Practical Matters: Vaginal cytology for ovulation timing: Not necessarily an accurate indicator

Vaginal epithelial cells are induced to divide by high serum estradiol concentrations present during proestrus in bitches. The rapid increase in the thickness of the vaginal epithelium leads to the death of the cells at the vaginal lumen; these appear as cornified or keratinized cells in vaginal cytology samples. The percentage of cornified vaginal epithelial cells progresses from 0% to 100% during proestrus, remains at 100% during estrus or standing heat, and abruptly falls to 0% with the onset of diestrus.

Margaret V. Root Kustritz, DVM, PhD, DACT

Veterinarians cannot identify an ovulation date or a bitch's fertility from vaginal cytology. All that can be identified is whether the bitch has been exposed to estradiol and at which stage of the cycle she is in at the time of sample collection. The average bitch ovulates two days after the onset of cytologic estrus, defined as complete cornification with more than 50% of the cells appearing anuclear. However, many normal bitches are not average, which makes vaginal cytology results too inaccurate to consistently determine the ovulation date. The ovulation date can be most accurately identified prospectively by measuring luteinizing hormone or progesterone concentrations in blood.

Margaret V. Root Kustritz, DVM, PhD, DACT

Department of Veterinary Clinical Sciences

College of Veterinary Medicine

University of Minnesota

St. Paul, MN 55108