Cross Yourself and Ward Off Evil: The Case Against the Plasma Chaser
A patient is partway through a routine therapeutic plasma exchange. The replacement fluid has been albumin the whole way through, as it usually is. But as the procedure nears its end, the last exchange volume gets switched to 2 units of FFP to "finish" the procedure. Nothing about the patient's clinical picture changed in that last liter. The switch happens because ending an exchange on plasma feels safer than ending it on albumin, even though nothing about the math supports that feeling.
I've started calling this the plasma chaser, and I think it's worth being honest about what it actually is. It isn't a treatment. It's a ritual. And rituals, unlike treatments, don't have to work to persist.
What the chaser is supposed to do
If you ask someone why the last unit or two gets switched to plasma, you'll usually get one of two answers: either they're worried about the coagulopathy the exchange is causing, or they're worried about oncotic pressure and albumin. Both are reasonable things to worry about. Neither is what finishing on 2 units of FFP actually addresses.
The tell is that the switch rarely comes with a stated target. It's not "switching to plasma for the last liter to correct a specific factor deficit." It's just a reflex, a way of not ending the procedure on a note that feels too far from whole blood. That vagueness is the first sign we're dealing with superstition rather than a plan.
The math doesn't work
Here's what a plasma exchange actually does to clotting factors: removal and replacement with a non-plasma fluid drops circulating factor levels substantially, often by 60% or more from baseline by the time the procedure is winding down. That depletion happens across the whole exchange. It doesn't reset just because the last bag is plasma.
A single unit of FFP raises any given factor level by roughly 2-3%. Finishing with 2 units buys back maybe 5-10 percentage points of factor activity. Against a 60% deficit built up over the entire procedure, that leaves the patient down by roughly half from where they started, essentially unchanged by the switch. The last 2 units don't correct the depletion the exchange caused; they barely register against it.
If the actual goal is meaningful coagulation correction, the literature is clear that weight-based dosing, 10-15 mL/kg, meaning 4 or more units for a normal-sized adult, is what correlates with real change. Finishing on 2 units is a gesture that looks like it's addressing the depletion and does almost nothing to it.
The oncotic math doesn't work either
If the real concern is albumin or oncotic pressure rather than coagulation, FFP is an especially poor tool for the job. Plasma albumin concentration is only about 3.5-5 g/dL to begin with, and 2 units is roughly 500 mL, a trivial volume against total plasma volume. If oncotic support is genuinely the goal, the replacement fluid already running, whatever concentration of albumin was chosen for the procedure, is the tool built for that purpose. Switching the last liter to FFP doesn't add meaningful oncotic support; it just changes what's in the bag for no measurable benefit.
The exposure is real even when the benefit isn't
This is the part that should bother us more than it does. Every unit of FFP is a separate donor exposure, and every donor exposure carries its own small but nonzero risk: allergic or anaphylactoid reaction, TRALI, TACO, alloimmunization. None of that risk scales down just because the switch happens at the end of a procedure that was otherwise entirely non-plasma.
So the actual trade being made by finishing on 2 units is this: real, if modest, risk, in exchange for a physiologic effect too small to touch what the exchange depleted. That's not a conservative choice. It's the illusion of one.
A framework that isn't ritual
There is a version of this that makes sense, and it isn't complicated. Before choosing what to run for any portion of the exchange, decide what you're actually treating:
If there's no clinically significant coagulopathy anticipated or observed, run albumin for the entire procedure, start to finish. There's no reason for the last liter to be different from the rest.
If the factor depletion is genuinely severe enough to warrant correction, replace with plasma at a volume that actually moves the deficit, not a token switch at the end.
If there's a real but moderate concern, a deliberate proportion of plasma throughout the exchange, scaled to the actual depletion, is defensible. Switching to 2 units only because the procedure is ending is not.
Each of those has a rationale you could say out loud in front of the patient. "Finishing on plasma, just in case" does not.
Decide, and commit
The plasma chaser survives because it feels like doing something without requiring you to answer the actual question: is this exchange causing a clinically significant factor deficit, or not? That question has an answer before the procedure even starts, and it doesn't change because the bag count is running low. Once you know the answer, the replacement fluid follows from it, start to finish, and the ritual has nothing left to justify.
So decide what your replacement fluid is for before the procedure begins; not as a hedge, not as a way to end things on the "safer-feeling" bag, but as a stated clinical judgment. Then commit to that choice for the whole exchange. If you can't say what you're correcting, you're not treating the patient. You're just crossing yourself and hoping.
