Background <p>Vascular access recirculation (AR) is an underrecognized but clinically significant cause of inadequate solute clearance during continuous kidney replacement therapy (CKRT). Access recirculation can be easily overlooked in the absence of circuit pressure alarms or obvious hemodynamic instability, potentially delaying diagnosis and compromising patient outcomes.</p> Case presentation <p>We describe a unique case of severe AR during CKRT with regional citrate anticoagulation (RCA) in a 75-year-old woman with long-standing diabetes mellitus and hypertension who underwent elective total pelvic exenteration and ileal conduit creation for cervical cancer. Her postoperative course was complicated by intraoperative haemorrhage, hypotension, and acute kidney injury, necessitating intensive care unit care, vasopressor support and CKRT-RCA via right internal jugular vein, and subsequently right femoral vein, non-tunnelled dialysis catheter. Following a period of improvement in solute control, she developed persistent high anion-gap metabolic acidosis and worsening uraemia despite uninterrupted CKRT-RCA. Notably, there were no circuit pressure alarms. There were progressively increasing systemic total and ionized calcium levels, with the total-to-ionized calcium ratio consistently below 2.5, necessitating cessation of intravenous calcium replacement. This raised suspicion of significant AR, which was confirmed by simultaneous blood sampling from the systemic circulation, access limb, and post-filter circuit sampling ports. The underlying cause of AR was identified as clots partially occluding the infrarenal inferior vena cava. After re-siting the dialysis catheter to a new site, circuit performance normalized, and biochemical parameters improved promptly.</p> Discussion <p>This case highlights a rare scenario where AR occurred without conventional warning signs, such as circuit pressure alarms. Sub-optimal solute and acid-base control, accompanied by a unique calcium profile, i.e. rising systemic total and ionized calcium levels, with a stable total-to-ionized calcium ratio, requiring decrement, and eventual discontinuation of intravenous calcium replacement, served as a diagnostic clue. These findings underscore the diagnostic value of targeted biochemical sampling from systemic, access, and circuit ports.</p> Conclusion <p>This report expands current understanding of AR in the context of CKRT-RCA and demonstrates how atypical biochemical patterns, particularly in calcium dynamics, can signal occult circuit dysfunction. Physicians should maintain a high index of suspicion and adopt strategic diagnostic sampling when faced with persistent treatment failure, even when standard CKRT circuit monitoring parameters appear unremarkable.</p>