Q. Can an O positive patient be given O negative blood in an emergency situation?
Rh blood group system consists of 50 defined blood group antigens among which the 5 antigens D, C, c, E and e are the most important. The commonly used term Rh factor, Rh positive and Rh negative refers to the D antigen only. Individuals either have, or do not have, the "Rh factor" on the surface of their red cells. This term strictly refers only to the most immunogenic D antigen of the Rh blood group system. This is usually indicated by Rh positive (have the D antigen) or Rh negative (do not have the D antigen) suffix to the ABO blood type. However, other antigens of this blood group system are also clinically relevant.
Generally speaking, if one is Rh negative, then Rh positive blood transfusion is contraindicated. However, in some cases there is a weak D antigen in some individuals, and they apparently are Rh negative. In serologic testing, D positive blood is easily identified. Units which are D negative are often retested to rule out a weaker reaction. This was previously referred to as Du, which has been replaced. In some cases, this phenotype occurs because of an altered surface protein that is more common in people of African descent. The testing is difficult, since using different anti-D reagents, especially the older polyclonal reagents, may give different results.
The practical implication of this is that people with this sub-phenotype will have a product labelled as "D positive" when donating blood. When receiving blood, they are sometimes typed as a "D negative", though this is the subject of some debate. Most "Weak D" patients can receive "D positive" blood without complications. However, it is important to correctly identify the ones that have to consider D+ or D-. This is important, since most blood banks have a limited supply of "D negative" blood and the correct transfusion is clinically relevant. In this respect, genotyping of blood groups has much simplified this detection of the various variants in the Rh blood group system.
However, for all practical purposes, no Rh positive blood transfusion is advised in Rh negative individuals.