This question came up in a discussion about a mass transfusion protocol.  The goal that we strive to meet is an equal ratio of packed red cells, platelets, and plasma.  Most in the discussion seemed to have a general awareness that apheresis platelets count as six units in this 1:1:1 scheme.  There was confusion as to why.  Let’s clear that up.

What is apheresis?

Apheresis is a procedure in which whole blood is drawn into a machine that separates it into its components.  One or more of those components are selectively removed. The rest is returned to the donor.  This can be used to harvest not only platelets, but also plasma, red cells, white cells, lipids, and so on.  This technology is most often used to obtain blood products from donors, though there are therapeutic uses as well.  Exchange transfusions, for example in the case of sickle cell crisis, involve removing damaged red cells via apheresis and replacing them with donor packed red blood cells.

What are apheresis platelets?

This refers to platelets obtained by apheresis. A machine collects whole blood from a donor through an IV and separates out the platelets. The plasma and blood cells are then returned to the donor. By definition, the total number is at least 3×10¹¹ platelets in about 250 ml of plasma. Apheresis-derived platelets are often referred to as “single donor platelets.” This manner of platelet collection allows for a comparatively large harvest because the bulk of the blood volume is returned to the patient.

What is the alternative way to obtain platelets?

The traditional way is to separate the platelets from donated whole blood. In this setting, each unit of platelets contains roughly 5.5×10¹⁰ platelets in 50 ml of plasma. This is the number that 1:1:1 mass transfusion protocols are based on. One unit of platelets derived in this manner, plus one unit of packed red cells, plus one unit of fresh frozen plasma would equal one unit of whole blood. To account for the lower number of total platelets, whole blood-derived platelets are dispensed in what is often called a “six-pack.” Six units of whole blood-derived platelets would have roughly the same total number of platelets as an apheresis unit in about the same amount of plasma.

Whole blood-derived platelets are often referred to as “random donor platelets” because there will be six different donors in the six-pack.

A benefit to collecting platelets by apheresis is that any number can be extracted because the bulk of the donors blood is returned to them. They lose very little circulating volume. The only way to increase the yield of whole blood-derived platelets from a given donor would be to take more whole blood. Six units of whole blood would have to be collected to obtain the same platelet yield as one apheresis donation. Clearly, this is not feasible.

Why six units? What is the meaning of the seemingly arbitrary 3×10¹¹ platelets per dose?

I don’t know where this number came from, but this amount raises a recipient’s platelet count by at least 50,000; which would take anyone from zero to a safe enough number to do an invasive procedure like a spinal tap (clearly this is an over simplification – don’t make clinical decisions without more thought than this statement). I suspect that is not a coincidence.

Why use one over the other?

The short answer is that you will use whatever your lab stocks. In the United States, most blood banks stock apheresis-derived platelets.

How does this play into a mass transfusion protocol?

Assuming the goal is a 1:1:1 ratio of PRBC:FFP:platelets, one must consider the fact that the “1” unit of platelets is one unit of whole blood-derived platelets.  A “six-pack” of random donor platelets (whole blood-derived) would count for six units in a 1:1:1 mass transfusion protocol.  An apheresis unit is the same.  It counts for roughly six.  So, to clear up any confusion, in terms of counting bags of blood products being hung: the actual ratio of PRBC:FFP:platelets would be roughly 6:6:1 whether using a six-pack of (“pooled”) random donor platelets or a unit of single donor platelets. Now you know why.

I hear people say “apheresis pack.” What is that?

That’s not a thing. It’s an amalgamation of “six-pack” (of random donor platelets) and “apheresis unit” (of single donor platelets). Nothing about apheresis-derived platelets comes in packs. The more precise term would simply be an apheresis unit.

1. Sloan, SR. Chapter 36: Transfusion Medicine. In: Orkin SH, Fisher DE, Look AT, Lux SE, Ginsburg D, Nathan DG, eds. Nathan and Iski’s Hematology and Oncology of Infancy and Children. 8th ed. Saunders/Elsevier; 2015:1127-1164.