Effects of cycle stage and sampling procedure on interpretation of endometrial cytology in mares

The aim of this study was to ascertain if (1) the stage of reproductive cycle influences cytological results, (2) cytology obtained from an endometrial biopsy is more accurate than cytology derived using the cytobrush, and (3) different methods used for evaluation of cytological samples produce similar diagnostic results. Material was collected from 46 mares in estrus, 48 in diestrus and from 33 mares in anestrus. Smears were evaluated using two criteria. In criterion I, a total of 300 cells were counted and the percentage of polymorphonuclear cells was recorded. In criterion II, the number of polymorphonuclear cells/high power field was determined. Endometrial biopsy samples were also examined histologically, and comparisons were made between cytological and histological results. Regardless of the method used for sample collection, endometritis was diagnosed significantly more frequently if smears were evaluated according to criterion I. The agreement between the number of polymorphonuclear cells infiltrated into the endometrial luminal epithelium and stratum compactum and the cytology results for smears evaluated according to criterion I was fair for anestrous mares, poor for diestrous mares, and moderate for mares in estrus. When cytological smears were evaluated according to criterion II the agreement between the histological and cytological and results was always poor. In summary, cytological evaluation of the mare endometrium should be based on counting the numbers of polymorphonuclear cells in relation to epithelial cells rather than counting the number of polymorphonuclear cells in relation to high power field.