Validation of serological assays for diagnosis of infectious diseases

Assay validation is a series of the following interrelated processes: an experimental process: reagents and protocols are optimised by experimentation to detect the analyte with accuracy and precision, and to ensure repeatability and reproducibility in the assay a relative process: its diagnostic sensitivity and diagnostic specificity are calculated relative to test results obtained from reference animal populations of known infection/exposure status a conditional process: classification of animals in the target population as infected or uninfected is conditional upon how well the reference animal population used to validate the assay represents the population to which the assay will be applied (accurate predictions of the infection status of animals from test results and predictive values of positive and negative test results are conditional upon the estimated prevalence of disease/infection in the target population) an incremental process: confidence in the validity of an assay increases over time when use confirms that it is robust as demonstrated by accurate and precise results (the assay may also achieve increasing levels of validity as it is upgraded and extended by adding reference populations of known infection status) a continuous process: the assay remains valid only insofar as the assay continues to provide accurate and precise results as proved through statistical verification. Therefore, validation of diagnostic assays for infectious diseases does not end with a time-limited series of experiments based on a few reference samples. Rather, it is a process that also requires constant vigilance and maintenance, along with reassessment of its performance characteristics for each population of animals to which it is applied. It is certain that the current movement to develop and implement accreditation criteria for veterinary diagnostic laboratories may be of little worth unless there is some assurance that the assays conducted in such laboratories are properly validated. Fully accredited laboratories may generate highly reproducible test results, but the results may still misclassify animals as to their infection status due to an improper assay validation process. Therefore, assay validation is foundational to the core product of veterinary diagnostic laboratories - test results and their interpretation.