- TMP calculator: enter the four dialyzer pressures read off the machine — blood inlet (PBi, arterial), blood outlet (PBo, venous), dialysate inlet (PDi) and dialysate outlet (PDo) — to get the mean blood and dialysate pressures and the transmembrane pressure (TMP). Optionally enter the dialyzer's KUF to estimate the theoretical ultrafiltration rate.
- Filtration-fraction calculator: choose the dilution mode (post- or pre-dilution HDF), then enter the blood flow (QB), hematocrit, substitution/replacement-fluid rate and net patient ultrafiltration rate to get the plasma-water flow and the filtration fraction (FF), classified against its mode-specific target.
- Read each result's interpretation band, then read both tools together — a normal TMP does not guarantee a safe FF, and a rising TMP often signals an excessive FF.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use at the dialysis machine to interpret pressures and convective settings: estimating TMP when a machine displays only the raw compartment pressures, troubleshooting a rising-TMP alarm, setting up or auditing a hemodiafiltration (HDF) prescription, and teaching the relationship between pressure, ultrafiltration, internal filtration and convective stress.
Appropriate use
Maintenance HD/HDF in which the operating pressures, blood flow, hematocrit and substitution volume are known. Pair with assessment of access flow, anticoagulation, dialyzer choice and the ultrafiltration-rate (mL/kg/h) calculation.
Caveats
Machine-displayed TMP is not universally comparable — manufacturers use different sensors and algorithms (some use only PBo−PDo). Treat the computed TMP as an estimate of the average gradient, not a manufacturer-exact value, and always trend it over time rather than reading a single number.
Pearls & Pitfalls
FF often matters more than TMP
Modern HDF practice targets the filtration fraction (≤25–30% post-dilution, ≤45–50% pre-dilution) rather than chasing a TMP number. A normal TMP does not guarantee a safe FF — but a steadily rising TMP frequently signals an excessive FF, hemoconcentration, or impending fiber clotting.
TMP is not uniform along the dialyzer
The displayed TMP is an average. Pressure is highest at the blood inlet, so internal filtration is greatest proximally and backfiltration may occur distally. At QB 400–500 mL/min internal filtration can reach ~30–40 mL/min, boosting middle-molecule clearance even in conventional high-flux HD.
Pitfalls
A progressively rising TMP at a stable UF suggests fiber clotting or excessive FF — not just "more water removal." Actual machine TMP runs higher than KUF×UF predicts because of blood viscosity, hemoconcentration, protein fouling and fiber resistance. Excessive TMP (>300 mmHg; common alarms 350–450) risks membrane stress and hemolysis.
Why Use It
TMP is the principal force driving net ultrafiltration, internal filtration and convective transport; FF expresses how hard the blood is being concentrated as it crosses the filter. Together they are the real-time markers of membrane resistance and convective stress — letting you keep convective therapy effective while avoiding hemoconcentration, clotting and hemolysis.
Transmembrane Pressure (TMP)
Enter the four dialyzer compartment pressures from the machine to compute the mean blood pressure, mean dialysate pressure and TMP, with an operating-range interpretation. Optionally enter the dialyzer's ultrafiltration coefficient (KUF) to see the theoretical UF rate at that TMP.
⚕ TMP = [(PBi + PBo) ÷ 2] − [(PDi + PDo) ÷ 2]. Machine-displayed TMP varies by manufacturer (some use PBo−PDo or a single internally-computed dialysate pressure). Actual TMP usually exceeds KUF×UF because of blood viscosity, hemoconcentration, protein fouling and fiber resistance. Trend over time; interpret with FF, hematocrit and dialyzer type.
Filtration Fraction (FF)
Filtration fraction is the convective volume removed across the filter as a percentage of the plasma-water flow entering it. Choose the dilution mode, then enter blood flow, hematocrit, substitution-fluid rate and net patient ultrafiltration to compute the plasma-water flow and FF, classified against its mode-specific safe target.
⚕ Plasma-water flow Qp = QB × (1 − Hct) × 0.93. FF = (substitution rate + net UF rate) ÷ Qp. For pre-dilution the substitution fluid dilutes blood upstream, so the denominator is increased (Qp + substitution rate) and a higher FF is tolerated. Targets: post-dilution ≤25–30%, pre-dilution ≤45–50%. Educational estimate — machine algorithms differ.
Next Steps
If TMP is high or rising during treatment:
- Look for hemoconcentration from aggressive ultrafiltration, fiber clotting, a high hematocrit, rising blood viscosity, protein fouling, a kinked venous line, or excessive substitution volume in post-dilution HDF.
- Check anticoagulation and inspect the dialyzer and lines; a rising TMP at a stable UF suggests clotting or an excessive filtration fraction.
- Compute the filtration fraction — if above target, reduce the substitution rate, raise QB, or switch toward pre-dilution.
If the filtration fraction exceeds its target:
- Lower the substitution/convective volume, or increase blood flow (QB) to raise plasma-water flow.
- Consider pre-dilution or mixed-dilution HDF, which tolerate a higher FF because blood is diluted upstream of the filter.
- Re-evaluate the dialyzer and treatment time, and confirm hematocrit — a high hematocrit lowers plasma-water flow and raises FF.
Evidence & References
Formulae
| Quantity | Equation |
|---|---|
| Transmembrane pressure (standard) | TMP = [(PBi + PBo) ÷ 2] − [(PDi + PDo) ÷ 2] |
| TMP (simplified, machine) | TMP = [(PBi + PBo) ÷ 2] − PD · or PBo − PDo |
| TMP ↔ ultrafiltration | UF = KUF × TMP ⇔ TMP = UF ÷ KUF |
| Plasma-water flow | Qp = QB × (1 − Hct) × 0.93 |
| Filtration fraction (post-dilution) | FF = (substitution rate + net UF rate) ÷ Qp |
| Filtration fraction (pre-dilution) | FF = (substitution rate + net UF rate) ÷ (Qp + substitution rate) |
Typical TMP ranges
| Modality / state | Typical TMP (mmHg) |
|---|---|
| Low-flux HD | 50–150 |
| High-flux HD | 80–250 |
| Pre-dilution HDF | 100–250 |
| Post-dilution HDF | 150–300 |
| Excessive | > 300 |
| Common alarm range | 350–450 |
Filtration-fraction targets
| Mode | Target FF |
|---|---|
| Post-dilution HDF | ≤ 25–30% |
| Pre-dilution HDF | ≤ 45–50% |
Notes & References
TMP is the pressure gradient across the dialyzer membrane that drives ultrafiltration and convective transport; displayed values are not comparable across manufacturers because of differing sensors and algorithms. The displayed TMP is an average — pressure is highest at the blood inlet, producing proximal internal filtration and possible distal backfiltration; at QB 400–500 mL/min internal filtration may reach ~30–40 mL/min, enhancing middle-molecule clearance. Filtration fraction has become the preferred safety metric in HDF: a normal TMP does not guarantee a safe FF, while a rising TMP often signals an excessive FF, hemoconcentration, or impending fiber clotting.
- Daugirdas JT, Blake PG, Ing TS, eds. Handbook of Dialysis. 5th ed. Wolters Kluwer; 2015 — chapters on hemodialysis apparatus, high-flux membranes and hemodiafiltration.
- Ronco C, Clark WR. Haemodialysis membranes. Nat Rev Nephrol. 2018;14(6):394-410.
- Tattersall JE, Ward RA; EUDIAL group. Online haemodiafiltration: definition, dose quantification and safety revisited. Nephrol Dial Transplant. 2013;28(3):542-550.
- Canaud B, et al. Recommendations for the conduct of online haemodiafiltration and the role of convective dose and filtration fraction. Semin Dial / EUDIAL consensus documents.
