Lack of correlation of vaginal impedance measurements with hormone levels in the rat.

Hormone levels vary in female rats depending on estrous cycle stage. Vaginal cytology is a reliable method of staging female rats, but vaginal impedance offers an alternative depending on application. We sought to correlate vaginal impedance in cycling female rats with hormone levels. Vaginal cytology was the standard for comparison and verification of estrous cycle stage. Female rats (n = 41) were evaluated twice daily for 15 days via vaginal cytology and impedance to evaluate two or three estrous cycles per rat. During the last 5 days of the study, selected anesthetized sampling groups (n = 3 or 4 rats per group) were bled terminally at each time point to allow hormone determinations concurrently with vaginal cytology and impedance. Rats with abnormal vaginal smears or discharges (n = 5) were evaluated for reproductive tract histology. Rats classified in estrus by vaginal cytology had significantly higher vaginal impedance values than did nonestrus rats, but vaginal impedance and estrous cycle stage as determined by vaginal cytology did not correlate. Because of small sampling size in nonproestrus groups, correlation between vaginal impedance and hormone levels was evaluated only in proestrus rats (n = 22) and was nonsignificant. No correlation occurred between vaginal impedance and hormone levels in unstaged rats (n = 41). Two animals evaluated for reproductive tract histology showed evidence of pseudopregnancy. Vaginal impedance may be useful in distinguishing estrus from nonestrus rats but may be limited for chronic estrous cycle monitoring because of the possible risk of inducing pseudopregnancy.