Several other methods, including radioimmunoassay (RIA) and fluorescent immunoassay (FIA), are similar to ELISA except that radionudeotides (usually 12SI or ,4C) are substituted for enzymes in RIA and fluoro-chromes are substituted for enzymes in FIA. Although RIA was formerly the key method for antigen detection for numerous infectious agents, including hepatitis B virus, it has been largely replaced by ELISA testing, which does not require use of radioactive substances.
An optical immunoassay (OIA) has recently been introduced to detect group A streptococci pharyngitis (Figure 9-16). It detects changes in the reflection of light occurring on the inert matrix after antigen and antibody have attached to it.
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