Frequency of ABO, Rh Subtypes, and Kell Antigen Among Blood Donors at Riyadh Regional Blood Bank: A Retrospective Cross-Sectional Study

Background/Objectives: Blood group antigen frequencies vary across ethnic populations and are critical for safe transfusion practice, inventory management, and alloimmunization prevention. This study aimed to determine ABO blood group, extended Rh phenotype (D, C, c, E, e), and K antigen frequencies among blood donors at the Riyadh Regional Blood Bank, and to examine their associations with donor ethnicity. Methods: This retrospective cross-sectional study analyzed 10,463 blood donors (January–December 2021) using the Immucor NEO Iris fully automated immunohematology analyzer. Donors were classified into five ethnic groups. Chi-square tests assessed associations (p < 0.05), the primary scientific contribution is the descriptive frequency data, with hypothesis testing presented as secondary and exploratory. Results: Most donors were male (95.2%). Blood group O+ was most prevalent (31.6%); AB− was rarest (0.9%). Rh D-positivity was 84.7%, lowest among Saudis (81.1%). The most frequent Rh phenotypes were CcDee (24.2%), ccDee (19.9%), and CCDee (17.7%). K antigen positivity was 13.7%. Significant associations were found between D antigen and ethnicity (p < 0.001), Rh phenotype and ethnicity (p = 0.003), and ABO and ethnicity (p < 0.001). A hypothesis-generating association between Rh phenotype and K antigen (p = 0.019) did not meet the Bonferroni-corrected threshold and is considered exploratory. Conclusions: This study provides, to our knowledge, one of the largest ethnically stratified blood group datasets from Riyadh to date. The high D-negativity among Saudi donors (18.9%) has direct implications for extended phenotyping protocols and rare donor registry development. The association between Rh phenotype and K antigen requires molecular confirmation before any biological or clinical conclusions can be drawn.