How do I calculate the appropriate continuous renal replacement therapy (CRRT) dose (effluent flow rate) for a patient?

CRRT Dose Calculation

Calculate the CRRT effluent flow rate as 20-25 mL/kg/hour based on the patient's actual body weight, which translates to a prescribed rate that should be increased by approximately 15-20% to account for downtime and filter efficiency decline. 1

The Standard Calculation

The KDIGO/KDOQI guidelines provide a Level 1A recommendation for delivering an effluent volume of 20-25 mL/kg/h for CRRT in acute kidney injury 1. This is the strongest evidence-based target we have, derived from two large randomized controlled trials (RENAL and ATN studies) that demonstrated no mortality benefit from higher doses (35-40 mL/kg/h) 1.

Step-by-Step Calculation:

1. Determine patient weight: Use actual body weight in kilograms

   • Exception: For severely underweight patients (BMI extremes), ideal body weight may better approximate effective volume of distribution 2

2. Calculate target effluent rate:

   • Minimum: 20 mL/kg/h × patient weight (kg) = mL/h
   • Target: 25 mL/kg/h × patient weight (kg) = mL/h
   • Convert to L/h by dividing by 1000

3. Adjust prescription upward: The prescribed dose should exceed the target by 15-25% because:

   • Delivered dose typically falls 15-32% short of prescribed dose due to circuit downtime, clotting, procedures, and filter efficiency decline 1, 3
   • In the RENAL trial, patients received only 85% of prescribed dose 3
   • Observational studies show patients may receive as little as 68% of prescribed dose 3

Practical Example

For a 70 kg patient targeting 25 mL/kg/h:

• Target delivered dose: 25 × 70 = 1,750 mL/h (1.75 L/h)
• Prescribed dose to account for 15% loss: 1,750 ÷ 0.85 = 2,059 mL/h (~2.1 L/h)

Alternative Quick Formula (When Generation Rate Unknown)

If you lack reliable data on the patient's urea generation rate, use this simplified approach 4:

K (L/h) = 1.2 × (patient weight in kg) ÷ (desired BUN goal in mg/dL)

For a typical goal BUN of 60 mg/dL, this simplifies to: K (L/h) = (2 × patient weight in kg) ÷ 100

This assumes moderate hypercatabolism and 60% body weight as urea distribution volume 4.

Critical Monitoring Requirements

You must frequently assess the actual delivered dose and adjust the prescription accordingly 1. This is a Level 1B recommendation. The prescribed dose is not the delivered dose 5.

Methods for Dose Verification:

• Flows-based method (easiest): Monitor actual effluent flow accounting for dialysate, replacement fluid, blood flow, and net ultrafiltration 5
• Urea clearance method (most accurate): Calculate current effective delivered dose through measured urea clearance, though this is more complex 5

Important Caveats

Do not exceed 25 mL/kg/h as your target delivered dose. The ATN and RENAL trials definitively showed that higher doses (35-40 mL/kg/h) provide no survival benefit and waste resources 1, 6. Recent data from 2024-2026 confirms that moderated effluent doses are both clinically appropriate and more sustainable 7.

The prescription must be individualized for fluid and electrolyte balance needs 1, but the base clearance calculation starts with the 20-25 mL/kg/h framework.

For intermittent RRT (not CRRT), the target is different: deliver a Kt/V of 3.9 per week 1.