Rh blood type is determined by the presence or absence of the D
antigen. People who are Rh positive have the D antigen and those
that are Rh negative lack this. Some people have a variant of the D
antigen, called Du. Presence of Du antigen causes lab results to
report the blood type as Rh negative, although the patient behaves
as an Rh positive would. Du negative means a "true Rh negative"
result.