Citrate accumulation during CRRT — FFICM MCQ

Correct answer: A — Citrate accumulation

Citrate accumulation (option B) is the correct diagnosis. In liver failure and shock, citrate metabolism (normally via the Krebs cycle in liver, kidney and muscle) is impaired, so citrate-calcium complexes accumulate in the circulation rather than being cleared. This causes a falling ionised calcium (more calcium chelated by unmetabolised citrate) despite escalating calcium replacement, a rising total calcium (reflecting the bound, unmeasured calcium-citrate complexes), and worsening metabolic acidosis because citrate itself is an acid load and its failure to be metabolised to bicarbonate removes a normal buffering step. A total-to-ionised calcium ratio rising above 2.5, alongside a raised lactate from hepatic and shock-related hypoperfusion, is the classic diagnostic pattern in this setting. Why the other options are wrong: B. Heparin-induced thrombocytopenia: this patient is on citrate, not heparin, anticoagulation, and HIT causes thrombocytopenia and thrombosis, not calcium derangement or acidosis. C. Filter clotting from under-anticoagulation: circuit pressures are explicitly acceptable here, and clotting would present with rising transmembrane pressure and circuit loss, not systemic calcium and acid-base changes. E. Hypercalcaemia of malignancy: this raises both total and ionised calcium together and is unrelated to CRRT circuit chemistry or acidosis. D. Respiratory alkalosis: this stem describes worsening acidosis, the opposite acid-base direction, and respiratory alkalosis does not explain the calcium pattern. Key point: a rising total-to-ionised calcium ratio (greater than 2.5) with worsening acidosis during regional citrate anticoagulation signals citrate accumulation, particularly in patients with liver failure or shock who cannot metabolise citrate.

Reference: Buckinghamshire Healthcare NHS Trust, Guideline 404FM.4: Deranged Physiology - Renal Intensive Care (CRRT), 'Signs of citrate accumulation include metabolic acidosis or worsening acidosis... increasing free (ionized) calcium but a rise in total calcium... rapidly rising lactate or lactate >5mmol/L', https://www.bucksformulary.nhs.uk/docs/Guideline_404FM.pdf