From the Conglutinating Complex Fixation Test to the Saiduldin Test: Development, Veterinary Evidence Base, and Prospects for Contemporary Validation

The Saiduldin Test (ST), originally developed as the Conglutinating Complex Fixation Test (CCFT), is a complement-dependent serological method based on specific antigen–antibody interaction followed by conglutinin-mediated secondary aggregation. This review aimed to systematize and critically evaluate the scientific evidence concerning the immunological foundations, historical development, standardization, and veterinary diagnostic applications of ST, with particular emphasis on bovine and ovine brucellosis, while also considering its applications to other infectious diseases and identifying priorities for contemporary validation. Scientific publications, dissertations, field-trial reports, patents, methodological instructions, and regulatory and technical documents reflecting the successive development of the method were analyzed. The most extensive evidence base concerns bovine brucellosis and includes large-scale field trials, comparisons with established serological methods, and longitudinal observations of animals with discordant test results. ST was subsequently adapted for other infections and animal species, including ovine brucellosis, bovine leukemia, camel surra, equine dourine, and ovine chlamydiosis. The conglutinin-dependent principle was applied both to the detection of specific antibodies and to the identification of circulating immune complexes. Historical investigations demonstrate the practical applicability and feasibility of standardizing ST and, in certain populations, higher serological detection rates than those obtained with some comparator tests. However, the absence of independent confirmation of infection status in most early studies and limitations of their designs prevent higher positivity rates from being interpreted as evidence of superior diagnostic sensitivity or specificity. Contemporary evaluation should therefore follow a predefined intended diagnostic purpose and include reagent standardization, assessment of analytical characteristics, diagnostic cut-off, sensitivity, specificity, repeatability, and interlaboratory reproducibility, together with prospective investigation of discordant results and seroconversion dynamics. The antibody-detection format of ST for bovine brucellosis represents the most appropriate initial model for validation. Stepwise independent validation would allow the diagnostic value of ST and its potential role in contemporary veterinary diagnostics to be determined objectively.