Adapting a porcine reproductive and respiratory syndrome virus (PRRSV) oral fluid antibody ELISA to routine surveillance.

Distinct from tests used in diagnostics, tests used in surveillance must provide for detection while avoiding false alarms, i.e., acceptable diagnostic sensitivity but high diagnostic specificity. In the case of the reproductive and respiratory syndrome virus (PRRSV), RNA detection meets these requirements during the period of viremia, but antibody detection better meets these requirements in the post-viremic stage of the infection. Using the manufacturer's recommended cut-off (S/P ≥ 0.4), the diagnostic specificity of a PRRSV oral fluid antibody ELISA (IDEXX Laboratories, Inc., Westbrook, ME, USA) evaluated in this study was previously reported as ≥ 97 %. The aim of this study was to improve its use in surveillance by identifying a cut-off that would increase diagnostic specificity yet minimally impact its diagnostic sensitivity. Three sample sets were used to achieve this goal: oral fluids (n = 596) from pigs vaccinated with a modified live PRRSV vaccine under experimental conditions, field oral fluids (n = 1574) from 94 production sites of known negative status, and field oral fluids (n = 1380) from 211 sites of unknown PRRSV status. Based on the analysis of samples of known status (experimental samples and field samples from negative sites), a cut-off of S/P ≥ 1.0 resulted in a diagnostic specificity of 99.2 (95 % CI: 98.8, 99.7) and a diagnostic sensitivity of 96.5 (95 % CI: 85.2, 99.2). Among 211 sites of unknown status, 81 sites were classified as antibody positive using the manufacturer's cut-off; 20 of which were reclassified as negative using a cut-off of S/P ≥ 1.0. Further analysis showed that these 20 sites had a small proportion of samples (18.0 %) with S/P values just exceeding the manufacturer's cut-off (x̄ = 0.5). Whereas the remainder of positive sites (n = 61) had a high proportion of samples (76.3 %) with high S/P values (x̄ = 6.6). Thus, the manufacturer's cut-off (S/P ≥ 0.4) is appropriate for diagnostic applications, but a cut-off of S/P ≥ 1.0 provided the higher specificity required for surveillance. A previously unreported finding in this study was a statistically significant association between unexpected reactors and specific production sites and animal ages or stages. While beyond the scope of this study, these data suggested that certain animal husbandry or production practices may be associated with non-specific reactions.