Background: Continuous renal replacement therapy (CRRT) with regional citrate anticoagulation (RCA) is now commonly used to treat acute kidney injury in critically ill patients. The concentration of citrate is not routinely measured, with citrate accumulation and/or toxicity primarily assessed using surrogate measures. Objectives: The aim of this study was to measure the concentration of citrate in plasma and ultrafiltrate in patients receiving CRRT with RCA using a modified commercial enzymatic assay. Methods: After meeting inclusion criteria, blood was sampled from 20 patients before, during, and after episodes of filtration. Using spectrophotometry, samples were tested for citrate concentration. Demographic and other clinical and biochemical data were also collected. Throughout, a 15 mmol/L solution of trisodium citrate was used as the prefilter anticoagulant. Results were analysed using STATA (v15.0) and presented as mean (SD), median (IQR), or simple proportion. Comparisons were made using either the Student t test or the Wilcoxon rank-sum test. Correlation was assessed using Pearson's r. Results: Twenty patients (17 males) were enrolled in the study. Mean (SD) age was 63.7 years (9.9). Median (IQR) ICU length of stay was 281 h (199, 422) with 85% undergoing intermittent positive pressure ventilation. Median APACHE 3 score was 95 (87, 117) with an overall 30% mortality rate. Median filtration time was 85 h (46, 149). No difference was found between pre- A nd post-filtration plasma citrate concentrations (79 μmol/L [50] vs. 71 μmol/L [42], p = 0.65). Mean citrate concentration during filtration was 508 μmol/L (221) with a maximum of 1,070 μmol/L. This was significantly higher than the pre/post levels (p < 0.001). Conclusions: Plasma concentrations of citrate rose significantly during episodes of CRRT using RCA returning back to normal after cessation of treatment.