Theoretically, any of the methods described for detecting antigen - antibody interactions can be applied directly to clinical specimens. The most common of these is immunofluo-rescence, in which antigen is detected on the surface of the organism or in cells present in the infected secretion. The greatest success with this approach has been in respiratory in-fections where a nasopharyngeal, throat washing, sputum, or bronchoalveolar lavage specimen may contain bacteria or viral aggregates in sufficient amount to be seen micro-scopically. Although the fluorescent tag makes it easier to find organisms, these methods are generally not as sensitive as culture. With some genera and species, the immunofluo-rescent detection of antigens in clinical material provides the most rapid means of diagno-sis, as with Legionella and respiratory syncytial virus.
Another approach is to detect free antigen released by the organism into body fluids. This offers the possibility of bypassing direct examination, culture, and identification tests to achieve a diagnosis. Success requires a highly specific antibody, a sensitive detec-tion method, and the presence of the homologous antigen in an accessible body fluid. The latter is an important limitation, because not all organisms release free antigen in the course of infection. At present, diagnosis by antigen detection is limited to some bacteria and fungi with polysaccharide capsules (eg, Haemophilus influenzae), Chlamydia, and to certain viruses. The techniques of agglutination with antibody bound to latex particles, CIE, RIA, and EIA are used to detect free antigen in serum, urine, cerebrospinal fluid, and joint fluid. Live organisms are not required for antigen detection, and these tests may still be positive when the causative organism has been eliminated by antimicrobial ther-apy. The procedures can yield results within an hour or two, sometimes within a few min-utes. This feature is attractive for office practice, because it allows diagnostic decisions to be made during the patient’s visit. A number of commercial products detect group A streptococci in sore throats with over 90% sensitivity; however, because these tests are less sensitive than culture, negative results must be confirmed by culture.