Q: What is the formula to anticipate excess citrate in Continuous Renal Replacement Therapy (CRRT) patient? 


Answer: Total Ca/iCa concentration

 Sodium citrate has a complex effect while CRRT. Citrate is not only an anticoagulant but a buffer generator as well as citric acid.

Na3 citrate + 3H2CO3 → citric acid (C6H8O7) + 3NaHCO3

Secreted citric acid mostly get metabolized in liver's mitochondria which may decreased in liver insufficiency. If citrate accumulates,  Ionized calcium (iCa) decreases and metabolic acidosis ensues (because bicarbonate continues to be removed by CRRT). Accumulation of citrate due to decreased metabolism can be predicted by ratio

 Total Ca/iCa concentration

A ratio of more than 2.1 or 2.5 is c/w a citrate concentration of greater than 1 mmol/l.

Also clinically, Citric acid accumulation cause increased anion gap metabolic acidosis.

Same formula can be applied in patients receiving a massive transfusion, as blood products contain substantial amount of sodium citrate.



References: 

1.  Bakker AJ, Boerma EC, Keidel H, Kingma P, van der Voort PH. Detection of citrate overdose in critically ill patients on citrate-anticoagulated venovenous haemofiltration: use of ionised and total/ionised calcium. Clin Chem Lab Med. 2006;44:962–966. 

2. Meier-Kriesche HU, Gitomer J, Finkel K, DuBose T. Increased total to ionized calcium ratio during continuous venovenous hemodialysis with regional citrate anticoagulation. Crit Care Med. 2001;29:748–752