Why is pre-transfusion testing necessary?

In daily life, we see a lot of scenes in movies and TV shows where a person needs a blood transfusion because of heavy bleeding. Movies and TV shows are based on real life. In real life, some people have also encountered situations where they need a blood transfusion. At this time, some people may have such questions: I need a blood transfusion, why do I need to draw blood samples for testing? Why do blood transfusions seem to be very simple in movies and TV shows, but there are many procedures for blood transfusions in real life? Why do I need to wait "a long time" before I can get a blood transfusion? Why do I or my family need to draw blood samples in advance for the so-called blood preparation when I need to have surgery? I know my blood type, why do I need to match blood before transfusion? Why can't the blood of donors with the same blood type be used directly?

In addition to red blood cell products, the blood products we commonly use include plasma and platelets. As the name suggests, red blood cell products are blood products with red blood cells as the main component. According to the patient's condition, appropriate red blood cell products will be selected for transfusion. There are no red blood cells in plasma. It is separated from whole blood and is yellow. The main purpose of plasma transfusion is to supplement coagulation factors, so some patients with poor coagulation function can consider plasma transfusion. The platelets used by clinical patients now are basically donated by blood donors directly as single-collected platelets. The whole blood of the blood donor is separated and only the platelet part is taken. This part of the platelets will be suspended in the plasma, so it will also appear yellow. Before transfusing blood products, the doctor will judge whether the patient needs to be transfused with blood products, as well as the type and amount of blood products to be transfused based on the patient's condition and examination indicators. This is actually one of the most basic examinations before transfusion.

We all know that blood transfusion plays an irreplaceable role in medicine, especially in emergency situations, which can effectively save the patient's life and even "pull the patient back from the brink of death". The purpose of blood transfusion therapy is safety and effectiveness, and its fundamental purpose is to save patients. All our work is based on this fundamental purpose. We must not only save patients, but also avoid adverse reactions as much as possible. To achieve the above goals, we need to select blood products with "same blood type" for transfusion.

How do we do it in actual work?

Before that, let's first clarify a concept. What is blood type? In scientific terms, blood type refers to the type of antigen on the surface of blood components (including red blood cells, white blood cells, and platelets). The blood type usually refers to the specific antigen type on the red blood cell membrane, and the most closely related to clinical practice is the red blood cell ABO blood type system and the Rh blood type system. The ABO blood type system is the most common A, B, O, and AB types we usually talk about. That is, the expression of a certain antigen on our red blood cells is the corresponding blood type. The Rh system is much more complicated than the ABO system. In real life, we only focus on the D antigen for the Rh blood type system. A word we often hear, "panda blood", refers to blood with negative RhD antigen. Because this type of people is relatively rare in my country, we use "panda blood" to call their blood. As of November 2022, humans have discovered 44 blood type systems. More blood type systems are being studied and continuously discovered.

So what do we mean by "same blood type"?

Strictly speaking, "blood type consistency" means that the patient who needs a blood transfusion and the donor have exactly the same blood types in more than 40 systems, even those that have not been discovered. Only in this way can it be safe for two people to transfuse blood to each other. However, this is impossible in daily work. How can we find this person in such a vast sea of ​​people? Even from a genetic point of view, it is impossible to find two people with the same blood type unless they are identical twins.

Since this is an "impossible task", how do we ensure the safety of blood in our daily work? At this time, we need to conduct pre-transfusion testing.

Pre-transfusion testing completed by the blood transfusion department generally includes blood type verification, antibody screening and cross-matching. Blood type verification is to find blood from donors with the same ABO blood type and RhD antigen as the patient, which can effectively improve the safety of transfusion. This is what we generally think of as "blood type consistency". Antibody screening can screen out irregular antibodies in the plasma of transfused patients, that is, red blood cell antibodies other than anti-A and anti-B. In this way, blood from donors that do not react with the recipient's blood can be found. When the antibody screening test is positive, we will think that the patient has irregular antibodies in his body, and we need to conduct antibody identification tests to determine what this antibody is. Then screen the patient for blood products with negative corresponding antigens for transfusion. However, the blood type systems involved in antibody screening are limited, and only antibodies of several blood type systems that are often seen and studied more thoroughly can be screened out. Some antibodies involved in blood type systems that appear less frequently in the population often cannot be screened out by antibody screening experiments. At this time, cross-matching can solve this problem. The cross-matching test directly reacts the plasma and red blood cells of the donor with the red blood cells and plasma of the recipient, simulating the in vivo environment to the greatest extent possible. It takes into account that the antibodies in the recipient's body should not destroy the transfused red blood cells, and that the antibodies in the donor's plasma should not destroy the recipient's red blood cells, thus eliminating the occurrence of blood incompatibility to the greatest extent possible. It is worth noting that regardless of whether the patient's antibody screening test is negative or positive, a cross-matching test must be performed to select compatible blood products for the patient for transfusion.

So, is the blood obtained after blood type compounding, antibody screening, and cross-matching absolutely safe? The answer is: No. Because there will be some other adverse reactions in the process of blood transfusion that are not caused by inconsistent blood types, such as: fever reaction, allergic reaction, transfusion-transmitted disease, transfusion-related lung injury, etc. In order to avoid these situations as much as possible, a series of other tests will be done on the patient before blood transfusion to determine whether the patient's physical condition can benefit from the blood transfusion process, or whether the patient's benefits outweigh the risks. Of course, the blood donor will also be tested before blood transfusion to ensure the safety of blood transfusion to the greatest extent. Therefore, even in an emergency, blood matching is required before blood transfusion, and blood with the same ABO blood type cannot be directly selected for direct transfusion.

Author: Gao Hongjinwen Civil Aviation General Hospital