A specialized and advanced immunological blood test is performed to determine the precise blood type in relation to the Rh factor, specifically to detect the weak D type (or Du). It relies on a more sensitive method (involving a second incubation phase, cell washing, and the use of human antiglobulin (HIA) – Coombs) to detect minute amounts of the D antigen on the surface of red blood cells that may not be detectable by conventional direct methods. Its aim is to accurately classify individuals as "Rh positive" or "Rh negative," which is crucial in transfusion medicine and obstetrics.