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.