Comparison of pre-filter and post-filter ionised calcium monitoring in continuous veno-venous hemodiafiltration (CVVHD-F) with citrate anti-coagulation

BACKGROUND: It is widespread practice during citrate anticoagulated renal replacement therapy to monitor circuit ionised calcium (iCa(2+)) to evaluate the effectiveness of anticoagulation. Whether the optimal site to sample the blood path is before or after the haemofilter is a common question. METHODS: Using a prospectively collected observational dataset from intensive care patients receiving pre-dilution continuous veno-venous haemodiafiltration (CVVHD-F) with integrated citrate anticoagulation we compared paired samples of pre and post filter iCa(2+) where the target range was 0.3–0.5 mmol.L(-1) as well as concurrently collected arterial iCa(2+). Two nested mixed methods linear models were fitted to the data describing post vs pre filter iCa(2+), and the relationship of pre, post and arterial samples. SETTING: An 11 bed general intensive care unit. PARTICIPANTS: 450 grouped samples from 152 time periods in seven patients on CRRT with citrate anticoagulation. RESULTS: The relationship of post to pre-filter iCa(2+) was not 1:1 with post = 0.082 + 0.751 x pre-filter iCa(2+) (95% CI intercept: 0.015–0.152, slope 0.558–0.942). Variation was greatest between patients rather than between circuits within the same patient or citrate dose. Compared to arterial iCa(2+) there was no significant difference between pre and post-filter sampling sites (F-value 0.047, p = 0.827) CONCLUSION: These results demonstrate that there is minimal difference between pre and post filter samples for iCa(2+) monitoring of circuit anticoagulation in citrate patients relative to the arterial iCa(2+) in CVVHD-F however compared to pre-filter sampling, post filter sampling has a flatter response and greater variation.