A precise cow-side point care system for the diagnosis of mastitis is critical for targeted antimicrobial therapy. Recently, several multiple-media culture systems became commercially available for on-farm identification of mastitis pathogens. However, the accuracy of these systems has not been thoroughly and independently validated against microbiological evaluations performed by referral laboratories. Therefore, the purpose of the present study was to evaluate the effectiveness of popular commercially available culture plates (Accumast, Minnesota Easy System, SSGN and SSGNC Quad plates) to identify pathogens associated with clinical mastitis in dairy cows. Milk samples from the affected quarter with clinical mastitis were collected aseptically. Samples were aerobically cultured with the on-farm culture systems and by 2 reference laboratories. Agreeing results from both reference laboratories were denoted as gold standard (GS). Accuracy (Ac), sensitivity (Se), specificity (Sp), positive and negative predictive values (PPV and NPV, respectively), and Cohen’s kappa coefficient (k) of on-farm plates were determined based on the GS culture of 211 milk samples. All 4 plates correctly identified ≥84.9% of milk samples with no bacterial growth. Accumast had greater values for all predictive factors and a substantial agreement (k = 0.79) with GS (Table 1). The inter-rater agreements of Minnesota, SSGN, and SSGNC with GS were moderate (0.45 ≤ k ≤0.55). Only Accumast correctly identified Staphylococcus aureus (Ac = 66.7% and Se = 100%). Likewise, Accumast had greater Ac for Streptococcus sp. (55%) than the remaining plate systems (33.3% ≤ AC ≤38.9%). Our findings suggest that Accumast was the most accurate on-farm culture system for identification of mastitis pathogens. Table 1. Predictive factors for identifying mastitis-related pathogens using four culture plates (n = 211)

Parameter	On-farm culture system
	Accumast	Minnesota	SSGN	SSGNC
Ac, %	89.6	73.5	79.1	75.8
Se, %	97.6	88.9	79.4	79.4
Sp, %	84.5	68.2	79.0	74.3
PPV, %	80.0	49.0	64.3	56.8
NPV, %	98.2	94.7	89.0	89.4
κ	0.79	0.45	0.55	0.48