The first diagnostic IgG-avidity (AVI) test was described in 1988 by Hedman et al. (#10). The IgG antibody has a weak avidity at an early stage of primary infection. By avidity measurement, the acuteness of an infection can be established with sensitivity and specificity equal to or higher than those of conventional IgM methods. In microbial diagnosis, IgM tests may give false or inappropriately positive results due to several reasons including secondary infections (exogenous reinfections or endogenous reactivation) or breakdown of vaccination immunity. In general, if the specificity of the conventional IgM test is e.g. 99% and that of the confirming test (AVI; ETS) is likewise 99%, the specificity of the test combination will be 99.99%.
 

2. PERFORMANCE AND COST OF AVIDITY DIAGNOSTICS
 

In performance the AVI assays are similar to conventional indirect IgG or IgM EIA tests. The classical gold-standerd method consists of titration of serum either manually or automatically and postserum washes with and without a protein denaturant. Avidity is calculated from EIA absorbances either manually from titration curves or by a computer connected to the EIA reader. Ordinary instruments like EIA, RIA and DELFIA can be used for measurement.

The reference AVI method ( #9), consumes more reagents than a conventional IgM EIA. Hence, AVI measurement is at its best in ruling in or out positive or unclear results by conventional methods. Recently, Dr. Hedman's team has developed a new AVI technique, which is as sensitive and specific as the reference method and twice as cost beneficial. The new, computerized AVI assay is suitable for diagnosis of toxoplasma and all the viruses listed above (Korhonen et al., Clin. Diagn. Lab. Immunol. 1999;6:725-8). If the test is carried out in 5 per cent of samples studied, the overall extra cost will be only approx. USD 1 per sample.