Blood Type Testing in the Laboratory
The standard laboratory test for determining a patient's blood type is ABO and RhD typing performed by serological hemagglutination methods, with forward typing (testing patient red cells against known antibodies) and reverse typing (testing patient serum against known red cells) to confirm ABO group. 1, 2
Standard Testing Methods
Primary Serological Testing
- Forward typing tests the patient's red blood cells against known anti-A and anti-B antibodies to identify which antigens (A, B, both, or neither) are present on the red cell surface 1, 2
- Reverse typing (back typing) tests the patient's serum/plasma against known A and B red cells to detect expected isoagglutinins (naturally occurring antibodies), which must match the forward typing results for confirmation 1, 2
- RhD typing determines presence or absence of the D antigen, classifying patients as Rh positive (D+) or Rh negative (D-) 1, 3
Testing Platforms
- Hemagglutination test (tube method) remains the most common traditional approach in clinical laboratories 2
- Microcolumn gel test (gel card technology) is widely used and provides standardized, objective results with reduced technical variability 1, 2
- Both methods are acceptable for routine ABO/RhD typing, though gel methods offer advantages in automation and interpretation 1
Extended Typing for Specific Populations
Transfusion-Dependent Patients
- Extended red cell antigen profiling beyond ABO/RhD should include Rh antigens (C/c, E/e) and K antigen for patients with sickle cell disease or other conditions requiring chronic transfusion 4
- This extended profile can be determined by either serological phenotyping or DNA-based genotyping 4
- DNA-based genotyping is preferred over serologic typing when recent transfusion or interfering antibodies are present, as it provides improved accuracy and can detect up to 40 antigens 4
Bleeding Disorder Evaluation
- ABO blood group typing is performed in 70% of initial workups for patients with suspected bleeding disorders, as blood type O individuals have naturally lower von Willebrand factor levels 4
Critical Pre-Analytical Considerations
Sample Collection and Handling
- Blood samples must be collected atraumatically to avoid hemolysis and cellular activation 5
- Samples should be processed at room temperature and plasma separated promptly 5, 6
- If testing is delayed beyond 2 hours, samples must be frozen at or below -40°C 5
Factors Affecting Results
- Patient factors that can alter antigen or antibody expression include recent transfusion (within 3 months), pregnancy, acute illness, inflammatory conditions, and certain medications 4, 5
- Technical factors include improper sample handling, temperature variations during transport, and delayed processing 5, 6
Compatibility Testing Before Transfusion
Antibody Screening
- Antibody screening detects unexpected antibodies in patient serum that could cause transfusion reactions 1, 3
- Positive antibody screens require antibody identification to determine specificity before compatible blood can be selected 4, 3
Crossmatch
- Crossmatch testing serves as the final compatibility check between donor red cells and patient serum before transfusion 1, 7
- This test detects incompatibilities not identified by ABO/RhD typing and antibody screening alone 1
Common Pitfalls to Avoid
- Relying on single typing method: Always perform both forward and reverse typing for ABO confirmation; discrepancies require investigation before transfusion 1, 2
- Ignoring recent transfusion history: Serological typing may be inaccurate within 3 months of transfusion due to circulating donor cells; genotyping should be used instead 4
- Inadequate sample handling: Temperature variations and delayed processing significantly affect test accuracy and can lead to false results 5, 6
- Missing extended typing needs: Patients requiring chronic transfusion need extended antigen profiling at first encounter to prevent alloimmunization 4, 3