Determination of an optimized cut-off value for the Neospora agglutination test for serodiagnosis in cattle

A definitive diagnosis of neosporosis in cattle implies the examination of the aborted fetus. However, in many instances fetal material is not available. Therefore, most diagnosis are based on serological tests. At the moment, there are no consensuses about the cut-off for serodiagnosis of neosporosis in cattle, for any test. The objective of the present study was to estimate the best cut-off for the Neospora agglutination test (NAT) for serodiagnosis in cattle. For that purpose, 246 serum samples from 4 groups of dairy cows (aborted Neospora positive; not aborted healthy; aborted other diseases and herds endemic neosporosis) were collected and antibodies anti-N. caninum were determined by NAT. Additionally, immunoblot (IB) was performed with sera from all cows of the endemic neosporosis group and the patterns of seroreactivity were contrasted with the NAT titers for this group of cows. Evaluation of the optimized sensitivity and specificity was calculated using Youden's J-statistics. The best Youden's J-statistic was obtained at 1:40 titer, presenting 100% of sensitivity and 90.4% of specificity with negative and positive predictive values of 100 and 75.0%, respectively. The comparison between NAT titers and the IB banding pattern support the results of the statistical analysis, i.e. titers of 1:40 and higher showed a complex pattern of bands, while titers lower than 1:40 did not precipitate any bands. These results indicate that 1:40 was an optimized NAT cut-off for serodiagnosis in cattle.