CVVHD Filtration Fraction Calculator
Calculate filtration fraction in CRRT modes, including CVVHD, with instant interpretation and chart visualization.
Clinical reminder: in pure CVVHD, convective ultrafiltration is usually only the net fluid removal component, so filtration fraction is often low. High filtration fraction risk is most relevant in high-convection settings (CVVH/CVVHDF).
How to Calculate Filtration Fraction in CVVHD: Expert Guide for ICU and Nephrology Teams
Filtration fraction is one of the most practical bedside safety calculations in continuous renal replacement therapy (CRRT). If you run CVVHD, CVVH, or CVVHDF in the ICU, understanding filtration fraction helps reduce filter clotting, avoid excessive hemoconcentration, and maintain prescribed clearance. Even though CVVHD is primarily diffusive, many teams still calculate filtration fraction to make sure the circuit remains in a low-risk operating range and to compare therapy settings across shifts.
In simple terms, filtration fraction is the proportion of plasma water that is being removed as ultrafiltrate while blood passes through the hemofilter. The higher that proportion, the thicker blood becomes inside the filter fibers, and the greater the chance of early circuit failure. Most CRRT programs target a low-to-moderate filtration fraction, often below 20% to 25%, especially when anticoagulation is suboptimal or vascular access is marginal.
Why this number matters clinically
- Helps predict filter lifespan and clotting risk.
- Supports dose delivery by reducing unscheduled downtime.
- Guides whether to increase blood flow, shift replacement strategy, or reduce convective load.
- Provides a common communication metric between nursing, pharmacy, nephrology, and critical care teams.
Core formula used in practice
The widely used bedside formula is:
Filtration Fraction (%) = Convective Ultrafiltration Rate / Plasma Water Flow Through Filter × 100
Where:
- Plasma Water Flow (mL/min) is approximately: Qb × (1 – Hct), with pre-filter replacement added when present.
- Convective Ultrafiltration Rate (mL/min) includes replacement volume crossing the filter plus net fluid removal.
For many protocols, denominator is written as: Qb × (1 – Hct) + Qpre (with Qpre in mL/min). This correction reflects dilution from pre-filter replacement fluid and often lowers the calculated filtration fraction.
How this applies specifically to CVVHD
CVVHD is predominantly a diffusion-based modality. That means dialysate flow drives most solute clearance, not convection. In pure CVVHD without replacement fluid, convective ultrafiltration is usually only the net fluid removal ordered for volume management. Therefore, filtration fraction is often much lower than in CVVH or CVVHDF.
Example: if net ultrafiltration is 100 mL/h (1.67 mL/min), blood flow is 180 mL/min, and hematocrit is 32%, plasma water flow is about 122.4 mL/min. Filtration fraction is roughly 1.4%, which is very low risk for hemoconcentration.
Step-by-step bedside workflow
- Record current blood flow (Qb) in mL/min.
- Record hematocrit as a decimal fraction (for 32%, use 0.32 in the formula).
- Identify pre-filter replacement rate (if any) and convert mL/h to mL/min.
- Determine convective ultrafiltration rate:
- CVVHD: usually net fluid removal only.
- CVVH: replacement (pre and post) + net fluid removal.
- CVVHDF: convective component (replacement + net UF), not dialysate.
- Calculate plasma water flow.
- Compute filtration fraction percentage.
- Interpret with your unit threshold policy (commonly <20% preferred).
Common interpretation bands used by many programs
| Filtration Fraction | Typical Interpretation | Operational Action |
|---|---|---|
| < 20% | Low hemoconcentration risk | Usually acceptable, continue monitoring pressure trends |
| 20% to 25% | Moderate risk zone | Review anticoagulation, consider higher Qb or more pre-dilution |
| > 25% | Higher clotting risk in many circuits | Lower convective load, increase Qb, optimize access and anticoagulation |
Real-world ICU context: why optimization matters
Severe AKI in critical illness is common, and CRRT is frequently used when hemodynamic instability limits intermittent dialysis. Epidemiologic data consistently show high illness burden and high mortality in this population, so avoiding avoidable circuit interruptions matters for solute and volume control.
| Critical Care Statistic | Reported Range | Clinical Relevance to FF |
|---|---|---|
| AKI incidence in ICU populations | Approximately 40% to 60% | Large patient volume exposed to CRRT decisions and filter management |
| ICU patients requiring RRT | Approximately 5% to 13% | Highlights need for reliable circuits and minimized downtime |
| Mortality in severe AKI requiring dialysis support | Commonly 35% to 60% depending on cohort | Supports meticulous attention to delivered therapy quality |
These ranges come from large ICU AKI cohorts and CRRT trials in the modern era. The exact values vary by case mix, sepsis burden, and inclusion criteria, but the practical message is consistent: treatment reliability matters, and filtration fraction control is one actionable part of reliability.
Frequent mistakes when calculating filtration fraction
- Including dialysate flow as convection in CVVHD. Dialysate contributes to diffusion, not convective ultrafiltration.
- Forgetting unit conversion. mL/h must be converted to mL/min to match Qb units.
- Ignoring pre-filter dilution. Pre-replacement changes denominator and can lower FF.
- Using outdated hematocrit values. Rapid ICU fluid shifts can meaningfully change plasma water flow.
- Applying one threshold to every machine and anticoagulation strategy. Unit protocol and local outcomes should drive final limits.
Practical adjustment strategy when FF is high
- Increase blood flow rate if vascular access permits.
- Shift replacement fluid toward pre-filter administration when clinically acceptable.
- Reduce convective dose intensity if above necessary target.
- Check access pressures and line position to reduce stasis and recirculation issues.
- Review anticoagulation method (for example, regional citrate protocol compliance).
- Recalculate after each major setting change and reassess transmembrane pressure trends.
CVVHD versus CVVH versus CVVHDF: quick comparison
In pure CVVHD, filtration fraction can be deceptively reassuringly low because convection is minimal. That does not remove the need for surveillance of access quality, transmembrane pressure, and anticoagulation. In contrast, CVVH and CVVHDF often run larger convective volumes, where FF management becomes central to filter longevity.
- CVVHD: mostly diffusion; FF usually low unless net UF is high.
- CVVH: high convection; FF can rise quickly if Qb is modest.
- CVVHDF: mixed mechanism; FF depends on how much of total dose is convective.
Documentation template for bedside teams
A simple standardized note can improve handoffs:
- Mode and prescription (Qb, Qd, replacement split, net UF goal)
- Most recent hematocrit and time obtained
- Calculated plasma water flow and filtration fraction
- Pressure trends (access, return, transmembrane)
- Anticoagulation status and any interruptions
- Action taken if FF above protocol target
Authoritative educational resources
For foundational AKI and critical care renal support references, review:
- NIDDK (.gov): Acute Kidney Injury overview
- NCBI Bookshelf (.gov): Acute Kidney Injury clinical reference
- UCSF (.edu): Continuous Renal Replacement Therapy handbook page
Final takeaway
If you remember only one point, make it this: filtration fraction is a controllable safety lever. In CVVHD it is usually low, but still worth calculating to verify circuit conditions and support consistent multidisciplinary communication. In convection-heavy prescriptions, keeping FF below your program threshold can markedly improve filter life and delivered dose reliability. Use a standardized formula, consistent units, and repeat calculations after any prescription change. The calculator above is designed to make that workflow fast and reproducible at the bedside.