Titre: The reciprocal of the highest serum dilution that causes macroscopic agglutination when serial dilutions of an antibody are tested against selected red cells.
Prenatal testing: mother's serum is tested at intervals during pregnancy, and the titre of her antibody is used to identify patients who should have amniocentesis. Because the titration results lead to a clinical manipulation which carries risks, care must be taken when doing the titration to minimize method variables as much as possible).
Identification of HTLA: High titre, low avidity antibodies (HTLA) react to a high titre when serially diluted, but react only weakly even when minimally diluted (1/1, 1/2, etc.) To identify them involves determining if they have a high titre. Specificities include anti-Ch, anti-Rg, anti-JMH, anti-Csa, anti-Yka, and others.
Today, we consider HTLA to be a misnomer because the antibodies do not necessarily have a low avidity for their antigens; rather their corresponding antigens (several on complement receptor, type 1) have a variable (often weak) expression on red cells.
Complex antibody identification: some antibodies cause universal agglutination when undiluted serum is used, but comparing titration results may indicate specificity. For example, in warm antibody type AIHA, the Rh antibodies of `broad specificity' can sometimes be further categorized by doing titrations against selected red cells such as R1R1, R2R2, and rr. If, for example, the titre with R2R2 cells was 4, but against the other two cells was 128, it is reasonable to assume that part of the antibody specificity is anti-e. Titration to identify antibodies is seldom done by routine blood banks.
Differentiation of pathological and harmless autoanti-1: at 4°C, the titre of harmless autoanti-1 is usually less than or equal to 64, whereas pathological anti-1 often has a titre greater than 1000.
Procurement of antisera: if a donor is found to have an irregular alloantibody, the donor centre may determine its titre to see if it is suitable for antisera.
Quality assurance of reagents: it is common procedure to determine the titre of an antiserum that will give 1+ or 2+ agglutination so that it can be used when doing Q.C. For example, to control that A1 or A2 cells are reactive, anti-A can be titrated to the point where it gives weak agglutination, and this dilution can be used to ensure the A antigens on the stored red cells are still reactive.
Titrations are only semiquantitative estimates of antibody reactivity due to several variables that affect their performance. Three main variables are the technologist, the red cells, and the method.
When comparative studies are done, such as in prenatal testing, a difference in titre of at least 2 tubes is required to be considered a significant difference. For example, if the titre changes from 32 to 64, this is not considered to be significant (difference of only 1 tube); however, a change from 32 to 128 would be significant (2 tube difference).
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