A diagnostic test using serum (the fluid, non-cellular part of blood) that detects antibody produced in response to infection. ELISA technology is widely used to diagnose infectious diseases. As for other serologic assays, the fundamental basis of the test is binding of antibody in the test sample (serum) to MAP antigens in the test container. An indicator system based on an enzymatic reaction generating a colored product distinguishes ELISAs from CF or AGID (other ways of detecting antibody). The amount of color produced in the reaction is directly related to the amount of antibody in the test sample. While the colored reaction resulting from the ELISA can be interpreted by the unaided eye, most often an instrument called and ELISA reader measures the amount of color in each well and reports the result as an optical density (OD) value. OD values for test samples can be compared to negative and positive control samples to derive ELISA results. For example, the ELISA kit for paratuberculosis sold in the USA by IDEXX uses the ratio between the OD value for the sample to the OD value for the positive control (provided in the kit). The resulting value, called the S/P ratio, standardizes results for tests done on different plates, or different days or different laboratories. The S/P is a reproducible, quantitative measure of the level of antibody in the original serum sample. S/P values can be used to gauge the probability a tested animal is infected once sufficient representative control populations of infected and non-infected animals are tested. See the section on diagnosis of MAP infections in dairy cattle in this website for more information. Also, see the information provided sponsors of this website with ELISA kits for paratuberculosis.