova and GEM was 0.25 mmol/L, between Nova and RAPIDPoint 0.19 mmol/L, and GEM and RAPIDPoint 0.06 mmol/L. The differences between the values measured 1 hour after the sample had been decapped were negligible.
Conclusion
This study demonstrates inconsistency among instruments when measuring iCa in CRRT blood samples. The Nova analyzer tends to give higher results while the Rapid Point and GEM are lower and with values closer together. Currently, there is no reference material or reference method established for low iCa in CRRT samples. This study helped our clinicians understand the differences that occur between the different analyzers for low levels of iCa in CRRT blood samples.
Low levels of total and ionized calcium in blood of COVID-19 patients