ABO discrepancies In ABO typing, the results of the forward and reverse grouping should always correspond with each other. An unexpected difference between the two results is termed an ABO discrepancy, and must be resolved before the person's blood type is reported. Forward grouping Weak reactions in the forward grouping may occur in people who belong to certain ABO subgroups—variant blood types characterized by decreased expression of the A or B antigens or changes in their structure. Weakened expression of ABO antigens may also occur in leukemia and Hodgkin's lymphoma. Weak reactions in forward grouping can be strengthened by incubating the blood and reagent mixture at room temperature or , or by using certain enzymes to enhance the antigen-antibody reactions. Occasionally, two populations of red blood cells are apparent after reaction with the blood typing antisera. Some of the red blood cells are agglutinated, while others are not, making it difficult to interpret the result. This is called a mixed field reaction, and it can occur if someone has recently received a blood transfusion with a different blood type (as in a type A patient receiving type O blood), if they have received a bone marrow or stem cell transplant from someone with a different blood type, or in patients with certain ABO subgroups, such as A3. Investigation of the person's medical history can clarify the cause of the mixed field reaction. People with cold agglutinin disease produce antibodies against their own red blood cells that cause them to spontaneously agglutinate at room temperature, leading to false positive reactions in forward grouping. Cold agglutinins can usually be deactivated by warming the sample to and washing the red blood cells with saline. If this is not effective, dithiothreitol can be used to destroy the antibodies. Cord blood samples may be contaminated with Wharton's jelly, a viscous substance that can cause red blood cells to stick together, mimicking agglutination. Wharton's jelly can be removed by thoroughly washing the red blood cells. In a rare phenomenon known as "acquired B antigen", a patient whose true blood type is A may show a weak positive result for B in the forward grouping. This condition, which is associated with gastrointestinal diseases such as colon cancer and intestinal obstruction, results from conversion of the A antigen to a structure mimicking the B antigen by bacterial enzymes. Unlike the true B antigen, acquired B antigen does not react with reagents within a certain pH range. Reverse grouping Infants under 3 to 6 months of age exhibit missing or weak reactions in reverse grouping because they produce very low levels of ABO antibodies. Therefore, reverse grouping is generally not performed for this age group. Elderly people may also exhibit decreased antibody production, as may people with hypogammaglobulinemia. Weak reactions can be strengthened by allowing the plasma and red cells to incubate at room temperature for 15 to 30 minutes, and if this is not effective, they can be incubated at . Approximately 20% of individuals with the blood type A or AB belong to a subgroup of A, termed A2, while the more common subgroup, encompassing approximately 80% of individuals, is termed A1. Because of small differences in the structure of the A1 and A2 antigens, some individuals in the A2 subgroup can produce an antibody against A1. Therefore, these individuals will type as A or AB in the forward grouping, but will exhibit an unexpected positive reaction with the type A1 red cells in the reverse grouping. The discrepancy can be resolved by<unk>  the person's red blood cells with an anti-A1 reagent, which will give a negative result if the patient belongs to the A2 subgroup. Anti-A1 antibodies are considered clinically insignificant unless they react at . Other subgroups of A exist, as well as subgroups of B, but they are rarely encountered. If high levels of protein are present in a person's plasma, a phenomenon known as rouleaux may occur when their plasma is added to the reagent cells. Rouleaux causes red blood cells to stack together, which can mimic agglutination, causing a false positive result in the reverse grouping. This can be avoided by removing the plasma, replacing it with saline, and re-centrifuging the tube. Rouleaux will disappear once the plasma is replaced with saline, but true agglutination will persist. Antibodies to blood group antigens other than A and B may react with the reagent cells used in reverse grouping. If a cold-reacting autoantibody is present, the false positive result can be resolved by warming the sample to . If the result is caused by an alloantibody, an antibody screen can be performed to identify the antibody, and the reverse grouping can be performed using samples that lack the relevant antigen. Weak D phenotype Approximately 0.2 to 1% of people have a "weak D" phenotype, meaning that they are positive for the RhD antigen, but exhibit weak or negative reactions with some anti-RhD reagents due to decreased antigen expression or atypical variants of antigen structure. If routine serologic<unk>  for RhD results in a score of 2+ or less, the antiglobulin test can be used to demonstrate the presence of RhD. Weak D<unk>  is also performed on blood donors who initially type as RhD negative. Historically, blood donors with weak D were treated as Rh positive and patients with weak D were treated as Rh negative in order to avoid potential exposure to incompatible blood. Genotyping is increasingly used to determine the molecular basis of weak D phenotypes, as this determines whether or not individuals with weak D can produce antibodies against RhD or sensitize others to the RhD antigen. Red cell antibody sensitization The indirect antiglobulin test, which is used for weak D<unk>  and typing of some red blood cell antigens, detects IgG bound to red blood cells. If IgG is bound to red blood cells in vivo, as may occur in autoimmune hemolytic anemia, hemolytic disease of the newborn and transfusion reactions, the indirect antiglobulin test will always give a positive result, regardless of the presence of the relevant antigen. A direct antiglobulin test can be performed to demonstrate that the positive reaction is due to sensitization of red cells.
Blood compatibility testing
Hematology analyzer
Calibrated automated thrombogram
Blood smear
Drug test
Viral disease testing
Iron tests
Clinical chemistry