Impression cytology of the healthy equine ocular surface: Inter-observer agreement, filter preservation over time and comparison with the cytobrush technique.

Background: The cytobrush technique is commonly used to sample the equine ocular surface. Impression cytology (IC) is an innovative noninvasive method, which allows for the collection of superficial layers of ocular epithelium.

Objectives: The aims of this study were to compare the cytobrush and IC techniques on healthy equine ocular surfaces, to assess the agreement between observers with different levels of expertise, and to test the preservability of filters over time.

Methods: Twenty-four horses were sampled within 10 minutes of slaughter using IC on the left eye and the cytobrush technique on the right eye. May-Grünwald-Giemsa stained specimens were evaluated by two observers with different levels of expertise. Morphologic features were evaluated using a 4-grade system. The IC samples were re-evaluated after 6 months to examine filter preservation.

Results: In IC samples, corneal and conjunctival cells were clearly separated. Goblet cells were found in five and 17 filters by observer 1 and 2, respectively. Using the cytobrush technique, corneal and conjunctival cells were present but mixed. Goblet cell cellularity, preservation, and enumeration were higher with the IC technique compared with the cytobrush technique (P = 0.013; P = 0.004; P = 0.031, respectively). The inter-observer agreement for the IC technique was moderate to fair. In 7/24 IC samples re-evaluated after 6 months, cellular morphology was impaired, and the overall score was significantly lower.

Conclusions: IC is an innovative noninvasive method, which allows for sample collection with higher cellularity and preservation. Moreover, the identification of goblet cells is easier. For these reasons, IC could be interesting and useful as a complementary diagnostic cytologic method in clinical practice.