Researchers identified B(A) blood group in 0.00055% of over 544,000 samples from Siriraj Hospital. B(A) shows trace A antigen in type B blood due to mutation. Discovery confirmed by University of Cambridge and hematology centers. Standard tests may misidentify B(A) due to subtle A activity. Patients with B(A) function normally outside transfusions or surgery. Mutation affects enzyme for antigen creation but details unknown. Finding suggests even rarer blood variants exist beyond standard tests. More research needed on B(A) and similar rarities. Study spanned 8 years under Janejira Kittivorapart at Mahidol University. Results published in Transfusion and Apheresis Science. Rare blood types complicate donor matching in emergencies. Advanced DNA methods enable detection of such variants. ABO system remains key for transfusions since early 1900s.