Vet. Res. 34 (2003) 341-352
DOI: 10.1051/vetres:2003009

Influence of age and purpose for testing on the cut-off selection of serological methods in bovine neosporosis

Gema Álvarez-García^a, Esther Collantes-Fernández^a, Eduardo Costas^b, Xabier Rebordosa^c and Luis Miguel Ortega-Mora<sup>a

a</sup>  Departamento de Sanidad Animal, Facultad de Veterinaria, Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain
^b  Departamento de Producción Animal, Facultad de Veterinaria, Universidad Complutense de Madrid, 28040 Madrid, Spain
^c  Laboratorios HIPRA, S.A., avda. La Selva, 135-17170 Amer (Girona), Spain

(Received 26 September 2002; accepted 20 January 2003)

Abstract
The aim of this study was to investigate the need for different cut-off points, according to animal age and the purpose of testing, for two of the most widely used serological techniques in bovine neosporosis, IFAT and a crude antigen ELISA (Civtest®, HIPRA). Therefore, the population reference sera used were defined using a combination of multiple criteria such as epidemiological/clinical and histopathological parameters and an immunoblot test. Firstly, foetuses and breeding cattle (heifers and cows) were considered as separate subpopulations for serological evaluation. Secondly, cut-off points for each serological technique (IFAT and ELISA) according to age group (foetuses and breeding cattle) and the different practical applications (detection of infection and abortion) were calculated following the receiver operating characteristics (ROC) analysis. Cut-off points were defined, for IFAT and ELISA for aborted breeding cattle and for IFAT alone in the case of the foetuses, assuming an equivalent cost of false positive and negative results. In infected breeding cattle, for IFAT and ELISA and in foetuses for ELISA, two possible cut-off values were obtained, one for a maximum sensitivity and one for a maximum specificity and the intervals of unclear results were defined. In this case, a cut-off value for equal sensitivity and specificity was also estimated. When cut-off points for infected breeding cattle, 1:100-1:250 for IFAT and 0.306-0.451 for ELISA were applied to a target population, optimal and similar negative and positive predictive values together with similar apparent and true prevalence results were observed suggesting the possibility of using both tests interchangeably.

Key words: Neospora caninum / foetus / breeding cattle / serology

Correspondence and reprints: Luis Miguel Ortega-Mora Tel.: (34) 91 3943713; fax: (34) 91 3943908;
    e-mail: luis.ortega@vet.ucm.es

© INRA, EDP Sciences 2003