Blood transfusion rates remarkably high on the spectrum of “things we do a lot, yet poorly understand.” In fact, transfusion is often shrouded in mystery for everyone, including internists, surgeons, even hematologists. The only ones who really seem to grasp it are transfusionists, the actual “blood bankers” (a board-certified specialty of pathology), and they are a rare beast, usually toiling invisibly in the background of our hospitals.
That’s too bad, because a nuanced understanding of transfusion medicine can add a lot to our craft. Like most things, clinicians may not use all of these nitty-gritty details on a daily basis, but it shouldn’t be a black box, either; you should have that knowledge tucked away, ready to be wielded when a complex situation presents itself. Let’s spend some time building that understanding.
Just what is banked blood?
Most blood we transfuse is donated blood, and perhaps you’ve been on the other side of this: you visited a donation site, such as those run by the American Red Cross, and after a screening process to ensure your blood is a suitable candidate for donation, you were phlebotomized.
What you donated was generally one unit of whole blood. What’s that? It’s about 500 ml (for you beer-drinkers, around a pint) of the same stuff circulating in your body. If you’re curious about its hematocrit, or its platelet count, or its white blood cell count, just consult the normal lab ranges for those values, because that’s what it is. It’s blood.
For various reasons, however, this blood is usually not left “whole.” Instead, it is centrifuged down to component blood products. Along with some practical benefits (involving issues like compatibility and shelf life), this allows multiple patients to benefit from a single blood donation. The usual resulting components:
- 1 unit of packed red blood cells (PRBCs). This is about 300–400 ml of red cells, floating in a small volume of plasma and saline, as well as some preservatives and citrate for anticoagulation. It’s kept refrigerated.
- 1 unit of fresh frozen plasma (FFP). This is about 275–300 ml of plasma, which is quickly frozen to maintain “freshness,” since some of its constituent factors are labile and don’t last long on the shelf. It’s stored frozen and thawed in a warm-water bath when ready for use, which takes around 20 minutes. One exception is busy trauma centers, which may keep a small amount of thawed (or never-frozen) liquid plasma on hand for emergencies.
- 1 unit of platelets (about 50 ml). Except… not really. In the US, these “whole blood derived” platelets are rarely prepared. Instead, most platelet products come from apheresis, a different donation process where a larger volume of blood is drawn, its platelets spun off, and the remaining blood returned to the donor. (You can do this at the donation center too, although it’s a longer appointment.) This results in a pooled, single-donor platelet “pack” which is equivalent to 5–6 “units” worth. In a system that uses mainly apheresis platelets, the platelets in whole blood are not needed, and are typically not even separated out; they stay where they are, mostly in the plasma, where they become largely inactivated by the freeze-thaw process. Platelets are stored at room temperature, as temperature extremes can cause theoretical problems with their function, although this unfortunately limits their shelf life. This makes platelet shortages common, as well as making them the highest-risk products for transfusion-related infections.
One final, less-used component is cryoprecipitate. “Cryo” is produced by a process of partial thawing and centrifugation of FFP, which allows certain clotting factors to precipitate into a highly-concentrated sludge. Since one unit of cryo is just a dribble, it’s usually pooled (similar to platelets), with typical doses for clinical use between 5–10 units. It primarily contains fibrinogen and factor VIII, along with von Willebrand factor, factor XIII, and some weird stuff called fibronectin. The “cryo-poor” plasma that’s left behind can be used for a few specialized applications, such as plasma exchange for TTP patients, but otherwise tends to be discarded.
A few points: you’ll notice that the idea of a “unit” is a relative measurement. A “unit” of whole blood is about a pint, but a “unit” of a component product is simply the volume derived from one unit of whole blood, and hence is smaller and somewhat variable. A unit of platelets, on the other hand, is an even more confusing concept, and I recommend referring to pooled apheresis bags as “packs” to avoid the confusion with true single units.
You may also note that if you combine the component products, the volume adds up to more than the original volume of whole blood. This is a good reminder that banked blood are not the same as fresh, whole blood, even if we transfuse each of its constituent components. It’s more dilute, due to the addition of anticoagulants and preservatives; it’s cold; it carries less oxygen and retains less 2,3-DPG; its pH is lower and the hemoglobin are less pliable; and of course, it conveys a variety of risks. None of this should make us hesitate to use the stuff when it’s needed—but it should be treated with an appropriate amount of respect, and is no substitute for simply keeping the patient’s own blood inside their body.
In Part 2, we’ll talk about cross-matching blood for transfusion.