Citrate anticoagulation for CRRT: don’t always trust the postfilter iCa results!

Citrate has been recommended as the first-line anticoagulant for continuous renal replacement therapy (CRRT) in critically ill patients. Compared with heparin, citrate anticoagulation is safer and more efficacious. Citrate inhibits the coagulation cascade by lowering the ionized calcium (iCa) concentration in the filter. Monitoring of systemic iCa concentrations is inherent to the protocol, and monitoring of postfilter iCa is recommended to adjust citrate flow and optimize anticoagulation. While systemic iCa targets are in the physiological range, postfilter iCa concentrations are targeted between 0.20 and 0.35 mmol/l. In a previous issue of Critical Care, Schwarzer et al. compared systemic and postfilter iCa measurements of patients receiving citrate-based CRRT between six devices. They highlight the unreliability of iCa concentrations in the postfilter range, because the instruments cannot be validated in the low iCa range. The maximum mean difference between two instruments was as high as 0.33 mmol/l (range 0.21–0.50 mmol/l). The authors call for dialysis companies to revise their protocols. However, the first implication of their study is that the accuracy of blood gas analyzers to measure iCa in the low range needs to improve; and, secondly, clinicians using citrate anticoagulation need to be aware that the postfilter iCa result may be falsely high or low. This is particularly relevant when frequent premature filter clotting is observed despite postfilter iCa results in the seemingly target range. In these situations, citrate flow can be safely increased up to 4 mmol/l blood flow under monitoring of signs of citrate accumulation.