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Abstract Background The gastrointestinal nematode (GIN) Ostertagia ostertagi can cause severe disease in first season grazers (FSG) and impaired performance due to subclinical infections in adult cows. Diagnostic methods to assess exposure include faecal egg count and detection of specific antibodies using antibody-ELISAs resulting in an optical density ratio (ODR). Using the ELISA test on bulk tank milk (BTM) allows for a herd level diagnosis. Appropriate use of diagnostic methods for evaluation of O. ostertagi exposure is required to optimize herd parasite surveillance and aid in a sustainable control regime. The aim of this study was to describe the relationship between different diagnostic tests used to assess GIN exposure in Norwegian production systems. A cross-sectional field study was carried out in twenty herds in Norway in the fall of 2020. Serum and faecal samples were taken from 380 individuals, of which 181 were FSG and 199 were cows. In addition, milk was collected from every cow and one BTM sample was taken from each herd. Faecal egg counts were performed. The distribution of ODR values in individual samples within and between herds and the associations between BTM ODR and individual ODR values were described. The data were analysed using visual assessment of scatter plots, Pearson correlation coefficients and linear regression. Results A high variability of the within-herd individual ODR values in serum and milk in every herd was detected. The ODR in BTM explained a low degree of the variation in the individual serum and milk samples. When plotting the ODR results in milk or serum according to four BTM categories, the distribution of ODR values were notably different in the highest and lowest BTM categories. The correlation between individual milk and serum samples was moderate (r = 0.68), while the highest correlation (r = 0.81) was between the BTM ODR and the group average individual milk samples. Conclusions A poor predictive ability for BTM ODR to assess individual ODR values in both FSG and cows was demonstrated. However, the study indicates that the evaluation by ELISA test on BTM to assess exposure to GIN could be useful in herds with a very high or low BTM ODR.