Association between quantitative bacterial culture of bronchoalveolar lavage fluid and antibiotic requirement in dogs with lower respiratory tract signs.

Background: Historically, positive bacterial cultures from the lower respiratory tract (LRT) have been considered clinically relevant when quantitative bacterial cultures of bronchoalveolar lavage fluid (BALF) were >1700 colony forming units (cfu)/mL. However, this threshold might not accurately predict a requirement for antibiotics.

Objectives: To study whether quantitative BALF bacterial culture results were predictive of antibiotic requirement in dogs with LRT signs.

Animals: Thirty-three client-owned dogs.

Methods: Cross-sectional study. Dogs with positive quantitative bacterial culture of BALF were included. Dogs were divided into 2 groups, depending on whether they had a LRT infection requiring antibiotics (LRTI-RA) or LRT disease not requiring antibiotics (LRTD-NRA), based on thoracic imaging features, presence of intracellular bacteria on BALF cytology, and response to treatment. Predictive effect of cfu/mL and BALF total nucleated cell count (TNCC) on antibiotic requirement, adjusting for ongoing or prior antibiotic therapy and age, were studied using logistic regression.

Results: Twenty-two and 11 dogs were included in the LRTI-RA and LRTD-NRA groups, respectively. The cfu/mL was not significantly predictive of antibiotic requirement, independent of ongoing or prior antibiotic treatment and age (LRTI-RA: median, 10 000 cfu/mL; range, 10-3 × 10 ; LRTD-NRA: median, 10  000 cfu/mL; range, 250-1.3 × 10 ; P = .27). The TNCC was not significantly predictive of antibiotic requirement when only dogs with bronchial disease were considered (LRTI-RA: median, 470 cells/μL; range, 240-2260; LRTD-NRA: median, 455 cells/μL; range, 80-4990; P = .57).

Conclusion And Clinical Importance: The cfu/mL is an inappropriate measure for determining whether antibiotics are of benefit in dogs with LRT signs.