[Comparison of four different staining methods for ear cytology of dogs with otitis externa]

Cytological examination is crucial for the diagnosis and classification of canine otitis externa. Staining should reveal micro-organisms as perpetuating factors of otitis externa. The aim of the study was to compare four different staining methods (Diff-Quik®, Diff-Quik® after dipping in acetone, Gram Quick stain® and a commercial rapid stain for otitis externa) for ear cytology of dogs with otitis externa and to investigate the agreement of cytology and culture.

In a study evaluating dogs with otitis externa, five ear swabs (one for culture and four for cytology) were taken from the horizontal part of the external auditory canal of 224 affected ears and compared semi-quantitatively.

Diff-Quik® with and without prior dipping in acetone as well as the Gram Quick stain® displayed a high degree of agreement in the detection of micro-organisms (cocci p = 0.2366; rods p = 0.4832; yeasts p = 0.1574), while the commercial otitis rapid stain revealed significantly less micro-organisms (p < 0.001 for all comparisons). The results of the first three stains corresponded to the culture results by >  70%; the agreement was lower with the commercial otitis rapid stain. The quickest and easiest method was staining with Diff-Quik®.

Diff-Quik® with or without prior dipping in acetone and the Gram Quick stain® had a high agreement in the detection of microorganisms and can thus be considered nearly equivalent for the diagnosis of otitis externa infectiosa. The commercial otitis rapid stain is less reliable.

Based on this study Diff-Quik® can be recommended for the routine cytology of ear swabs. Additionally, a culture may be indicated and must be interpreted in the context of the cytology.