Serological tests may be expressed qualitatively or semi quantitatively In the

Serological tests may be expressed qualitatively or

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Serological tests may be expressed qualitatively or semi-quantitatively. In the former the result will appear either as positive or negative. This type of result is especially found with screening tests where large numbers of serum specimens are tested at a specific serum dilution. The serum dilution can vary from 1:10 in an indirect fluorescent antibody test (IFA), to 1:500 or more in various enzyme-linked immunosorbent assays (ELISA). The semi-quantitative method involves serial two-fold dilutions of sera in a diluent buffer. The highest dilution giving a positive result with a particular serological test is then taken as the end point and expressed as a titre. In the case of ELISAs, the amount of antibody is directly proportional to the intensity of the colour produced when the specific substrate is added to the enzyme marker and the amount of antibody may therefore be expressed in terms of international units rather than as a semi-quantitative titre. For the diagnosis of recent infection, immunoglobulin G (IgG ) titres may be determined in paired sera. The first serum specimen is collected as early as possible after onset of disease, and the second, 10- 14 days later. These specimens are respectively referred to as acute and convalescent sera. An increase in the titre of two dilutions or more, employing two-fold dilutions of the sera, is significant and
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