CRRT Filtration Fraction Calculator
Estimate filtration fraction quickly using blood flow, hematocrit, and replacement/ultrafiltration settings.
How to Calculate Filtration Fraction in CRRT: Practical, Bedside Guide for ICU Teams
Filtration fraction (FF) is one of the most useful safety metrics in continuous renal replacement therapy (CRRT). It tells you what proportion of plasma water is being removed as ultrafiltrate while blood passes through the hemofilter. If FF climbs too high, hemoconcentration inside the filter increases, transmembrane pressure often rises, and circuit clotting risk increases. If FF is kept within a reasonable range, filter life is usually better and treatment delivery becomes more reliable.
In daily ICU practice, many interruptions in CRRT happen because a circuit clots before planned filter change. Knowing how to calculate FF correctly helps you adjust blood flow, replacement strategy, or ultrafiltration targets before clotting occurs. This guide walks through the equation, clinical interpretation, worked examples, and common mistakes.
Core Formula Used at the Bedside
A common practical formula in convection based CRRT is:
Filtration Fraction (%) = Total Convective Ultrafiltration Rate / Plasma Water Flow Through Filter × 100
To estimate plasma water flow:
Plasma Water Flow (mL/h) = [Blood Flow (mL/min) × (1 – Hematocrit)] × 60
When you use pre-filter replacement, that fluid dilutes blood before it reaches the membrane, so many units use:
Adjusted Plasma Flow = Plasma Water Flow + Pre-filter Replacement Rate
For a simple practical estimate in CVVH or convection-heavy CVVHDF:
Total Convective UF Rate ≈ Pre-filter Replacement + Post-filter Replacement + Net Fluid Removal
Step by Step: How to Calculate Filtration Fraction in CRRT
- Record blood flow (Qb) in mL/min from the CRRT machine.
- Get current hematocrit as a decimal (30% becomes 0.30).
- Calculate plasma water flow: Qb × (1 – Hct) × 60 for mL/h.
- Add pre-filter replacement fluid rate if your protocol uses adjusted plasma flow.
- Calculate total convective ultrafiltration rate from therapy settings.
- Divide ultrafiltration rate by adjusted plasma flow.
- Multiply by 100 to report FF as a percent.
Worked Clinical Example
Suppose an ICU patient is on CVVHDF with:
- Blood flow: 180 mL/min
- Hematocrit: 30%
- Pre-filter replacement: 1000 mL/h
- Post-filter replacement: 500 mL/h
- Net fluid removal: 150 mL/h
First, plasma water flow:
180 × (1 – 0.30) × 60 = 7560 mL/h
Adjusted plasma flow:
7560 + 1000 = 8560 mL/h
Convective ultrafiltration:
1000 + 500 + 150 = 1650 mL/h
Filtration fraction:
FF = 1650 / 8560 × 100 = 19.3%
Interpretation: this is generally within a common safety range and less likely to cause excessive hemoconcentration, assuming no other circuit issues.
Why Filtration Fraction Matters So Much
- Filter life: higher FF can accelerate hemofilter clotting, shortening circuit survival.
- Dose delivery: frequent clotting reduces delivered therapy and can compromise solute and fluid targets.
- Nursing workload: repeated circuit changes increase staff burden and downtime.
- Blood loss: each premature circuit failure can increase blood loss risk, especially in unstable patients.
- Cost: reduced filter life increases consumable use and treatment interruptions.
Comparison Table: Landmark CRRT Dose Trials and Practical Context
| Trial | Population | Dose Comparison | Main Outcome | Relevance to FF Practice |
|---|---|---|---|---|
| RENAL Study (NEJM 2009) | 1508 critically ill adults | 25 vs 40 mL/kg/h effluent | 90-day mortality: no significant difference | Supports avoiding unnecessarily high intensity settings that may raise convective burden without clear survival gain. |
| ATN Study (NEJM 2008) | 1124 critically ill adults | Intensive vs less intensive RRT strategy | 60-day mortality: no significant difference | Reinforces balanced prescription and reliable delivery rather than aggressive, interruption-prone settings. |
| KDIGO AKI Guidance | International guideline framework | Delivered CRRT dose often targeted around 20 to 25 mL/kg/h | Focus on adequate, consistent delivery | Lower interruption rates from reasonable FF can help maintain intended delivered dose. |
Comparison Table: Bedside FF Scenarios
| Scenario | Qb (mL/min) | Hct | Pre + Post + Net UF (mL/h) | Estimated FF | Typical Interpretation |
|---|---|---|---|---|---|
| Balanced CVVHDF | 180 | 0.30 | 1650 | 19.3% | Generally acceptable in many protocols |
| Higher convection, low blood flow | 140 | 0.35 | 2200 | 36.5% (approx) | High risk for hemoconcentration and clotting |
| Increased Qb with same convection | 220 | 0.30 | 1650 | 15.4% (approx) | Lower FF, often improves filter tolerance |
How to Reduce a High Filtration Fraction
- Increase blood flow if vascular access supports it.
- Shift a larger share of replacement to pre-filter when clinically appropriate.
- Reduce convective ultrafiltration rate if treatment goals allow.
- Reassess aggressive net fluid removal targets in unstable hemodynamics.
- Confirm anticoagulation strategy is appropriate (regional citrate or systemic approach per protocol).
- Check catheter position and access pressures to avoid low effective blood flow.
Frequent Calculation Errors in ICU Practice
- Unit mismatch: mixing mL/min and mL/h without conversion is very common.
- Ignoring hematocrit: using blood flow alone overestimates denominator and underestimates FF risk.
- Forgetting pre-dilution effect: pre-filter replacement changes concentration dynamics in the filter.
- Using prescribed instead of delivered flow: interruptions and alarms reduce delivered rates.
- Not recalculating after setting changes: every major prescription adjustment should trigger a fresh FF estimate.
Clinical Context: FF Is Important, But Not the Only Variable
A circuit can clot at modest FF if there are access problems, severe inflammation, high fibrinogen states, inadequate anticoagulation, or repeated blood flow instability. Likewise, some patients tolerate moderate FF better when access is excellent and anticoagulation is robust. The practical approach is to treat FF as a high value warning signal inside a broader circuit surveillance strategy that includes transmembrane pressure trend, filter pressure trend, effluent pressure stability, and delivered dose tracking.
What Range Should You Aim For?
Many centers aim for an FF under 20%, with caution when trending above 20 to 25%. Some protocols tolerate slightly higher values depending on modality and anticoagulation method, but persistent high FF should prompt action. A stable circuit that runs for the intended duration with predictable delivered dose is usually a sign that FF and related settings are balanced.
Authority Sources for Further Reading
- National Library of Medicine (NIH): Continuous Renal Replacement Therapy review
- NCBI Bookshelf: Continuous Renal Replacement Therapy overview
- National Institute of General Medical Sciences (.gov): Acute Kidney Injury background
Bottom Line
If you are asking how to calculate filtration fraction in CRRT, the key is to use a consistent bedside formula with correct unit conversion and frequent recalculation after prescription changes. Keep FF in a conservative range, monitor pressures and delivered dose, and adjust blood flow, replacement distribution, and ultrafiltration goals early. Doing this well can reduce circuit loss, improve treatment continuity, and support safer ICU renal support.