[Comparative study of antitoxoplasmic IgG isotypes titrated by high sensitivity direct agglutination and indirect immunofluorescence: consequence of the choice of methods on the expression of results in international units].

The use of International Units per ml (IU/ml) to express antitoxoplasmic IgG antibody titers in the various diagnostic systems presently proposed, is misleading owing to discrepancies in the values found from one test to the other for a given serum. The authors compared the results of high sensitivity direct agglutination (HSDA) to those of indirect immunofluorescence (IIF) in two studies, systematic and longitudinal, dealing with 158 sera stratified for values ranging from 102,400 to 5 IU/ml. Discordances between the methods, which are greater for high values, prompt the use of low-titer sera for standardization. From the systematic study, a correlation table was established and proposed to convert the HSDA results into the theoretical corrected values close to those that would be obtained by IIF. Although this may be of interest in maintaining a coherent language, this table has its limits, particularly in acute episodes where the various antibody kinetics vary and amplify further the discrepancies. In such situations, it seems advisable for both clinicians and biologists to raise any equivocal kept going by what is termed as International Units. Consequently, if the titers obtained by one method cannot be correlated to those of the technique of reference (IIF, Dye test), on the basis of using I.U., it would be appropriate to express the results in units related to the method or kit used (e.g. U/ml/HSDA for high sensitivity direct agglutination). Finally, whatever the technique, it is still mandatory to conserve a significant threshold value of protective immunization, common and identical to those classically adopted (8-12 IU/ml).