Red Blood Cell Antigen Genotyping

Testing is useful in determining allelic variants predicting red blood cell (RBC) antigen phenotypes for patients with recent history of transfusion or with conflicting serological antibody results due to partial, variant, or weak expression antigens. Also useful as an aid in management of hemolytic disease of the fetus and newborn (HDFN).

Prevalence and/or Incidence

Erythrocyte alloimmunization occurs in up to 58% of sickle cell patients, up to 35% in other transfusion-dependent patients, and in approximately 0.8% of all pregnant women.

Symptoms

Transfusion reactions or HDFN can occur due to alloimmunization:

• Intravascular hemolysis: hemoglobinuria, jaundice, shock
• Extravascular hemolysis: fever and chills
• HDFN: fetal hemolytic anemia, hepatosplenomegaly, jaundice, erythroblastosis, neurological damage, hydrops fetalis

Clinical presentation is variable and dependent upon the specific antibody and recipient factors

Pathophysiology

Extravascular Hemolysis

Removal of antibody-bound RBCs via splenic macrophages

Intravascular Hemolysis

• Rare for non-ABO antibodies
• Donor RBCs are destroyed by the recipient's antibodies while they are still inside blood vessels
• Hemoglobin released into plasma and excreted in urine; bilirubin accumulates in blood

HDN-Mediated Disease

• Caused by antigen-antibody mediated red-cell hemolysis from previously transferred maternal antibodies
• Examples of antibodies associated with HDFN:
  • Rh (Ant-D, -C, -c, -E, and -e)
  • Kell (Anti-K, -k, -Kp^a, -Kp^b, -Js^a and -Js^b)
  • Duffy (Anti-Fy^a and -Fy^b)
  • MNS (Anti-M, -N, -S, -s, and -U)
  • Kidd (Anti-Jk^a and -Jk^b)
• Anti-D is the most common cause of HDFN, followed by anti-c, anti-K, and anti-E; anti-Jk^a and -Jk^b is rare cause
• Antibodies cross the placenta and cause immune destruction of RBCs in fetus, leads to hydrops fetalis

Genetics

See Genes and Variants Tested table below

Inheritance

Typically codominant for RBC antigens; autosomal recessive for HbS  

Sensitivity/Specificity

• Clinical sensitivity and specificity
  •  >99% for C, c, E, e, K, k, Jk^a, Jk^b, Fy^a, Fy^b, M, N, S, s
  • Unknown for Kp^a, Kp^b, Js^a, Js^b, Lu^a, Lu^b, Di^a, Di^b, Co^a, Co^b, Do^a, Do^b, Jo^a, Hy, LW^a, LW^b, Sc1, Sc2, U, V, VS, HbS
• Analytical sensitivity and specificity
  • >99% for C, c, E, e, K, k, Jk^a, Jk^b, Fy^a, Fy^b, M, N, S, s
  • Unknown for Kp^a, Kp^b, Js^a, Js^b, Lu^a, Lu^b, Di^a, Di^b, Co^a, Co^b, Do^a, Do^b, Jo^a, Hy, LW^a, LW^b, Sc1, Sc2, U, V, VS, HbS

Results

• Predicted phenotype for each RBC antigen tested will be provided
  • If identified, weak alleles will be reported
• Possible variant
  •  Reported for allele combinations that have not widely been reported in the literature
• Indeterminate
  • Abnormal signal intensities may result in inability to predict genotype resulting in indeterminate results for all tested antigens and HbS

Limitations

• Only the variants listed will be interrogated
• Does not test for Rh D, a major cause of HDFN 
• Patients who have had allogeneic hematopoietic stem cell transplants may have inconclusive results on this test
• Diagnostic/phenotype prediction errors can occur due to
  • Rare variants affecting primer/probe binding
  • Molecular events that affect the blood-group antigen expression and phenotypes that are not detected by this assay (ie, certain null phenotypes or other phenotypes with altered expression)
• This assay is not designed to diagnose sickle cell disease