All tube tests are graded (e.g., 4+, 3+, 2+, 1+, -) where 4+ is one solid clump of red cells and 1+ is the presence of many small clumps (agglutinates).
Helps detect the presence of more than one antibody. For example, if some cells react 2+ and others react 4+, the 2+ reaction may be one antibody reacting with one antigen, whereas the 4+ reaction may be two antibodies reacting (both antigens are on the same cell).
Helps detect dosage effect of antigens. Dosage is a phenomenon in which having a gene present in the homozygous state results in an increased number of antigens per red cell than if the gene was only present in the heterozygous state. For example:
Helps detect abnormal reactions in the ABO BGS. When ABO grouping, we expect strong agglutination (3+ or 4+). If weak reactions are present (2+ or less), this may indicate a weak subgroup (e.g., A3) or a weak antigen due to a disease state (e.g., weakened A phenomenon due to leukemia or acquired B phenomenon due to infection).
Helps detect spurious (false positive) reactions. Antigen typings of patient cells are read in comparison to positive/negative controls. If a test on patient cells is noticeably weaker than the positive antiserum control, the patient test may be a false positive. For example, if a positive control (antiserum plus heterozygous cell) is 3+, but the test using the patient's cells is 1+, we should be suspicious and ask, "Why is the test so much weaker?"
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