Frequency distributions for B. ovis ELISA values from negative (n = 2,535) and positive (n = 589) sera with fitted distribution curves. Reproduced with the permission of the authors and publisher from Reichel MP., Ross G., Drake J., Jowett JH. 1999. Performance of an enzyme-linked immunosorbent assay for the diagnosis of Brucella ovis infection in rams. New Zealand Veterinary Journal, 47,71-76.
Serological tests are important test methods used to assist in the diagnosis of infectious diseases. They are usually done to determine the level of antibody that is present in a serum sample, but in some tests an antibody preparation is used to demonstrate the presence of antigen in a serum or tissue sample.
The level of antibody present is usually critical to the interpretation of the test because in most tests, the level of antibody must exceed a critical level before the test is regarded as a positive test. When a new test is developed an essential part of the process is the establishment of standards for interpretation. It is important to determine the cut-off (threshold) levels for classifying test results as negative, suspicious and positive. However, it is also important that test results are not just mechanically classified as positive or negative, according to test titres. Careful consideration should be given to the epidemiological principles that are important for interpreting the results of a particular case and the reason for which the test is being done.
The following topics will be covered in this sub-module:
Determination of positive/negative cut-off points
Pathogenesis of diseases and test interpretation
False positive, and specific, non-specific and cross reactions
About The Instructor
Dr Jannie Crafford
Dr Jannie Crafford
Senior lecturer, Department of Veterinary Tropical Diseases, Faculty of Veterinary Science, University of Pretoria