Pediatric CRRT · Bedside tool
The Dilution Factor
Extracorporeal circuit–related hemodilution in pediatric CRRT — a predictable, quantifiable pharmacokinetic modifier, estimable at the bedside before the first loading dose.
Concept
Circuit volume meets small blood volume
Pediatric CRRT circuits hold between 58 and 276 mL of extracorporeal blood volume (ECBV) depending on filter and set. That volume is effectively added to the patient's intravascular compartment at the moment of connection. Against a total circulating blood volume of roughly 75–80 mL/kg, the smaller the child, the larger the relative expansion — and the larger the initial dilutional effect on drug concentrations.
The dilution factor expresses this geometrically: the fraction of the original intravascular drug concentration preserved within the circuit-expanded volume. A dilution factor of 0.80 corresponds to an initial central concentration roughly 20% lower than would be expected from body-weight dosing alone.
Formula
The calculation
Bedside calculator
Estimate dilution for this patient
Results
Dilution profile across weight
Summary
All filters at this weight
| Manufacturer | Hemofilter | ECBV (mL) | DF (75–80) | DF (mid) | Dilution (%) |
|---|
Worked example
3 kg infant · Baxter Prismaflex HF20 · amikacin 7.5 mg/kg
Scenario
Amikacin PK
Why it matters
Clinical implications
- Priming threshold. When ECBV exceeds ~10% of circulating blood volume, blood priming is already recommended — yet the pharmacokinetic consequence of the same volume expansion is routinely under-recognised.
- Hydrophilic antimicrobials. Drugs with a limited volume of distribution and time-sensitive pharmacodynamic targets — aminoglycosides, β-lactams — are most vulnerable to initiation-phase dilution.
- Early peak matters. Standard weight-based dosing may fall short of PD targets in the first dosing interval. Early TDM, not delayed TDM, is the detection step.
Anticipate before you measure. The dilution factor turns an invisible pharmacokinetic perturbation into a number on the prescription. It does not replace TDM — it tells you when to order the first level, and how aggressively to adjust if that level sits below target.
Beyond TDM
MIPD, TCI, and population PK
Model-Informed Precision Dosing (MIPD) and Target Concentration Intervention (TCI) integrate population pharmacokinetic (popPK) models with individual patient data to enable real-time dose optimisation, accounting for covariates such as weight, renal function, and CRRT parameters.
Incorporating the ECBV-to-estimated-blood-volume ratio as a covariate on the initial central volume of distribution (Vc) within popPK models offers a mechanistic basis for circuit-aware loading strategies. A recent prospective study has demonstrated the feasibility of implementing an MIPD-based framework within a pediatric intensive care setting — laying the groundwork for prospective validation of dilution-factor–adjusted dosing.
Methodology & assumptions
How this tool computes dilution
- Vblood is estimated as (75–80 mL/kg) × weight, following New et al. 2016 transfusion guidelines. The 75–80 band is rendered as the ribbon around the central curve.
- VECBV is taken as the total filter and set priming volume (Table 4, Stitt et al. 2022), not the hemofilter alone.
- Prime-infused scenario. The prime fluid is assumed to enter the patient at connection; the effective intravascular volume therefore equals Vblood + VECBV.
- Scope of the estimate. The dilution factor describes initial geometric dilution at connection. It does not account for redistribution, protein binding shifts, adsorption onto circuit materials, or on-treatment clearance.
- Not modelled: extension lines, connectors, blood warmers — each adds to effective ECBV and increases the real dilutional effect beyond what this tool shows.
- Not modelled: hematocrit-dependent plasma-volume corrections.
- Weight parsing. Values entered > 100 are interpreted as grams and converted to kilograms (e.g. 3500 → 3.5 kg).
- Disclaimer. This tool supports clinical reasoning — it does not substitute for therapeutic drug monitoring, clinical pharmacology consultation, or individual patient assessment.
References
Key sources
- Neumayr TM, Bayrakci B, Chanchlani R, et al. (2024) Programs and processes for advancing pediatric acute kidney support therapy in hospitalized and critically ill children: a report from the 26th Acute Disease Quality Initiative (ADQI) consensus conference. Pediatr Nephrol 39:993–1004.
- New HV, Berryman J, Bolton-Maggs PHB, et al. (2016) Guidelines on transfusion for fetuses, neonates and older children. Br J Haematol 175:784–828.
- Stitt G, Dubinsky S, Edginton A, et al. (2022) Antimicrobial dosing recommendations in pediatric continuous renal replacement therapy: a critical appraisal of current evidence. Front Pediatr 10:889958.
- Abdul-Aziz MH, Alffenaar JC, Bassetti M, et al. (2020) Antimicrobial therapeutic drug monitoring in critically ill adult patients: a Position Paper. Intensive Care Med 46:1127–1153.
- Bayraktar I, Kasikci M, Benek Z, et al. (2026) A prospective feasibility study evaluating the implementation of Model-Informed Precision Dosing in critically ill children. Frontiers 17.
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