The quick and dirty on ear canal cytology

Continuing with my focus on diagnostic tips, today I’ll share my approach to ear canal cytology in dogs and cats.  As you know already, this is a simple, but valuable test that should be recommended whenever otitis externa is suspected, and on follow up examinations for those pets. I like to take samples from the junction of the vertical and horizontal canals.  You may venture into the deeper horizontal canal if the pet is immobilized and you are confident that you are not jamming more debris deeper into the canal. This means a light touch on the cotton-tipped swab.

Once you’ve collected your swabs, roll each on opposite sides of the slide. I insist on frosted slides so that I can easily keep track of where I’ve placed what.  It also avoids the frustration (most of the time!) of trying to focus with your oil-immersion lens on the wrong side of the slide.

The next point is where I might save you or your technician a few minutes a day. When staining ear cytology slides, you can skip heat fixing and the first two stain jars.  That’s right, go straight to the blue/purple stain of your modified Wright’s stain kit.  A blinded study found that the interpretation of ear cytology samples did not change when you bypass these steps.  If your practice does 20 ear cytology stains per day and this saves you or your technician 1-2 minutes, it adds up over the course of a year.

Scan the slide and find representative areas to hone in on.  Using oil immersion so you can get a better look at any bacteria, scan at least 10 fields then score the exudate with this scale, which is derived from some recent research papers:

Scoring ear cytology

Score Yeast per oil field Bacteria per oil field WBCs per oil field
1 (normal) 0 <1 none
2 (gray zone) 0-1 1-4 rare
3 (abnormal) >1 ≥5 >0.5
Malassezia on ear canal cytology

Malassezia on ear canal cytology

Always interpret the samples in light of your clinical examination of the ear canal. For example, yeast can be found in normal ears, but it would be unlikely that an ear with abnormal numbers of white blood cells is clinically normal.

I hope this helps you get the most out of your ear cytology samples!

Jon Plant, DVM, DACVD

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About skinvet

Jon Plant, DVM, is a Diplomate of the American College of Veterinary Dermatology, founder of SkinVet Clinic and developer of RESPIT, regionally-specific immunotheray for atopic dermatitis of dogs and cats. He is a member of the International Committee on Atopic Diseases of Animals, the past President of the Portland Veterinary Medical Association and the Dermatology Section Editor of the Journal of the American Animal Hospital Association.
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