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Continuous Renal Replacement Therapy (CRRT) dose refers to the prescribed effluent flow rate, expressed in millilitres per kilogram of body weight per hour (mL/kg/h), which quantifies the intensity of solute clearance delivered to a critically ill patient with acute kidney injury (AKI) in the intensive care unit. Unlike intermittent haemodialysis which operates over 3-4 hours, CRRT runs continuously for 24 hours a day, providing slower, gentler solute and fluid removal that is better tolerated in haemodynamically unstable patients. The effluent in CRRT comprises the ultrafiltrate (fluid and small solutes pushed across the membrane by pressure) plus any dialysate flow passing counter-currently through the filter. The total effluent rate is the sum of replacement fluid rate plus dialysate rate. The KDIGO AKI guidelines (2012) recommend a delivered CRRT dose of at least 20-25 mL/kg/h. The landmark RENAL trial (2009) compared 40 mL/kg/h versus 25 mL/kg/h and found no mortality benefit from higher intensity, establishing 25 mL/kg/h as the standard of care. A crucial distinction exists between prescribed dose and delivered dose — circuit downtime due to clotting, filter changes, procedures, and nursing interventions means the actual delivered dose is typically 65-80% of the prescribed dose. To reliably deliver 25 mL/kg/h, most protocols prescribe 30-35 mL/kg/h to compensate for expected downtime. Anticoagulation (regional citrate preferred, or systemic heparin) and pre- versus post-dilution mode further influence actual solute clearance.
Effluent Dose (mL/kg/h) = (Replacement Fluid Rate [mL/h] + Dialysate Rate [mL/h]) / Patient Weight [kg]; Delivered Dose = Prescribed Dose × Circuit Uptime Fraction
- 1Determine the CRRT modality: CVVH (haemofiltration, convection only — uses replacement fluid), CVVHD (haemodialysis, diffusion only — uses dialysate), or CVVHDF (combined haemodiafiltration — uses both); this determines which flows contribute to effluent
- 2Determine the patient's actual body weight; in oedematous or obese patients, ideal body weight or adjusted body weight may be used as per local protocol to avoid over-prescribing
- 3Set target effluent dose based on clinical indication — minimum 20-25 mL/kg/h for standard AKI; some centres target 30-35 mL/kg/h prescription to compensate for expected circuit downtime and achieve a delivered dose of 25 mL/kg/h
- 4Calculate required total effluent flow: multiply target dose (mL/kg/h) by patient weight (kg) to get total mL/h; split between replacement fluid and dialysate flows based on chosen modality
- 5Specify pre-dilution versus post-dilution for replacement fluid — pre-dilution reduces haemoconcentration and extends circuit life but reduces effective solute clearance by 15-20% (a correction factor must be applied); post-dilution achieves higher clearance but causes haemoconcentration and faster filter clotting
- 6Choose anticoagulation strategy: regional citrate anticoagulation (RCA) chelates calcium in the circuit to prevent clotting while calcium is replenished systemically — preferred for most patients due to superior circuit lifespan; systemic heparin is an alternative but carries bleeding risk
- 7Monitor delivered dose regularly by calculating actual effluent volume collected over 8-12 hour periods; adjust prescribed dose upward if delivered dose is consistently below target due to circuit downtime
Prescribe ~33 mL/kg/h to reliably deliver 25 mL/kg/h
With 75% circuit uptime, prescribing 33 mL/kg/h ensures the patient actually receives approximately 25 mL/kg/h of effective clearance over 24 hours. This is standard practice based on the RENAL trial target.
Pre-dilution reduces effective solute clearance — must apply correction factor
Pre-dilution dilutes the blood before it enters the filter, reducing the concentration of solutes available for filtration. The correction factor accounts for this, showing the effective clearance is ~18% less than the raw effluent volume would suggest.
Higher CRRT doses justified for specific indications (hyperammonaemia, rhabdomyolysis)
While the RENAL trial showed no mortality benefit from 40 versus 25 mL/kg/h for general AKI, certain specific indications (hyperammonaemia, severe intoxication, rhabdomyolysis) may benefit from higher-intensity clearance to reduce specific toxin levels more rapidly.
Use IBW or adjusted body weight in obese patients to avoid over-prescription
Adipose tissue has lower metabolic activity and lower urea generation than lean mass. Using actual body weight in severely obese patients over-estimates required clearance and increases risks from high-volume fluid exchanges.
Setting CRRT prescriptions for ICU patients with AKI to achieve the KDIGO target of 25 mL/kg/h delivered dose, representing an important application area for the Renal Replacement Dose in professional and analytical contexts where accurate renal replacement dose calculations directly support informed decision-making, strategic planning, and performance optimization
Calculating the required prescribed dose to compensate for predictable circuit downtime and consistently deliver the target dose over 24 hours, representing an important application area for the Renal Replacement Dose in professional and analytical contexts where accurate renal replacement dose calculations directly support informed decision-making, strategic planning, and performance optimization
Monitoring and comparing prescribed versus delivered dose during clinical rounds to identify and correct systematic under-dosing, representing an important application area for the Renal Replacement Dose in professional and analytical contexts where accurate renal replacement dose calculations directly support informed decision-making, strategic planning, and performance optimization
Adjusting CRRT intensity for specific indications — hyperammonaemia, drug overdose, or severe rhabdomyolysis — that require higher clearance than the standard AKI dose, representing an important application area for the Renal Replacement Dose in professional and analytical contexts where accurate renal replacement dose calculations directly support informed decision-making, strategic planning, and performance optimization
Guiding antibiotic and drug dose adjustments in patients receiving CRRT to maintain therapeutic drug levels and avoid treatment failure or toxicity, representing an important application area for the Renal Replacement Dose in professional and analytical contexts where accurate renal replacement dose calculations directly support informed decision-making, strategic planning, and performance optimization
Liver Failure and Citrate Accumulation
{'title': 'Liver Failure and Citrate Accumulation', 'body': 'In severe liver failure, regional citrate anticoagulation carries the risk of citrate accumulation because the liver normally metabolises citrate. Citrate toxicity manifests as a widening gap between total calcium and ionised calcium (total Ca rises while ionised Ca falls), high anion gap metabolic acidosis, and cardiac arrhythmias. Monitor the total-to-ionised calcium ratio (> 2.5 suggests accumulation). Switch to heparin or prostacyclin if citrate accumulation is detected.'}
Hyperammonaemia in Liver Failure or Metabolic Disease
{'title': 'Hyperammonaemia in Liver Failure or Metabolic Disease', 'body': 'CRRT is frequently used for rapid ammonia removal in acute liver failure, urea cycle disorders, and post-liver transplant hyperammonaemia. Standard dosing (25 mL/kg/h) achieves approximately 10-15% ammonia reduction per hour. Higher doses (45-60 mL/kg/h) or combining with plasma exchange significantly accelerates ammonia clearance. Target ammonia < 100 micromol/L to prevent irreversible cerebral injury.'}
Drug Overdose and Toxin Removal
{'title': 'Drug Overdose and Toxin Removal', 'body': 'CRRT is effective for removal of small water-soluble drugs with low protein binding and small volume of distribution (e.g., lithium, metformin, salicylate, some alcohols). High-dose CRRT (40-60 mL/kg/h) combined with maximum blood flow rates maximises toxin clearance. For larger molecules or highly protein-bound drugs, haemoperfusion cartridges combined with CRRT may be more effective.'}
Fluid Overload as Primary Indication
{'title': 'Fluid Overload as Primary Indication', 'body': 'In critically ill patients with severe fluid overload (> 10% fluid overload, defined as (fluid in - fluid out) / admission weight × 100), the ultrafiltration component of CRRT can be set independently of the effluent dose to achieve net negative fluid balance. The net ultrafiltration rate (NUF) should typically not exceed 1-2 mL/kg/h to avoid haemodynamic compromise from rapid fluid removal.'}
Paediatric CRRT Dosing
{'title': 'Paediatric CRRT Dosing', 'body': 'In paediatric patients, CRRT is dosed by weight (mL/kg/h) using the same principles. However, circuit prime volume can represent a significant fraction of circulating blood volume in small children (< 8-10 kg), requiring circuit priming with packed red blood cells to avoid haemodilution. Paediatric CRRT requires specialist equipment with smaller filter sizes, lower blood flow rates, and careful attention to filter pressure alarms.'}
| Study / Guideline | Doses Compared | Outcome | Recommendation |
|---|---|---|---|
| RENAL Trial 2009 | 25 vs 40 mL/kg/h | No mortality difference | 25 mL/kg/h standard |
| ATN Study 2008 | 20 vs 35 mL/kg/h (IHD/CRRT) | No difference | Lower intensity sufficient |
| KDIGO AKI 2012 | Guideline recommendation | Target 20-25 mL/kg/h delivered | Prescribe 30-35 to account for downtime |
| Hyperammonaemia | 45-60 mL/kg/h | Accelerated ammonia removal | Higher dose for specific indications |
| Rhabdomyolysis | 35-45 mL/kg/h | Faster myoglobin clearance | Higher dose for massive rhabdomyolysis |
| RCA vs Heparin meta-analysis | Anticoagulation comparison | RCA superior filter life | RCA first-line if no contraindication |
What did the RENAL trial show about CRRT dose?
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.
Why is the prescribed dose higher than the target delivered dose?
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.
What is the difference between CVVH, CVVHD, and CVVHDF?
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.
Why is regional citrate anticoagulation preferred over heparin?
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).
What body weight should be used for CRRT dose calculation?
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.
How does pre-dilution affect CRRT clearance?
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.
When should CRRT be initiated in AKI?
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.
Can CRRT remove antibiotics and medications?
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).
Pro Tip
Always calculate the delivered dose retrospectively by dividing the actual effluent volume collected over 24 hours by the patient's weight. If the delivered dose is consistently below 20 mL/kg/h, increase the prescribed rate or address the cause of excessive downtime (anticoagulation, access, alarms).
Did you know?
The concept of continuous renal replacement therapy was developed in the late 1970s by Peter Kramer, who first described continuous arteriovenous haemofiltration (CAVH) in 1977 using arterial blood pressure alone to drive ultrafiltration. The shift to pump-driven veno-venous access (CVVH) in the 1980s made the therapy safer, more controllable, and suitable for haemodynamically unstable ICU patients.
References
- ›RENAL Trial — NEJM 2009: Intensity of Continuous Renal-Replacement Therapy in Critically Ill Patients
- ›KDIGO Clinical Practice Guideline for Acute Kidney Injury 2012
- ›ATN Study — NEJM 2008: Intensity of Renal Support in Critically Ill Patients with Acute Kidney Injury
- ›Ostermann M et al — Recommendations on Acute Kidney Injury Biomarkers from the AKI7 Working Group (ADQI 21)
- ›STARRT-AKI Investigators — Timing of Initiation of Renal-Replacement Therapy in Acute Kidney Injury (NEJM 2020)