Introduction Blood donors are individuals who give blood, plasma and other blood components voluntarily, anonymously, without any compensation and purely as an act of solidarity. Blood cannot be produced artificially, so therefore blood donors are the sole source of blood. Various blood products may be produced from donated blood and must be safe for curing patients. That is why all blood products and blood doses must be tested for causes of transmitted diseases. ABO and Rh (D) blood type are also determined and the presence of irregular anti erythrocyte antibodies. Blood types have hereditary characteristics which are important to define in transfusion medicine. ABO system of blood types is the most important and it is composed of four blood types: A, B, O and AB. The Rh system is the largest and the antigen that is most important is antigen D. Individuals who have antigen D are Rh (D) positive and those who do not possess it are Rh (D) negative. Rh (D) negative individuals may have a weaker expression of antigen D and therefore Du must be defined in order to determine if it is due to weak D antigen. Objective The objective of this thesis is to show the importance of conducting the Du variant test properly and the sensitivity of this test in blood donors. Data sources and methods Mandatory immunohematology testing is performed before each donation of blood and blood products. Medical instructions and recommendations must exist in the laboratory and be available to all medical staff who conduct immunohematology testing. Samples for immunohematology testing must be conducted in test tubes which have anticoagulant EDTA solution. Blood samples may be from new or repeat donors. With new blood donors it is mandatory to determine ABO and Rh (D) blood type, Rh phenotype, Kell antigen and IAT. Repeat blood donors are tested for ABO and Rh (D) blood type. IAT, Rh phenotype and Kell antigen are only tested if the results are not recorded. For donors who are Rh (D) negative, weak antigen (Du test) is determined. Weak D antigen is determined in a microcard by IAT. Results are based on the principle of agglutination and are recorded in the e-Delphyn program. Discussion In this discussion the cases of Du positive and Du negative blood donors are shown. Conclusion D antigen is highly immunogenic and may elicit the production of antibodies and a strong immune response. Because of this, it is extremely important to detect the existence of weak D antigen expression in blood donors and resolve whether it is a weak or partial D. Conducting the Du test properly and interpreting the results correctly prevents Rh immunization in Rh negative recipients.