Chapter 10 Pretransfusion Testing

Pretransfusion Compatibility Testing

• Misidentification of the recipient is the most common cause of error (47%).
• The requisition form and patient's wristband identification must be compared to prevent wrong sample collection.
• It may be useful to use barcode printing rather than handwritten forms.
• Hemolysis of the sample must be avoided, as it can mask hemolysis caused by antigen-antibody complexing with complement.
• Serum or plasma may be used for pretransfusion testing.
• Before leaving the recipient's bedside, tubes must be labeled, and printed labels compared with the request form and wristband.
• Blood samples should not be taken from IV tubing lines; if necessary, disconnect the line for 5–10 minutes and discard the first 10 ml of blood.
• The technician must check the sample to ensure it matches the request form and unlabeled samples require a new sample be obtained.
• Samples for donor testing should be collected at the same time as the full donor unit, should be fully labeled.
• RBCs from the donor for pretransfusion compatibility testing can be prepared from the segment tubing from which the donor blood was collected.
• Donor and recipient samples must be stored for a minimum of 7 days following transfusion for re-evaluation of expected transfusion reaction.
• Donor samples must be tested for ABO and Rh group (including a test for weak D) and antibody screen.
• Tests to prevent disease transmission must also be performed.
• All results must be clearly indicated on all product labels appearing on the unit.
• Records must be maintained for all results obtained in testing patient samples.
• The record should include ABO, Rh and unexpected antibody screen test results.
• Patient samples can be used for crossmatch for 7 days, except if the patient has been transfused in the past 14 days.
• A fresh sample, no more than 24 hours old used, can be used for 72 hours after transfusion.

Selection of Appropriate Donor Units & Crossmatch Testing

• Blood for transfusion units should be close to the expiry date, and units should be visually checked for any abnormal or unusual appearance.
• A crossmatch is only a part of the pretransfusion compatibility test that includes testing the patient's serum against the donor's blood "RBCs" in all phases of the indirect test.
• The main function of the serologic crossmatch is to serve as a final check of ABO compatibility between the donor and recipient.
• It may also detect the presence of an antibody in the patient's serum that will react with the antigens on the donors RBCs not detected in the antibody screen.
• Current AABB standards state that ABO incompatibility detection tests are sufficient if the antibody screen is negative for clinically significant antibody.
• It is also sufficient if there is no historical record exists for the detection of clinically significant antibody.
• The serologic crossmatch test includes an immediate spin crossmatch and an antiglobulin crossmatch.
• Positive results show agglutination or hemolysis, and no agglutination yields negative results.
• All work should be signed by the technologist performing the test.
• A positive result in the crossmatch test requires explanation, and the recipient should not receive a transfusion until the cause of the incompatibility has been determined.

Investigation of Incompatible Major Crossmatches

Crossmatch Positive (+), Auto-control Negative (-), Antibody screen Negative (-)

• Possible Interpretation: Incorrect ABO grouping of patient or donor
  • Comments and Resolve: Repeat ABO grouping: verify identity of sample.
• Possible Interpretation: Patient's serum may contain an ABO antibody
  • Comments and Resolve: Check patient's sample for subgroups; check patient's transfusion and transplantation histories; passive Anti-A, -B, -AB.
• Possible Interpretation: Alloantibody in patient's serum reacting with antigen donor's red cells but not present on screening cells
  • Comments and Resolve: Perform antibody identification tests on patient's serum and repeat crossmatch using units negative for the corresponding antigen.
  • If studies are noninformative and patient is incompatible with only 1 unit, locate other compatible units. Antibody to low incidence antigen.
• Possible Interpretation: Donor unit may have a positive DAT
  • Comments and Resolve: Perform DAT on donor unit; if positive, do not use the unit.

Crossmatch Positive (+), Auto-control Negative (-), Antibody screen Positive (+)

• Possible Interpretation: Alloantibody in patient's serum reacting with antigens on donor's cells and screening cells
  • Comments and Resolve: Perform antibody identification studies on patient's serum and repeat crossmatch using units negative for the corresponding antigen.
  • If unable to identify antibody specificity, consult a reference laboratory.

Crossmatch Positive (+), Auto-control Positive (+), Antibody screen (+)

• Possible Interpretation: Both an autoantibody and alloantibody may be present in the patient's serum
  • Comments and Resolve: Perform autoadsorption of patient serum to remove autoantibody (if not recently transfused), perform antibody identification tests, repeat compatibility tests using autoadsorbed serum

Potential Abnormalities

• Abnormalities in patient's serum owing to imbalance of A/G ratio
  • Plasma expanders
  • Caprylate antibodies "Anti-reagent in AHG"
  • Contaminants
• Seen rouleaux use saline replacement technique. Replace specimen. Use caprylate-free reagents.
• Repeat tests using fresh saline, new bottles of reagent, clean test tubes.

Computer Crossmatch

• The computer crossmatch compares recent ABO serologic results and interpretations on file for both the donor and the recipient being matched, and it determines compatibility based on this comparison.
• The patient must be antibody screen negative for a recent sample, and if there is no history of unexpected antibody, a patient should have at least two records of ABO/Rh type in the file.

Compatibility Testing in Special Circumstances

• In an emergency, the patient's ABO and Rh group should be determined to give group-compatible blood.
• If no time to obtain and test a sample, group O Rh negative packed cells can be used.
• For Rh-negative patients needing large amounts of blood, conversion to Rh-positive may be applicable if the patient is a man or a woman beyond childbearing age.
• Pretransfusion testing should be completed, and any incompatible result should be reported to the physician and the blood bank medical director.
• When the recipient has received large amounts of alternative blood group, additional blood group specific units may agglutinate at immediate spin due to passive expected antibody; in this case, additional units should be of the alternative blood group.
• Current standards do not require a crossmatch test as part of the compatibility testing for the transfusion of plasma products.
• In intrauterine transfusion, crossmatch testing is performed using the mother's serum sample; O Rh-negative packed RBCs should be selected for the transfusion.
• Blood for exchange or transfusion of a neonate (20 days old) should be compatible with any maternal antibodies, either regular (Anti-A and Anti-B) or irregular (IgG) that could cross the placenta. The blood should be as fresh as possible and not older than 7 days.
• Massive transfusion occurs when the patient receives blood volume equal to their total blood volume within 24 hours.
• Prolonged clotting time is due to coagulation abnormalities with disease or medication.
• Complete coagulation of a sample with prolonged clotting time can often be accelerated by adding thrombin, one drop of thrombin 50U/ml to 1 ml of plasma.
• Use Protamine sulfate for heparin effects specimen.
• Autologous transfusion refers to a donor donating for oneself for a future procedure.
• Autologous donation is useful for rare blood groups; no tests for unexpected antibody and tests for transmitted diseases are required in this case.

Important Points for Sample Collection and Compatibility Testing

• Fatal transfusion reactions are mostly caused by clerical errors.
• Samples and forms must contain the patient's full name and unique identity number.
• Writing must be legible and indelible.
• Date of collection must be written on the sample.
• Collect the Sample within 3 days of scheduled transfusion.
• A blood bank specimen must have two unique patient identifiers, the date of collection, and the initials or signature of the person who collected the sample.
• Confirm blood type of donor.
• Check patient records for results of previous tests; perform ABO grouping, Rh typing, and antibody screening on the patient.
• Select the donor unit based on the ABO group and Rh type of the patient; further consider the presence of antibodies in the patient by antigen-typing donor unit for the corresponding antigen.
• Perform immediate spin or antiglobulin crossmatch based on current or historical serologic results.
• Electronic crossmatch can replace immediate spin crossmatch when two blood types are on file for the patient, and the antibody screen is negative.
• Positive results in the crossmatch may be caused by incorrect ABO grouping of patient or donor, alloantibody or autoantibody in patient reacting with the corresponding antigen on the donor RBCs, donor having a positive DAT, abnormalities in patient serum such as increased protein concentration (rouleaux), or contaminants in test system.

Compatibility Testing in Special Circumstances

Emergencies

• May select uncross matched, group O, Rh-negative packed RBCs.
• Use uncross matched, group O, Rh-positive packed RBCs if the patient is male or a female beyond childbearing years.
• May provide type-specific, uncross matched RBCs.

Plasma products units

• No compatibility testing required.

Transfusion to fetus

• Perform Compatibility testing using the mother's sample.
• The Donor unit must lack antigen against maternal antibody.
• Select Group O Rh-negative donor when fetal type is unknown or when type is known but not compatible with mother's type.

Transfusion to infant

• Use Maternal sample for compatibility testing.
• Initial sample from infant typed for ABO (front type) and Rh.
• The donor unit selected should be compatible with both mother and baby.