Importance of using guarded techniques for the preparation of endometrial cytology smears in mares

Material for endometrial cytology can be collected by veterinarians using guarded or unguarded swabs, or digitally with a gloved hand, and is an important diagnostic tool in establishing the endometrial health of mares prior to breeding. The aim of this study was to determine whether the use of unguarded endometrial samples is a reliable indicator of the presence of neutrophils in the uterus. Duplicate endometrial smears were collected from 41 genitally normal, non-pregnant fertile mares by both double-guarded swabs (DGS) and in an unguarded manner by digital scraping (DS) of the endometrium. In 17 of the 41 mares, smears were also collected from the cranial vagina by DS. Cytological samples were collected from a further seven non-pregnant mares at different reproductive stages, and tissues (vestibule, vagina and cervix) from four reproductively normal mares were examined histologically after slaughter to detect the presence of neutrophils. Only 3/41 (7.3%) of the DGS endometrial smears had neutrophils present compared to 36/41 (87.8%) of the DS endometrial smears. The percentage of neutrophils in DGS endometrial smears ranged from 0 to 6% (mean = 0.41%), whereas those in the DS smears ranged from 2 to 90% (mean = 22.02%). Neutrophils were present in all vaginal smears (17/17, range=3-56% (mean = 22.18%)). There was a significant positive correlation (r = 0.84; P < 0.001) between the percentage of neutrophils in the vagina and in the DS endometrial smears. More neutrophils were found in the cervix, vagina and vestibule than in endometrial smears during the cycle (P<0.05). Neutrophils were also observed in tissue collected from the cervix, vagina and vestibule from reproductively normal mares at post-mortem. In conclusion, endometrial smears collected using unguarded techniques are very likely to be contaminated with neutrophils transferred from the vagina potentially leading to incorrect diagnosis of endometritis. When collecting samples for endometrial cytology it is important to use guarded techniques to ensure that only the endometrium is sampled to avoid contamination with cells carried over from other areas of the reproductive tract.