RATs are
immunochromatographic assays which give results that can be seen with the naked eye (with or without special illumination, such as a UV lamp). They are qualitative in nature, although within a certain range it is possible to make rough order of magnitude estimates of viral load from the results. RATs are generally screening tests, with relatively low
sensitivity and specificity, thus results should be evaluated on the basis of confirmatory tests like
PCR testing or western blot. One inherent advantage of an antigen test over an antibody test (such as antibody-detecting
rapid HIV tests) is that it can take time for the
immune system to develop antibodies after
infection begins, but the foreign antigen is present right away. Although any diagnostic test may have
false negatives, this latency period can open an especially wide avenue for false negatives in antibody tests, although the particulars depend on which disease and which test are involved. A rapid antigen test typically costs around US$5 to manufacture. For both
typhoid and
HIV, the antigen test is able to detect the infection earlier than the antibody test can. As the infection progresses, however, the antigen test becomes less sensitive than the antibody test. In HIV, this is because blood levels of immunoreactive p24 antigen diminish as the infection progresses. There are several rapid combination antigen/antibody tests for HIV. == References ==