Review of critical factors affecting analytical characteristics of serological and molecular assays

Analytical characteristics of diagnostic tests, encompassing estimates of repeatability, analytical specificity (ASp) and analytical sensitivity (ASe), are determined during Stage 1 of the World Organisation for Animal Health (OIE) Assay Validation Pathway. Repeatability (an estimate of assay precision and robustness), ASp (measuring only what an assay is intended to measure) and ASe (synonymous with the lower limit of detection) are fundamental parameters that determine future test performance. Importantly, these parameters provide the basis for deciding whether a prototype assay progresses to the next stage of the validation pathway (determination of diagnostic characteristics) or is withdrawn in favour of alternative tests with better analytical performance characteristics. Implicit in the successful development and validation of any assay is a sound understanding of the target pathogen, the disease pathogenesis in susceptible hosts, the fundamental technical principles that underlie each test system, and the assay's intended use. Factors that affect the analytical characteristics of diagnostic assays are numerous and may vary according to each assay type. In this paper, the authors consider the main factors affecting the analytical characteristics of serological and molecular assays, using, as examples, the development of an enzyme-linked immunosorbent assay to detect antibodies against capripoxviruses, and the comparative assessment of three quantitative real-time polymerase chain reactions to detect African swine fever virus DNA. The authors demonstrate that comprehensive and well-designed experiments are required to develop and optimise assays with favourable analytical characteristics. The underlying principles are broadly applicable to all assay types and, when conducted with appropriate rigour, provide the foundations for high-quality diagnostic tests that are fit for their intended purpose(s).