Variability of laboratory identification and antibiotic susceptibility reporting of Pseudomonas spp. isolates from dogs with chronic otitis externa.

The purpose of this study was to evaluate interlaboratory variation in isolation and antibiotic susceptibility pattern of Pseudomonas spp. as reported to veterinarians for cases of canine chronic bacterial otitis externa. Twenty-six dogs with unilateral or bilateral bacterial otitis externa from multiple referral practices were included in this prospective study. Triplicate samples collected simultaneously from the same location in the external ear canal were randomly submitted to three laboratories for culture and susceptibility testing. Pseudomonas spp. were isolated from 18 of 34 (53%) ears. All three laboratories agreed on the presence of Pseudomonas spp. in 15 (83.3%) ears sampled. However, two laboratories agreed on two (11.1%) occasions, and on one occasion (5.5%) Pseudomonas spp. were identified in only one laboratory. Minimum inhibitory concentration (MIC) susceptibilities to 11 antibiotics were compared between laboratories B and C. Using laboratory-defined susceptibility of sensitive (S), intermediate (I) and resistant (R), none of the 16 Pseudomonas spp. with MIC data reported had identical patterns of antibiotic susceptibility. Agreement in susceptibility to individual antibiotics was observed in 13 of 16 (81%) occasions for amikacin and gentamicin, 10 of 16 (63%) occasions for ticarcillin, and nine of 16 (56%) for enrofloxacin. These results indicate that Pseudomonas spp. were identified by all three laboratories chosen for this study in 83% of the time. Moreover, antibiotic susceptibility patterns and MIC values reported to veterinarians may not agree between laboratories. Veterinarians should interpret bacterial culture and susceptibility results with multiple caveats including variability between laboratories.