This one’s a little controversial, but some evidence exists that taking swabs for culture from cases of otitis externa can lead to poor reproducibility.
Swabs taken from the same ear may produce different isolates, and even when the isolates agree, they may have different (antibiotic) susceptibility patterns (Bloom, 2015; Graham-Mize et al, 2004; and Schick, 2007).
Bloom (2015), therefore, suggested using rational topical therapy based on cytological findings was more valuable than using bacterial culture and sensitivity. He also only advised repeat cytology if the ear was not clinically resolving or neutrophils and bacteria were present on initial cytology.
He only takes culture and sensitivity where rods are found on cytology, there are proliferative changes and the ear is responding poorly to therapy – a rare set of circumstances in Paul Bloom’s experience.
This approach was supported by a study in which the authors evaluated whether any correlation was noted between topical antibiotic selection, in vitro bacterial antibiotic sensitivity, and clinical response in 17 cases of canine otitis externa complicated by Pseudomonas aeruginosa (Robson et al, 2010).
VIDEO: How to properly swab an ear for aural cytology (source: YouTube).
References
Bloom P B (2015). Diagnosis of Otitis Externa in the Real World, Proceedings of the North American Veterinary Conference, Orlando, Florida: 944-953.
Graham-Mize C A and Rosser E J Jr (2004). Comparison of microbial isolates and susceptibility patterns from the external ear canal of dogs with otitis externa, J Am Anim Hosp Assoc 40(2): 102-108.
Robson D C, Burton G G and Basset R J (2010). Correlation between topical antibiotic selection, in vitro bacterial antibiotic sensitivity and clinical response in 17 cases of canine otitis externa complicated by Pseudomonas aeruginosa, (Abst). In 25th Proceedings of the North American Veterinary Dermatology Forum, 2010: 245.
Schick A E, Angus J C and Coyner K S (2007). Variability of laboratory identification and antibiotic susceptibility reporting of Pseudomonas spp isolates from dogs with chronic otitis externa, Vet Dermatol 18(2): 120-126.