Subclinical mastitis is one of the most important infectious diseases, and higher levels of oxidative stress in the peritpartumperiod were related to higher risk of mastitis. The objective of the present study was to evaluate the diagnostic value of milk oxidative stress variables in early lactation of dairy cow with subclinical mastitis. A total of 925 milk samples from quarters of cows during (7 ± 3) days post-calving from two dairy herds were included in the present study. Bacterial culture, somatic cell count (SCC), DTNB (dithiobis nitrobenzoic acid for oxidative capacity), and FRAP (ferric reducing antioxidant power for antioxidatant capacity) have been assayed. Receiver-operating characteristic (ROC) analysis was used to assess the accuracy diagnostic value of each test using bacterial culture as gold standard. The accuracy of DTNB assay was comparable to that observed for SCC (AUC = 0.724 vs. AUC = 0.739). Sensitivity and specificity of DTNB at the concentration of 2.00 μmol s−1 were also comparable to those for SCC at 165 × 103 SCCml−1 (59.1 and 80.7% vs. 56.9 and 82.2%, respectively). Concentrations of both DTNB and FRAP were significantly higher in milk samples of quarters infected with S. aureus, when compared to non-infected quarters and quarters infected with other bacteria. The results showed that colorimetric measurement of DTNB could be a valuable indicator of subclinical mastitis, especially infection caused by S. aureus.

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