# Validation of a rapid on-farm culture system for group classification of clinical mastitis-causing pathogens

## TL;DR

A rapid on-farm test for classifying mastitis pathogens in dairy cows was found to be suboptimal, with a high risk of false negatives.

## Contribution

This study evaluates the diagnostic accuracy of a rapid test tube culture system for classifying mastitis pathogens in dairy cattle.

## Key findings

- The rapid test has a lower analytical sensitivity than conventional culture methods.
- The test's overall accuracy for Gram group classification was 57%.
- Using the test for treatment decisions could lead to incorrect antibiotic use due to false negatives.

## Abstract

Summary: The objective of this test accuracy study was to evaluate a rapid test tube milk culture for classification of Gram group of clinical mastitis (CM) pathogens as compared with conventional milk culture with identification using matrix-assisted laser desorption/ionization time-of-flight. An accurate method with a shorter turnaround time than conventional milk culture may aid in on-farm implementation of selective therapy protocols for CM. Overall, the performance of the test evaluated was suboptimal for the classification of mastitis pathogens from CM samples.

Summary: The objective of this test accuracy study was to evaluate a rapid test tube milk culture for classification of Gram group of clinical mastitis (CM) pathogens as compared with conventional milk culture with identification using matrix-assisted laser desorption/ionization time-of-flight. An accurate method with a shorter turnaround time than conventional milk culture may aid in on-farm implementation of selective therapy protocols for CM. Overall, the performance of the test evaluated was suboptimal for the classification of mastitis pathogens from CM samples.

•The test evaluated is prone to false negatives.•The test evaluated has a lower analytical sensitivity than the reference method.•The test is suboptimal for the classification of mastitis pathogens from CM samples.

The test evaluated is prone to false negatives.

The test evaluated has a lower analytical sensitivity than the reference method.

The test is suboptimal for the classification of mastitis pathogens from CM samples.

The objective of this diagnostic accuracy study was to compare the use of a rapid tube test system (MastDecide [MD]; Quidee GmbH, Homberg, Germany) to aerobic milk culture for group classification of mastitis pathogens in dairy cattle. A total of 204 milk samples from cows with clinical mastitis (CM) were collected across 60 dairy herds in Ontario, Canada. Samples were collected by dairy producers and their staff and transported refrigerated to the University of Guelph Animal Health Laboratory (Guelph, ON, Canada) on the day of collection. Each sample was tested via MD (gram-positive, gram-negative, or “no growth”) and via aerobic milk culture followed by MALDI-TOF (reference method [RM]). The MD and RM results were interpreted at 14 and 24 h after incubation, respectively. An additional assessment was performed regarding the accuracy of intramammary antibiotic treatment decisions (gram-positive: to treat; gram-negative and “no growth” samples: not to treat). Test performance characteristics (overall accuracy, sensitivity [Se], specificity [Sp], and positive [PPV] and negative predictive value [NPV]) as well as Cohen's kappa coefficient (κ) were calculated. The RM results were classified as gram-positive (n = 107, 53%), gram-negative (n = 20, 10%), “no growth” (n = 51, 25%), other pathogens (n = 21, 10%), and mixed growth (gram-positive and gram-negative; n = 5, 3%). The Se of the MD test was 58% (95% CI: 47%–67%), 40% (19%–64%), and 61% (50%–72%) for gram-positive, gram-negative, and “no growth,” respectively. The Sp of the MD test was 73% (63%–81%), 84% (78%–89%), and 75% (66%–82%) for gram-positive, gram-negative, and “no growth,” respectively. The PPV of the MD test was 68% (57%–78%), 22% (10%–39%), and 62% (50%–73%) for gram-positive, gram-negative, and “no growth,” respectively; and the NPV was 63% (54%–72%), 93% (87%–96%), and 74% (65%–82%) for gram-positive, gram-negative, and “no growth,” respectively. The overall test accuracy for Gram stain classification analysis was 57%, with a κ of 0.30. Regarding intramammary antibiotic treatment, the MD test yielded a correct decision relative to the RM for 65% of the cases. Using this on-farm test as a primary guide in CM treatment decisions should be approached with caution, as the risk of false negatives may affect the ability to effectively treat CM cases.