Correlation of Classical Swine Fever (CSF) Antibody Protective Level Detected by Enzyme-Linked Immunosorbent Assay and Serum Neutralization Test

Protective antibody against classical swine fever virus (CSFV) is an important parameter used for disease monitoring in CSFV-endemic areas. Enzyme-linked immunosorbent assay (ELISA) is a simple and practical serological assay for CSFV antibody detection. However, the use of CSFV ELISAs in CSFV antibody-positive herds, i.e., CSFVvaccinated or -previously infected herds, was limited by an application of the test results. This study aimed to evaluate the correlation and estimation of protective antibody levels detected by ELISA and serum neutralization assay. A total of 522 negative and positive serums were tested by SN and ELISA in parallel. Comparisons of sample-to-positive (S/P) values among the level of SN titers, correlation, and agreement between two assays were evaluated. There were statistically significant differences (p<0.001) between the mean S/P values among three distinct levels of SN titers, i.e., negative (SN titer <2), below protective level (SN titer <32), and at protective level (SN titer ≥32). There was a strong positive relationship (rs = 0.89; p<0.001) and excellent agreement between the S/P values and SN titer (Kappa value = 0.91). The correlated S/P values at 1.767±0.479 are suggested to be at the protective level. Therefore, ELISA S/P results could provide an estimation of the protective antibody levels that correlated with the serum neutralization assay.