Whenever a discrepancy exists between front cell grouping, it must be resolved. We never guess the patient's ABO group as an incorrect guess could be potentially fatal to the patient. Discrepancies encompass both results that are unexpectedly positive or negative, and results that are weaker than usual (i.e., weaker than 3+ or weaker than 1+, depending on lab policy). Should blood be required before discrepancies can be resolved, group O red cells and group AB plasma must be transfused.
ABO discrepancies can be grouped into four categories, which are discussed in detail below. For each category, only some of the potential causes are addressed; many more exist.
For brevity, discrepancies have been arbitrarily classified into categories such as "weak or missing antibodies," "extra antigens," etc. These categories should be named more properly as "weak or missing reactivity in serum grouping tests," "extra reactivity in cell grouping tests," etc.
Although not mentioned for each discrepancy discussed, the first step in resolving a discrepancy would be to repeat the ABO group using a fresh aliquot of washed red cells. Reactions that were unexpectedly weak or negative would be read microscopically. Some labs may then incubate at 4°C since ABO antibodies react best in the cold. As QC at 4°C an autocontrol would be set up (to detect a possible autoanti-I). Some labs may include a test with anti-A,B.Weak or Missing Antibodies