CRRT Dose Calculator

The RENAL trial (Randomized Evaluation of Normal versus Augmented Level of Replacement Therapy, NEJM 2009) randomised 1508 critically ill patients with AKI to receive CVVHDF at either 25 or 40 mL/kg/h. There was no significant difference in 90-day mortality (44.7% vs 44.7%), recovery of kidney function, or requirement for renal support at 90 days. This established 25 mL/kg/h as the evidence-based standard dose, with higher intensity not justified for routine AKI.

CRRT circuits are interrupted for filter changes (typically every 24-72 hours), procedural interventions (CT scans, surgery, line changes), filter clotting events, alarms, and nursing cares. These interruptions mean that actual uptime is typically 65-85% of the 24-hour day. To reliably deliver 25 mL/kg/h, most units prescribe 30-35 mL/kg/h, accepting that downtime will reduce the delivered dose to the target range.

CVVH (continuous veno-venous haemofiltration) removes solutes by convection — hydrostatic pressure pushes water and solutes across the membrane; replacement fluid is given to maintain fluid balance. CVVHD (haemodialysis) removes solutes primarily by diffusion — dialysate flows counter-currently to blood; no replacement fluid is needed. CVVHDF combines both mechanisms with both replacement fluid and dialysate, typically achieving better overall clearance, especially for middle molecules.

Regional citrate anticoagulation (RCA) delivers citrate (a calcium chelator) into the circuit to anticoagulate only within the filter, while calcium is replenished intravenously to maintain systemic coagulation. Multiple RCTs and meta-analyses show RCA extends filter lifespan by 50-100% compared to heparin, reduces bleeding risk, and may improve patient outcomes. RCA is the recommended approach in KDIGO guidelines unless contraindicated (severe liver failure, citrate accumulation risk, inability to monitor ionised calcium).

There is no universal consensus. Most guidelines and trials used actual body weight. However, in obese patients (BMI > 30), adipose tissue contributes little to urea generation, so using actual weight overestimates required clearance. Many centres use ideal body weight or adjusted body weight (IBW + 0.4 × (actual - IBW)) in obese patients. Locally agreed protocols should be followed.

In pre-dilution CVVH, replacement fluid is infused before the filter, diluting the blood and reducing solute concentrations presented to the membrane. This reduces effective clearance by approximately 15-20% compared to the raw effluent volume. A correction factor must be applied: corrected clearance = effluent rate × (blood flow / (blood flow + pre-dilution rate)). Pre-dilution is used to reduce haemoconcentration and extend filter life, particularly at high effluent doses or in hyperviscous states.

KDIGO 2012 AKI guidelines recommend starting CRRT when AKI is complicated by life-threatening fluid overload, hyperkalaemia (K > 6 mmol/L despite medical management), severe metabolic acidosis (pH < 7.15), uraemic encephalopathy or pericarditis, or drug/toxin elimination. Timing (early versus late initiation) has been investigated in the STARRT-AKI (2020) and IDEAL-ICU (2018) trials, both showing no benefit from routine early initiation; clinical indication-based timing is recommended.

Yes — CRRT significantly alters drug pharmacokinetics for renally cleared and moderately protein-bound drugs. Antibiotics particularly affected include vancomycin, piperacillin-tazobactam, meropenem, gentamicin, and fluconazole. Dosing must be adjusted based on CRRT effluent rate, drug molecular weight, protein binding, and volume of distribution. Specialist pharmacist input and therapeutic drug monitoring are essential in patients on CRRT to avoid under-dosing (treatment failure) or over-dosing (toxicity).