> Heterogeneous assays are those that require separation of the bound fraction from the free.
> The titre of an antibody in the context of immunoassay is the dilution of serum used in the assay.
This approach to immunoassay is characterized by the fact that the antibody is present in excess and is generally also labelled. Because the labelled antibody is in excess there is no requirement for the setting up of an equilibrium since all of the test antigen may be sequestered by the excess of antibody.
In immunometric assays, unlike competitive systems, the amount of labelled antibody bound is directly proportional to the amount of unlabelled antigen present rather than inversely proportional.
For each of these types of immunoassay, in order to observe the ratio of bound to free in the final reaction mixture, a method has to be employed which will separate these fractions and various ingenious ways have been devised to do this. Techniques which require the separation of free from bound fractions are referred to as heterogeneous assay systems.
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