CRRT Filtration Fraction Calculator
Calculate filtration fraction for CVVH and CVVHDF to estimate hemoconcentration and filter clotting risk.
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Enter CRRT settings and click calculate.
Expert Guide: Calculating Filtration Fraction in CRRT
Filtration fraction in continuous renal replacement therapy (CRRT) is one of the most practical bedside metrics for keeping circuits open, reducing unplanned downtime, and improving dose delivery consistency. In simple terms, filtration fraction (FF) tells you what percentage of plasma water is being removed as ultrafiltrate while blood crosses the hemofilter. When FF is pushed too high, blood proteins and cells become progressively concentrated inside the filter fibers, transmembrane pressure rises, and the circuit is more likely to clot early. For ICU teams trying to avoid repeated circuit loss, blood loss, and nursing interruptions, this single number can be very actionable.
Clinically, many units aim for filtration fraction at or below about 20%, and often keep it below 25% as an upper ceiling depending on anticoagulation strategy, catheter performance, and filter type. While the exact threshold varies by protocol, the concept is universal: lower FF generally means lower hemoconcentration stress in the filter. This is especially important in convective therapies like CVVH and mixed therapies like CVVHDF, where ultrafiltration rates can be substantial.
Core Formula Used at the Bedside
A practical formula for predilution and postdilution CRRT settings is:
Filtration Fraction (%) = [Total Convective Ultrafiltration Rate] / [Plasma Water Flow Through Filter] x 100
Total Convective Ultrafiltration Rate (mL/min) = (Pre-filter replacement + Post-filter replacement + Net UF) converted from mL/hr to mL/min.
Plasma Water Flow Through Filter (mL/min) = [Blood flow x (1 – Hematocrit)] + Pre-filter replacement flow.
This formulation reflects a key bedside truth: pre-filter replacement fluid dilutes blood before it reaches the membrane, reducing hemoconcentration and often lowering clotting risk. That is why increasing predilution is a common strategy when a unit is seeing frequent filter loss from high hemoconcentration patterns.
Why filtration fraction matters operationally
- Circuit longevity: Higher FF usually increases clotting probability and shortens filter life.
- Dose reliability: Frequent clotting reduces delivered dose versus prescribed dose.
- Resource use: More filter changes increase consumable costs, nursing workload, and interruptions to therapy.
- Patient impact: Repeated downtime can delay fluid control and toxin clearance goals in unstable patients.
Step-by-Step: How to Calculate Filtration Fraction Correctly
- Record blood flow (Qb) in mL/min from the machine prescription and confirm actual delivered flow.
- Record hematocrit as a decimal for formula work (for example, 30% = 0.30).
- Capture pre-filter replacement in mL/hr and convert to mL/min by dividing by 60.
- Add convective outputs: pre-filter replacement + post-filter replacement + net UF (all in mL/hr, then convert to mL/min).
- Compute plasma water flow: Qb x (1 – Hct) + pre-filter replacement flow (mL/min).
- Divide and multiply by 100 to obtain FF percentage.
- Interpret in context with anticoagulation status, access function, and transmembrane pressure trends.
Example: if Qb = 180 mL/min, hematocrit = 30%, pre-filter replacement = 1200 mL/hr, post-filter replacement = 400 mL/hr, and net UF = 100 mL/hr:
- Pre-filter replacement = 20 mL/min
- Total convective UF = (1200 + 400 + 100) / 60 = 28.3 mL/min
- Plasma water flow = 180 x 0.70 + 20 = 146 mL/min
- FF = 28.3 / 146 x 100 = 19.4%
In that profile, FF is within a common target range.
Interpreting Filtration Fraction Bands in Practice
| FF Band | Typical Clinical Interpretation | Reported Circuit Clotting Trend (Representative ICU Cohorts) | Median Filter Life Trend |
|---|---|---|---|
| <20% | Preferred in many protocols, especially if clotting history exists. | Often lower clotting burden, commonly around 10% to 20% per circuit depending on anticoagulation. | Often in the 24 to 48 hour range when access and anticoagulation are adequate. |
| 20% to 25% | Acceptable in some patients but requires closer monitoring of pressures and downtime. | Intermediate risk, commonly around 20% to 35% circuit clotting in mixed ICU populations. | Frequently 18 to 30 hours, with significant unit-to-unit variation. |
| >25% | Higher hemoconcentration stress zone. Consider mitigation quickly. | Commonly higher clotting rates, often 35% to 60% where anticoagulation is limited. | Can fall to 8 to 20 hours in vulnerable settings. |
These ranges reflect patterns repeatedly reported across ICU practice literature. Exact values differ by catheter quality, membrane type, anticoagulation protocol, blood flow stability, and local nursing workflow.
Important distinction: filtration fraction versus effluent dose
FF is a circuit stress and hemoconcentration metric. Effluent dose (mL/kg/hr) is a clearance prescription metric. They are related operationally but they are not interchangeable. You can raise effluent dose and still keep FF reasonable if you increase blood flow, use predilution thoughtfully, and optimize anticoagulation. Conversely, a moderate prescribed dose can still produce high FF if blood flow is low and post-filter convection is high.
Data From Landmark CRRT Dose Trials and Why It Matters for FF Strategy
Large RCTs comparing higher versus standard CRRT intensity did not show a mortality advantage for very high effluent prescriptions. That evidence supports practical dosing while focusing on delivery reliability, which is where FF control helps.
| Trial | Sample Size | Dose Comparison | Main Mortality Finding | Operational Relevance to FF |
|---|---|---|---|---|
| RENAL Study (2009) | 1508 ICU patients | 25 vs 40 mL/kg/hr effluent | 90-day mortality 44.7% vs 44.5% (no significant benefit of higher intensity) | Supports avoiding unnecessary high convective stress when it compromises circuit life. |
| ATN Study (2008) | 1124 critically ill patients | Intensive vs less-intensive renal support strategy | 60-day mortality 53.6% vs 51.5% (no significant mortality improvement) | Reinforces balancing prescription goals with consistent delivery and downtime reduction. |
How to reduce high filtration fraction at the bedside
1) Increase blood flow when feasible
Because plasma water flow depends strongly on blood flow, even modest increases in Qb can lower FF meaningfully. This requires reliable access and careful hemodynamic judgment.
2) Shift replacement toward predilution
Pre-filter replacement expands flow entering the membrane and reduces hemoconcentration. Many units use this intentionally when recurrent clotting occurs without clear access failure.
3) Reassess post-filter convection load
High post-filter replacement plus net UF can push FF above target. If clinically acceptable, redistributing convection toward predilution can improve filter survival.
4) Optimize anticoagulation pathway
Regional citrate anticoagulation often extends filter life and may allow stable operation at settings that otherwise clot rapidly with no anticoagulation. Monitor calcium and acid-base status per protocol.
5) Confirm catheter function and access pressures
Poor access causes intermittent low flow and stasis, which worsens clotting independent of FF. Always interpret FF along with machine pressure trends and line performance.
Common mistakes when calculating filtration fraction
- Unit errors: mixing mL/hr and mL/min without conversion.
- Ignoring predilution in denominator: this overestimates FF risk profile.
- Adding dialysate as convective UF: dialysate is diffusive flow and should not be included in convective FF numerator.
- Using outdated hematocrit values: anemia, transfusion, and bleeding can shift FF materially.
- Interpreting FF alone: TMP trends, filter pressure, alarms, and delivered dose still matter.
Recommended workflow for ICU teams
- Calculate FF at CRRT initiation and after every major prescription change.
- Document FF with transmembrane pressure and filter pressure trends.
- Set local escalation triggers, for example FF above 22% with rising TMP.
- Use a structured response: verify access, adjust predilution, adjust blood flow, review anticoagulation, reassess goals.
- Audit delivered dose versus prescribed dose weekly to identify hidden downtime from filter loss.
Clinical context: AKI burden and the importance of dependable CRRT delivery
Acute kidney injury in the ICU is common and carries high morbidity. In many critical care settings, CRRT is selected for hemodynamically unstable patients because it allows gradual fluid and solute control. Since outcomes are influenced by consistency of therapy delivery, preventing avoidable circuit clotting is operationally important. Filtration fraction calculation is one of the fastest ways to identify preventable circuit stress before repeated downtime happens.
Authoritative references and further reading
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK): Acute Kidney Injury overview
- NCBI Bookshelf (NIH): Continuous Renal Replacement Therapy overview
- MedlinePlus (.gov): Kidney failure and dialysis patient education resource
Use this calculator as a bedside decision support aid, not as a replacement for institutional protocol or specialist judgment. CRRT management should always be individualized to patient hemodynamics, anticoagulation suitability, fluid goals, and evolving ICU conditions.