In this study, 34 ears of 26 dogs with chronic unilateral or bilateral ear infections were cultured. Samples were randomly sent to 1 of 3 diagnostic laboratories for bacterial culture and sensitivity. A Pseudomonas species was isolated from 18 of 34 ears (53%) and in 50 of 102 (49%) samples submitted. When laboratories were compared, one isolated Pseudomonas species in 16 of 34 ears and the other two laboratories isolated Pseudomonas species in 17 of 18 ears. Overall, the three laboratories agreed on the presence of Pseudomonas in cultures in 83.3% of cases. All three laboratories agreed on isolation in 15 of 18 ear samples (83.3%). Tube dilution testing with minimum inhibitory concentration (MIC) results was requested from all laboratories, but one did not comply. Data from this laboratory were not included in the analysis. When culture data from the remaining two laboratories was compared, none of the 16 samples isolated from these two laboratories had identical culture results. Both laboratories reported resistance for amoxicillin/clavulanic acid and first-generation cephalosporins in 16 of 16 cases. For chloramphenicol, laboratories agreed on resistance in 14 of 16 cases (88%); for ampicillin, ceftiofur, and tetracycline, laboratories agreed on resistance in 13 of 16 cases (81%). Resistance to trimethoprim sulfa was reported by both laboratories in 11 of 16 ears (69%). For amikacin and gentamicin, laboratories agreed on susceptibilities in 13 of 16 isolates (81%). Both laboratories agreed on susceptibility or resistance to ticarcillin and enrofloxacin in 9 of 15 instances (60%). The results of this study showed that laboratory isolation, susceptibility patterns, and MIC may not agree among laboratories. Study funded by Bayer Animal Health

COMMENTARY: It's important for veterinarians to recognize that laboratories vary  in identification of not only Pseudomonas and but also susceptible and resistant classification. Because Pseudomonas otitis is such an important secondary cause of otitis in dogs, veterinarians should also use their clinical acumen in choosing treatment protocols.

Variability of laboratory identification and antibiotic susceptibility reporting of Pseudomonas spp. isolates from dogs with chronic otitis externa. Schick AE, Angus JC, Coyner KS. VET DERMATOL 18:120-126, 2007.