- Enter the patient's body weight in kg and select sex (this sets the blood-volume factor: 70 mL/kg for males, 65 mL/kg for females).
- Enter the hematocrit (Hct) in %. The estimated total blood volume and plasma volume update automatically.
- Set the number of plasma volumes to exchange per session (default 1.0; a typical TPE prescription is 1.0–1.5).
- Read off the estimated plasma volume, the exchange volume (mL), and the approximate fraction of a macromolecule removed (≈ 1 − e−N) to plan the session and the replacement fluid.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use this estimate when planning a therapeutic plasma exchange (TPE) session — to size the exchange volume and the replacement fluid, and to anticipate how much of a pathogenic circulating macromolecule (autoantibody, immune complex, cryoglobulin, paraprotein) a given prescription will clear. A standard session exchanges 1–1.5 plasma volumes; the estimated plasma volume drives the volume of replacement fluid (usually 5% albumin, with fresh frozen plasma added for coagulopathy or for TTP).
Nephrology indications for TPE
ANCA-associated vasculitis with severe AKI or diffuse alveolar hemorrhage, anti-GBM (Goodpasture) disease, thrombotic thrombocytopenic purpura (TTP), antibody-mediated transplant rejection, recurrent focal segmental glomerulosclerosis (FSGS), and cryoglobulinemia. The estimate helps size the session and choose the replacement fluid.
What it is — and is not
This is a planning estimate, not an exact measurement. Plasma volume depends on hydration, body composition, and disease state, so treat the result as a guide for prescribing volume and replacement fluid — verify the actual apheresis prescription against your machine, the indication category (ASFA), and institutional protocol. Removal of macromolecules also depends on the molecule's distribution between the intravascular and extravascular compartments and on re-equilibration between sessions.
Pearls & Pitfalls
Diminishing returns: 1–1.5 plasma volumes
Removal follows first-order kinetics: the fraction of a circulating macromolecule cleared by an exchange of N plasma volumes is ≈ 1 − e−N. So 1.0 PV removes ~63%, 1.5 PV ~78%, and 2.0 PV ~86%. Because the gain per extra volume falls off steeply, most prescriptions stop at 1.0–1.5 plasma volumes — exchanging more wastes replacement fluid and prolongs the session for little additional clearance.
Quick estimate
The Kaplan shortcut, EPV ≈ 0.065 × weight (kg) × (1 − Hct), gives the same answer as this calculator when the blood-volume factor is ~65 mL/kg. A useful rule of thumb is that estimated plasma volume is roughly 35–45 mL/kg of body weight in an adult of normal hematocrit — handy for a bedside sanity check of the computed exchange volume.
Pitfalls
(1) TPE removes clotting factors — monitor fibrinogen (and the INR) over a series of sessions, and time invasive procedures accordingly. (2) It also depletes immunoglobulins and removes protein-bound drugs, so review timing of essential medications. (3) Citrate anticoagulation can cause hypocalcemia (perioral tingling, paresthesias, arrhythmia) — monitor ionized calcium and supplement. (4) The estimate ignores hydration and body-composition extremes; a markedly volume-overloaded or cachectic patient will deviate from the prediction.
Why Use It
Sizing a plasma exchange correctly matters: the plasma volume sets how much replacement fluid is needed, and the number of plasma volumes exchanged determines how much of the pathogenic macromolecule is cleared. Estimating plasma volume from weight, hematocrit, and sex — rather than guessing — lets you prescribe a defensible exchange volume, anticipate albumin or fresh-frozen-plasma needs, and explain to the team why a session targets 1–1.5 plasma volumes rather than more. Because removal follows first-order kinetics (≈ 1 − e−N), the calculator also makes the diminishing return of larger exchanges explicit, which supports both efficient prescribing and informed consent.
Plasma Volume & TPE Exchange Volume
Enter body weight, hematocrit, and sex to estimate the plasma volume, then set the number of plasma volumes to exchange. The exchange volume and the approximate fraction of a macromolecule removed (≈ 1 − e−N) update automatically.
⚕ Estimated plasma volume = body weight × (70 mL/kg male / 65 mL/kg female) × (1 − Hct/100); macromolecule removal ≈ 1 − e−N. This is a planning estimate, not an exact measurement, and does not account for hydration or body-composition extremes. For licensed clinicians; verify the apheresis prescription against the ASFA indication category, the machine, and institutional protocol.
Next Steps
Use the estimated plasma volume and exchange volume to prescribe the session and its replacement fluid.
- Choose the replacement fluid. Use 5% albumin for most indications; add fresh frozen plasma when there is coagulopathy, for TTP, for diffuse alveolar hemorrhage, or before an invasive procedure. The exchange volume sets how much replacement is needed.
- Set the number of plasma volumes. Standard sessions exchange 1.0–1.5 plasma volumes; because removal follows ≈ 1 − e−N, larger exchanges add little. Plan the series (number and frequency of sessions) by the ASFA indication category.
- Anticipate complications. Monitor fibrinogen and the INR across the series, watch for citrate-induced hypocalcemia (check ionized calcium, supplement), and review the timing of immunoglobulin-based or protein-bound medications.
- For dialysis-circuit prescribing, see the dialysis prescription and dialysis adequacy (Kt/V) calculators.
Evidence & References
Formulas
| Quantity | Formula |
|---|---|
| Total blood volume, TBV (mL) | weight (kg) × (70 male / 65 female mL/kg) |
| Estimated plasma volume, EPV (mL) | TBV × (1 − Hct/100) |
| Kaplan shortcut | EPV ≈ 0.065 × weight (kg) × (1 − Hct) |
| Exchange volume (mL) | EPV × N (plasma volumes) |
| Fraction removed | ≈ 1 − e−N |
Removal vs. plasma volumes exchanged
| Plasma volumes (N) | Approx. fraction removed |
|---|---|
| 1.0 | ≈ 63% |
| 1.5 | ≈ 78% |
| 2.0 | ≈ 86% |
Replacement fluid is usually 5% albumin; fresh frozen plasma is added for coagulopathy, for TTP, for diffuse alveolar hemorrhage, or before an invasive procedure. Because removal follows first-order kinetics, most prescriptions target 1.0–1.5 plasma volumes.
References
- Kaplan AA. Therapeutic plasma exchange: a technical and operational review. J Clin Apher. 2013;28(1):3–10.
- Padmanabhan A, Connelly-Smith L, Aqui N, et al. Guidelines on the Use of Therapeutic Apheresis in Clinical Practice — Evidence-Based Approach from the Writing Committee of the American Society for Apheresis: The Eighth Special Issue. J Clin Apher. 2019;34(3):171–354.
- Reeves HM, Winters JL. The mechanisms of action of plasma exchange. Br J Haematol. 2014;164(3):342–351.
