Predicted Hematocrit Calculator
Estimate hematocrit after CPB priming, hemodilution, RBC transfusion, or volume addition/removal using volume-balance assumptions.
Predicted Hct on CPB
Dilutional hematocrit calculator
Estimate dilutional hematocrit at CPB initiation.
Clinical context
Predicted hematocrit during CPB is an estimate based on red cell mass and volume-balance assumptions. Hemodilution from CPB priming volume or crystalloid addition lowers Hct, while RBC transfusion increases red cell volume and volume addition/removal changes the denominator. Actual measured Hct may differ because of bleeding, ultrafiltration, hemolysis, transfusion timing, sampling differences, fluid shifts, and measurement variability.
- Use measured blood gas or laboratory Hct whenever clinical decisions depend on the result.
- A predicted Hct value does not directly represent oxygen delivery.
- Oxygen delivery also depends on pump flow, arterial saturation, hemoglobin concentration, temperature, metabolic demand, and microcirculatory conditions.
- When Hct is near a local transfusion threshold, recheck measured Hct after adequate mixing time.
About this predicted hematocrit calculator
This CPB predicted hematocrit calculator estimates hematocrit changes from cardiopulmonary bypass priming, hemodilution, RBC transfusion, fluid addition, and volume removal. It can be used before bypass to estimate dilutional hematocrit at CPB initiation, or during bypass to estimate how added crystalloid, RBC products, or ultrafiltration may change the on-pump hematocrit. The CPB hematocrit calculator is intended for perfusionists, cardiac anesthesia teams, cardiac surgery clinicians, and trainees who need a quick estimated blood volume, CPB prime volume, and red cell mass balance estimate.
Calculation method & assumptions
Estimated blood volume
EBV (mL) = Weight (kg) × EBV coefficient (mL/kg)
The EBV coefficient is an approximation and should be adjusted to patient type and local practice. Neonates, infants, children, adult males, and adult females may use different estimated blood volume coefficients.
Pre-CPB hemodilution model
Patient RBC volume = EBV × Pre-CPB Hct / 100
RBC product volume = RBC units × Volume per unit
RBC volume added = RBC product volume × Unit Hct / 100
Final estimated volume =
EBV
+ Prime volume
+ RBC product volume
+ Additional crystalloid/colloid added
− Ultrafiltration removed
Predicted Hct (%) = [(Patient RBC volume + RBC volume added) / Final estimated volume] × 100
Prime volume should represent the non-patient volume added to the circuit. If RBC product volume is entered separately, avoid double-counting it in the prime volume unless the calculator is intentionally configured that way.
Additional crystalloid or colloid increases total volume without adding red cell volume. In this simplified model, ultrafiltration decreases total volume without removing red cell volume.
On-pump adjustment model
Base CPB volume = Patient blood volume + Initial prime volume
Current total volume = Base CPB volume + Net I/O change from CPB base
Current RBC volume = Current total volume × Current Hct / 100
RBC product volume = RBC units × Volume per unit
RBC volume added = RBC product volume × Unit Hct / 100
Final estimated volume = Current total volume + Crystalloid/colloid added + RBC product volume − HF/UF removed
Predicted post-adjustment Hct (%) = [(Current RBC volume + RBC volume added) / Final estimated volume] × 100
In this simplified model, ultrafiltration or hemofiltration decreases total volume but does not remove red cell volume. RBC transfusion increases both total volume and red cell volume. Crystalloid or colloid addition increases total volume without adding red cell volume.
Target Hct helper
The target helper compares alternative ways to reach a target hematocrit from the current on-pump state: RBC only, RBC with matching HF/UF removal, or HF/UF only. RBC unit conversion uses the Vol/unit (mL) and RBC Hct (%) entered in the RBC Addition fields.
Limitations
- This is an estimate based on complete-mixing and volume-balance assumptions.
- It is intended for planning and educational support, not as a replacement for measured Hct.
- Actual Hct may differ due to bleeding, ultrafiltration, hemolysis, transfusion timing, sampling differences, fluid shifts, and measurement variability.
- It does not estimate oxygen delivery.
- It does not replace blood gas or laboratory hematocrit, transfusion guidelines, institutional protocols, or clinician judgment.
- Confirm clinically relevant values with measured blood gas or laboratory hematocrit.
FAQ
How is predicted hematocrit calculated?
Predicted hematocrit is estimated using red cell volume and total circulating volume assumptions. The calculation approximates how priming volume, transfusion, or crystalloid volume changes may affect hematocrit.
What is a hemodilution calculator?
A hemodilution calculator estimates how hematocrit changes when blood is diluted by priming solution, crystalloid, or other volume additions. In CPB, this is commonly used to anticipate hematocrit after circuit priming.
Can this predict the exact intraoperative hematocrit?
No. This calculator provides an estimate based on entered volumes and assumptions. Actual hematocrit can differ because of bleeding, hemolysis, ultrafiltration, transfusion timing, sampling differences, fluid shifts, and measurement variability.
Does this replace clinical judgment or blood gas measurement?
No. Results should be used as a planning aid only and should be confirmed with measured hematocrit or blood gas values during clinical care.
How does priming volume affect hematocrit?
A larger crystalloid priming volume increases total circulating volume without adding red cell volume, which lowers the predicted hematocrit through hemodilution.
Selected references
References are selected for CPB hematocrit, hemodilution, oxygen delivery context, and patient blood management. This page avoids medication-only references that are not directly relevant to predicted hematocrit.
- Tibi P, McClure RS, Huang J, et al. STS/SCA/AmSECT/SABM Update to the Clinical Practice Guidelines on Patient Blood Management. Ann Thorac Surg. 2021;112(3):981-1004.
- Wahba A, Kunst G, De Somer F, et al. 2024 EACTS/EACTAIC/EBCP Guidelines on cardiopulmonary bypass in adult cardiac surgery. Br J Anaesth. 2025;134(4):917-1008.
- Casselman FPA, Lance MD, Ahmed A, et al. 2024 EACTS/EACTAIC Guidelines on patient blood management in adult cardiac surgery in collaboration with EBCP. Eur J Cardiothorac Surg. 2024/2025.
Related tools
Related tools for CPB planning: Priming Volume Calculator, BSA Calculator, and DO₂i Calculator.