User:DiverDave/Pretransfusion compatibility testing

Pretransfusion compatibility testing refers to the testing that is performed prior to a blood transfusion, to determine if the donor's blood is compatible with the blood of an intended recipient. Transfusion of incompatible blood may result in a severe acute hemolytic transfusion reaction in the recipient, characterized by activation of the complement system, massive hemolysis of the transfused blood, renal failure, circulatory shock, and even death. This risk can be minimized if the donor unit is both ABO and Rh compatible.

Type and screen
The American Association of Blood Banks Standards for Blood Banks and Transfusion Services requires that certain procedures be performed before elective transfusion of blood components to a recipient:
 * Positive identification of the recipient and recipient blood sample.
 * ABO and Rh typing of the recipient's blood and resolving any ABO discrepancies.
 * Performing an antibody screen on the recipient's serum for clinically significant antibodies.
 * Results of previous antibody testing, if any, should be compared with those of current testing. Any history of clinically significant antibodies, even if undetectable now in the patient, dictates an antiglobulin phase crossmatch needs to be done between the recipient's serum and the donor's cells.

All of these requirements are accomplished by the type and screen. The term "Type and Screen" is used for the testing that (1) determines the blood group (ABO and Rh compatibility) and (2) screens for alloantibodies. This battery of tests takes about 45 minutes to complete. The blood bank technologist also determines whether the patient has any special requirements (eg. need for washed, irradiated or CMV negative blood), or any history of a previously identified alloantibody. Patients who have, or have a history of clinically significant alloantibodies in their serum should receive antigen-negative phenotyped red blood cells in order to avoid a transfusion reaction.

Once the type and screen has been completed, potential donor units will be selected based on ABO and Rh compatibility, special requirements, and presence or absence of alloantibodies.

Crossmatch
Crossmatch is usually performed only after other, less complex tests have not excluded compatibility. Blood compatibility is determined not only by the blood types (O, A, B, AB), but also by blood factors, (Rh, Kell, etc.).

Cross-matching involves mixing a sample of the recipient's serum with a sample of the donor's red blood cells and checking if the mixture agglutinates, or forms clumps. If agglutination is not obvious by direct vision, blood bank technicians usually check for agglutination with a microscope. If agglutination occurs, that particular donor's blood cannot be transfused to that particular recipient. In a blood bank it is vital that all blood specimens are correctly identified, so labeling has been standardized using a barcode system known as ISBT 128.

Crossmatch can be done serologically or electronically. If no alloantibodies are suspected or detected during the type and screening process, either the immediate spin or CAC (computer assisted crossmatch) methods are sufficient for crossmatch.

Serological crossmatch
In serological crossmatch, red blood cells from the donor unit are tested against the plasma of the patient in need of the blood transfusion. If the patient’s serum contains antibodies against the antigens present on the donor red blood cells, agglutination will occur. Agglutination is considered a positive reaction, indicating that the donor unit is incompatible for that specific patient. If no agglutination occurs, the unit is deemed compatible and is safe to transfuse.

There are two phases to the complete serological crossmatch:
 * 1. immediate spin phase; and
 * 2. antiglobulin phase

Immediate spin crossmatch
The patient's ABO and Rh type is determined and the serum is screened for the presence of clinically significant alloantibodies. If the antibody screen is negative, and there is no record of such antibodies having been detected in the past, the compatibility test may be abbreviated to include only the immediate spin phase, which demonstrates ABO incompatibility.

In the immediate spin method, two drops of patient serum are tested against a suspension of donor cells in a test tube and spun in a serofuge. Agglutination or hemolysis in the test tube is a positive reaction and the unit should not be transfused.

Antiglobulin crossmatch
A positive antibody screen warrants an antibody panel/investigation, which includes a direct antiglobulin test (DAT). The antibody panel consists of commercially prepared group O red cell suspensions from donors that have been phenotyped for commonly encountered and clinically significant alloantibodies. The patient's serum is tested against the various donor cell suspensions using an enhancement method, eg Gel or LISS. Based on the reactions of the patient's serum against the donor cells, a pattern will emerge to confirm the presence of one or more antibodies. Not all antibodies are clinically significant (i.e. cause transfusion reactions, HDN, etc).

The complete crossmatch with a 37oC incubation and antiglobulin phase should be done on patients whose immediate spin crossmatch is positive, or whose antibody screen is positive in Coombs, or whose records show a previously-detected clinically significant antibody, even though it may not be currently detectable.

If an antibody is suspected, potential donor units must first be screened for the corresponding antigen by phenotyping them. Antigen negative units are then tested against the patient plasma using an antiglobulin/indirect crossmatch technique at 37 degrees Celsius to enhance reactivity and make the test easier to rea

If the antibody screen is negative, it is not necessary to perform the antiglobulin phase of the crossmatching procedure.

Electronic crossmatch
Electronic crossmatch is essentially a computer-assisted analysis of the data entered from testing done on the donor unit and blood samples drawn from intended recipient. This includes ABO/Rh typing of the unit and of the recipient, and an antibody screen of the recipient. Electronic crossmatch can only be used if a patient has a negative antibody screen, which means that they do not have any active red blood cell atypical antibodies, or they are below the detectable level of current testing methods. If all of the data entered is compatible, the computer will print a compatibility label stating that the unit is safe to transfuse.

ABO compatibility requirements for blood components
Ideally, all blood components for transfusion to any recipient would be identical in ABO and Rh type. However, some blood types (e.g., AB negative) are uncommon, and this can result in a scarcity or even complete unavailability of identical blood components. At a minimum, transfused donor red cells must be ABO compatible with the patient's plasma and whatever antibodies may be present. Transfused plasma must be ABO compatible with the recipient's red cells.

Emergencies
Patients should ideally receive their own blood or type-specific blood products to minimize the chance of a transfusion reaction. Risks can be further reduced by cross-matching blood, but this may be skipped when blood is required for an emergency.

Type-specific blood
In an urgent situation, blood grouping can be done easily and quickly in 2 or 3 minutes in the laboratory on glass slides with appropriate reagents, by trained technical staff. This method depends on the presence or absence of agglutination, which can usually be visualized directly, although occasionally a light microscope may be needed. If laboratory services are not available, another system of deciding which type of blood to use in an emergency is the bedside card method of blood grouping, where a drop of the intended recipients' blood is added to dried reagents on a prepared card. This method may not be as reliable as laboratory methods, which are preferable.

Uncrossmatched blood
In certain emergency settings (such as life-threatening exsanguination), there may be inadequate time even for simple determination of a patient's ABO or Rh antibody group. Under such circumstances, it may be necessary to transfuse donor units of blood that have not been crossmatched. Though this approach carries a higher risk of an antibody-mediated transfusion reaction, it may be lifesaving, and so the risk-benefit ratio is favorable.

In this setting, group O red blood cells are chosen. Either Rh positive or Rh negative units may be transfused, though Rh negative are preferred for children and women of childbearing age. Where possible in such cases, it is preferable to obtain a pre-transfusion blood sample from the patient so that a proper type and screen can be performed. This makes it possible to transfuse units of the correct type (and also for the blood bank to preserve its inventory of valuable type O red blood cells), in the event that further transfusion is necessary.