52 Pre-warming Technique

Principle

The pre-warm technique can be used to eliminate the reactivity of cold-reacting, clinically insignificant antibodies, while still allowing for the detection of warm-reacting, clinically significant antibodies.

Prior to testing, the patient’s plasma and test red cells are warmed separately to 37°C. The pre-warmed plasma and cells are used to perform testing. For antibody screening, incubate at 37°C, wash with warm saline to remove unbound antibody, and then carry through the antihuman globulin phase using an Antiglobulin reagent. By maintaining strict adherence to a 37°C temperature, only clinically significant antibodies capable of reacting at body temperature should be detected. This technique can also be used with ABO and Rh testing where the tubes with reagents are prewarmed and then combined with the prewarmed patient plasma or prewarmed, washed patient red cells to obtain accurate results.

Related Policies

This procedure applies to all test protocols that require the pre-warm technique to eliminate the reactivity of cold-reacting antibodies. (ie, ABO, Rh testing, antibody screening, antibody identification, and crossmatching)

Specimen

Serum or plasma may be used. Specimen age must comply with policy and procedures of the institution.

Reagents

1. Normal saline(37°C)
2. AHG reagent. (polyspecific or anti-IgG may be used)
3. Screening/Panel Cells or A1 and B cells ..
4. LISS additive- if using LISS IAT method.
5. CC’s for addition to negative AHG tests.

Procedure

1. Label tubes with patient name and test identification (cell or antisera)
2. Pre-warm a bottle of saline to 37°C.
3. Add 1 drop of saline suspended red cells to each, appropriately labeled, clean tube.
4. Place the tubes containing red cells and a labeled tube of the patient’s plasma and a BB transfer pipette at 37°C; incubate for 5-10 minutes.
5. Transfer 4 drops of pre-warmed plasma using the warm pipette to each tube containing the pre-warmed red cells. Mix without removing the tubes from the incubator.
6. Incubate at 37°C for the appropriate length of time (depending on method). If performing ABO or Rh testing using pre-warm, no incubation is required. Tubes are spun at this point and then results are read and recorded.
7. Without removing the tubes from the incubator, fill each tube with pre-warmed saline (37°C). Centrifuge and wash three or four times with 37°C saline. Completely decant the last saline wash.
8. Add 2 drops of AHG to the washed red cells.
9. Mix thoroughly but gently and centrifuge for 20 seconds.
10. Gently re-suspend the cell button and examine macroscopically, with the use of an optical aid, for agglutination.
11. Grade and record results immediately while holding tube in hand.
12. Confirm validity of negative tests by adding CCs.

Interpretation

1. If an antibody was initially reactive in the routine indirect AHG test but is non-reactive in the pre-warmed AHG test, the antibody is considered to be cold reactive, clinically insignificant because it is non-reactive at 37oC.
2. If an antibody is still reactive in the pre-warmed AHG test, it is likely clinically significant and should be identified. Provide Antigen negative, XM compatible RCs for transfusion.
3. The absence of agglutination and hemolysis is a negative test result.
4. There must be agglutination in the last step with the IgG-sensitized red cells (CCs) confirming the presence of active antiglobulin reagent in the test mixture. If there is no agglutination or results are indiscriminate, repeat the negative testing. Record reaction strength of positive CC results (1-4+).

Notes 

1. This procedure need not be performed when reactions are only present at IS with the exception of interference in ABO and/or Rh testing with cold antibodies.
2. Pre-warming may not be effective in eliminating the reactivity of an extremely potent cold antibody with wide thermal amplitudes of 30-37°C, and the pre-warmed AHG test may still be positive. A cold autoadsorption procedure may be required.
3. This procedure does not allow for the detection of alloantibodies that agglutinate at 37°C or lower and/or do not react in the antiglobulin phase.