Clinical Theriogenology 2022; 14: 370 Ovarian remnant syndrome in small animals: case series Kiki Mullikin,a Michael Byron,b Janice Chen,c Soon Hon Cheong,b Cathy Gartley,c Mariana Diel de Amorimb aLaurel Highlands Animal Health, Somerset, PA bCornell University College of Veterinary Medicine, Ithaca, NY cOntario Veterinary College, University of Guelph, Guelph, Canada Abstract Our objective was to report the utility of various diagnostic tests in identifying ovarian remnant syndrome (ORS) in dogs and cats. Medical records from 2 referral teaching hospitals (Health Sciences Centre at the Ontario Veterinary College and Cornell Universi- ty Hospital for Animals) were examined and 48 animals (31 dogs and 17 cats) were chosen. Data included were based on sufficient clinical or diagnostic evidence of ORS. Histopathology was used as the confirmatory test for ORS. There was no difference between the proportions of dogs versus cats diagnosed with ORS. Similarly, there was no difference in the proportions of ovarian remnants (OR) between large, medium, or small dogs, or the side (right, left, or bilateral) in which OR was diagnosed. Vaginal cytology and transabdominal ultrasonography findings, and serum progesterone concentrations had the highest chance of correctly iden- tifying an OR prior to exploratory surgery. Transabdominal ultrasonography had strong agreement with OR location identified at surgery. Presumptive intraoperative diagnosis of OR was possible in 39/41 cases (95.1%). Auxiliary diagnostic testing should be recommended to confirm functional ovarian tissue before surgery to reduce unnecessary surgery. Additionally, transabdominal ul- trasonography examination may reduce surgical time since ovarian remnant location has strong agreement with surgical findings. Keywords: Dogs, cats, ovarian remanent, anti-Müllerian hormone, progesterone concentrations, transabdominal ultrasonography Introduction Ovarian remnant syndrome (ORS) is defined as the presence of functional gonadal tissue in a bitch or queen that had undergone a previous ovariectomy or ovariohysterectomy (OHE).1,2 Clinical presentations of ORS include: mamma- ry gland enlargement or masses, pollakiuria and stranguria, vulvar or vaginal masses, polyuria and polydipsia, recurrent urinary tract infection, and vaginal discharge and pseudopreg- nancy.3,4 Presence of functional ovarian remnant tissue is most often the result of incomplete surgical removal. It can also be the result of dropping a tissue fragment into the surgical site or failing to identify and remove ectopic ovarian tissue.4-6 Studies on surgical complications of postOHE reported that 17 - 43% of the referral cases, presented for postoperative complica- tions after ovariohysterectomy, were for ORS.7,8 Animal’s age, breed, body condition, whether the surgical procedure was elective or emergent, and the experience of the surgeon were not associated with a higher likelihood of ORS.4,9 However, in a larger scale and in a more recent study that examined the medical records of 1,880 bitches, higher weight was asso- ciated with more postOHE complications, including ORS.10 Relating to this aspect of surgical access, the right ovary (lo- cated more cranially) was the more common site for finding an ovarian remnant compared to left.11,12 Additionally, reports vary on whether ORS is more common in dogs or cats,4,9 with some authors suggesting a higher likelihood in dogs due to the presence of its unique ovarian bursa.3,13 The mean time to recurrence of estrous behavior following spay is reportedly 1.3 years in dogs and 3 years in cats.1,3,4 Most commonly reported clinical signs are associated with proestrus and estrous signs.3 Vaginal discharge may or may not be present depending on the amount of uterine stump left at OHE; other differentials for vaginal discharge such as coag- ulopathies, vaginitis, trauma, vaginal neoplasia, and vaginal foreign bodies should be ruled out.4 Diagnosis of ORS can be presumptive based on clinical signs or supported by ancillary testing such as, vaginal cytology, measuring estradiol and progesterone concentrations (before and after stimulation with gonadotropin releasing hormone [GnRH] or human chorionic gonadotropin [hCG]) or lu- teinizing hormone (LH) or anti-Müllerian hormone (AMH) or via ultrasonography.1,3,9,11,14 Vaginal cytology (based on abundance of superficial epithelial cells) is the cheapest and Clinical Theriogenology 2022; 14: 371 fastest method for presumptive diagnosis of ORS in dogs and cats presenting with proestrus or estrus clinical signs.1,4 Vag- inal cytology results are useful to determine the presence of functional ovarian tissue and endogenous estrogen secretions; however, exogenous and adrenal estrogen exposure should be ruled out.15 Additionally, vaginal cytology in cat presents both a challenge to obtain and to interpret.16 Serum estradiol con- centrations > 20 pg/ml are indicative of ORS in a OHE dog. However, reliability of estradiol as a marker of ORS is limited because it is not produced solely by the ovaries; furthermore, it varies on a daily, weekly, and seasonal basis, and also there is variability among dogs.17 Serum LH concentrations are ex- pected to be elevated in spayed animals due to lack of negative feedback of estradiol;1 however, a false positive could occur if the test is performed on the day of an LH peak in dogs, or false negative if the test is performed during the period of short- er day length in cats.18 Baseline progesterone concentrations are very reliable as it is an indication of active luteal tissue in the dog, and remain elevated for 4 months of the year in most dogs. However, in queens, progesterone concentrations are not useful indicator of the presence of functional ovari- an tissue unless ovulation has been induced.9 Anti-Müllerian hormone in adult females is secreted exclusively by granulosa cells of the ovary and therefore has been used as a serum bio- marker for ORS.19 However, AMH results can be inconclusive in remnant ovarian tissues that have corpora lutea. Therefore, AMH and progesterone tests should be performed together for higher accuracy. Additionally, AMH has not been thorough- ly investigated in prepubertal bitches that could lead to false negative results. Despite its limitations, AMH and progester- one have become the gold standard diagnosis of ORS prior to surgical exploration.14 Use of transabdominal ultrasonography in detecting ovarian remnants continues to be debated; the successful diagnosis of ORS depends on the skill of the examiner, the size of the remnant, and estrous cycle stage.1,19 A study reported that ul- trasonography was more helpful as an ancillary test to histo- ry and clinical signs rather than a replacement to hormonal testing.3 Additionally, in 32 dogs with ORS transabdominal ultrasonography was able to identify a remnant in 30 out of 31 dogs (97%) that had a remnant confirmed via histopathol- ogy examination.11 Exploratory laparotomy/laparoscopy is the treatment of choice for ORS and histopathology is considered the defini- tive diagnosis.1,11 Our objective was to describe the most com- mon clinical features, signalment of animals diagnosed with ORS and diagnostic tools utilized. Materials and methods Medical records from 2 referral veterinary teaching hospitals from 1984 to 2020 were reviewed to identify dogs and cats with a history of previous OHE that presented with clinical signs of estrus and/ or clinical signs that led to the presump- tive diagnosis of ORS. Cases were included if sufficient clin- ical or diagnostic evidence of an OR was present. Diagnosis of ORS was via vaginal cytology, serum estradiol concentra- tions, serum progesterone concentrations (> 0.5 ng/ml),14 measurement of progesterone concentrations after an gonad- otropin stimulation test, AMH concentrations, identification of ovarian remnant via transabdominal ultrasonography by a board-certified radiologist or radiology resident, identifica- tion of an ovary during an exploratory laparotomy/laparosco- py, histopathology, or any combination of these techniques. Cases referred for ORS but did not have confirmation of ovari- an remnant via histopathology, at surgery, or via vaginal cytol- ogy in combination with progesterone analysis were excluded. Following information, if available, was collected: species, breed, size of breed (small, medium or large), age at pre- sentation, age at OHE, time from spay to clinical signs, pre- senting complaint, clinical signs observed by the owner and the attending veterinarian, results of vaginal cytology, serum estradiol, progesterone, LH, or AMH concentrations, transab- dominal ultrasonography findings, side of suspected ovarian remnant via ultrasonography (right, left or both), surgical ap- proach for exploratory laparotomy (ventral midline or lapa- roscopy), side of the remnant at surgery (right, left or both), and histopathological diagnosis. Data analyses Statistical analysis was performed with JMP Pro software ver- sion 16 (SAS Institute Inc., Cary, NC, USA), with a level of sig- nificance of p < 0.05. Fischer’s Exact test was performed to test the differences in the proportions of species, breed size, and ovarian remnant side at surgery. A Kappa coefficient of agree- ment was used to determine the correlation between ovarian remnant side diagnosed via transabdominal ultrasonography to surgery findings. Results Forty-eight animals (31 dogs and 17 cats) were diagnosed with ORS. Dog population consisted of small, medium, and large breed dogs that included 10 Labrador retrievers and 12 mixed breed dogs. Remaining breeds were Akita, Boston Ter- rier, Brittany Spaniel, Brussels Griffon, Kerry Blue Terrier, Pug, Rottweiler, German Short-haired Pointer, and West Highland White Terrier. In the cat population, 13 were Domestic Short- hair, with the remainder comprised of a Himalayan, a Bengal, and 2 Domestic Longhair cats. The median and interquartile range age at presentation was 2.3 (1.4 - 5.7) years, whereas the age at presentation for dogs and cats respectively was 3.5 (1.6 - 5.9) years and 1.5 (1 - 3.5) years. The median and interquar- tile time between previous ovariohysterectomy and presenta- tion of clinical signs resembling ORS was 0.5 (0.3 - 2) years, whereas for dogs and cats the median and interquartile range were 1.0 (0.3 - 2.8) years and 0.5 (0.3 - 0.8) years, respectively. There was no difference in the proportion of dogs versus cats diagnosed with ORS or between the proportion of large breed dogs (36.7%) compared to medium (36.7%) and small dogs (26.7%). There was no difference in the proportions of the location of the OR, where 8.5% of the cases were identified in both locations, 40.4% were identified on the left side and the remaining 51.0% were on the right. As per owners, 45 of 48 animals (93.8%) exhibited classical signs of ORS. These signs included bloody vaginal discharge, vulvar swelling, and attracting males in dogs and rolling, vo- calizing, and attracting males in cats (Table 1). Of those ani- mals presenting for classical signs of estrus, vaginal cytology Clinical Theriogenology 2022; 14: 372 was performed on 24 animals (50.0%), and 12/24 of these animals had a cytological finding consistent with estrous smear (50.0%). The remaining 12 vaginal cytology were clas- sified as proestrus, diestrus, or inconclusive. Seven of those cy- tology had concurrent progesterone concentrations consistent with ovarian function (> 0.5 ng/ml) and 4 had progesterone concentrations that were < 0.5 ng/ml. Progesterone concentra- tions were not analyzed in 1 remaining dog. Table 1. Number of affected animals and percentage of affected cases out of total number of cases by presenting clinical signs Clinical sign Number affected Percent affected Dogs (n = 31) Vulvar swelling 23 74.2 Bloody vaginal discharge 20 64.5 Cats (n = 17) Vocalizing 13 76.5 Dogs and cats combined (n = 48) Mammary development 10 20.8 Attracting males 6 12.5 Abdominal/mammary neoplasia 3 6.3 Surgical complications of previous OHE 2 4.2 Serum estradiol concentrations were measured in 4 animals (8.5%); 2 of these had serum estradiol concentrations > 20 pg/ml. Baseline serum progesterone concentrations were mea- sured in 16 dogs (51.6%). Of these 16 dogs, 10 had serum pro- gesterone concentrations > 0.5 ng/ml (62.5%). Measurement of serum progesterone concentrations after stimulation with hCG or GnRH was performed in 6 cats and 1 dog (14.6%). Se- rum progesterone concentrations increased > 0.5 ng/ml in all 6 cats, but not in the dog in response to hCG or GnRH stimu- lation; however, this dog had positive estradiol concentrations and ovarian remnant was identified via surgery and histopa- thology. An in-house LH test was performed in 5 animals (3 dogs and 2 cats) that correctly identified ORS in 4/5 (80%) of animals and subsequently confirmed via histopathology. In 1 case ORS was not identifiable via LH concentrations and pro- gesterone concentrations of this dog was < 0.2 ng/ml; however transabdominal ultrasonography and surgery correctly identi- fied OR that was confirmed via histopathology. AMH assays were performed in 6 animals (5 dogs and 1 cat) (12.5%); 3/5 dogs tested positive, whereas the other 2 were inconclusive. However, progesterone concentrations on these were > 0.5 ng/ ml and surgery/histopathology confirmed the presence of an ovarian remnant. The single cat that had the AMH performed was positive for AMH and that was confirmed via histopathol- ogy. Diagnostic examinations and clinical findings in 48 cases are listed (Table 2). Cases that were correctly diagnosed with various diagnostic tests compared to histopathology confir- mation of OR are identified (Table 2). Table 2. Diagnostic examinations and clinical findings Case Species Breed Age at presentation (years) Presenting complaint Vaginal cytology* P4 † (ng/ ml) AMH LH Ultrasonography findings Surgical approach OR at Surgery Surgery/histology findings 1 Dog Labrador retriever 5.9 Estrous signs Ventral Midline Right (+) Right ectopic OR, bilateral paraovarian cyst (+) 2 Dog Mixed 7.3 Caudal abdominal mass Renal Cyst (-) Ventral Midline Both (+) Ovarian tissues and uterine horn 3 Dog Akita 1.4 Estrous signs Inc 27.66 Ventral Midline Right (+) Right entire ovary normal (+) 4 Dog Mixed 9.4 Estrous signs Diestrus 0.13 Uterine stump (-) Ventral Midline Right (+) Right granulosa cell tumor (+) 5 Dog Labrador retriever 4.3 Estrous signs 0.38‡ Ventral Midline Left (+) Mass resembling ovarian tissue§ Clinical Theriogenology 2022; 14: 373 Case Species Breed Age at presentation (years) Presenting complaint Vaginal cytology* P4 † (ng/ ml) AMH LH Ultrasonography findings Surgical approach OR at Surgery Surgery/histology findings 6 Dog Labrador retriever 5.6 Estrous signs Right ovary (+) 7 Dog Rottweiler 6.8 Postspay complication Inconclusive (-) Ventral Midline Left (+) Left ovarian tissue (+) 8 Dog Labrador retriever 3.5 Estrous signs Ventral Midline Right (+) Right OR w/ CL and follicles (+) 9 Dog Mixed 2.2 Estrous signs Ventral Midline Right (+) Right OR w/ CL (+) 10 Dog Mixed 1.7 Estrous signs Right ovary w/ CL (+) Ventral Midline Right (+) Right ovary w/ CL, follicles (+) 11 Dog Mixed 2.0 Estrous signs Intact right ovary and uterine horn remnant (+) Ventral Midline Right (+) Right ovary, uterine tube 12 Dog Mixed 1.3 Estrous signs Estrus Ventral Midline Right (+) OR with CL, uterine tube (+) 13 Dog Mixed 13.4 Estrous signs Caudal abdominal mass (-) Ventral Midline Left (+) Left OR with granulosa cell tumor (+) 14 Dog Brittany Spaniel 0.8 Estrous signs Estrus 1.68 Right ovary w/ cystic structures (+) Ventral Midline Right (+) Right entire ovary w/ CL and follicles (+) 15 Dog Boston Terrier 6.1 Estrous signs Proestrus 0.03 + 16 Dog Labrador retriever 2.2 Estrous signs Proestrus/ Diestrus 0.61 Inc Right OR (+) Laparoscopy Right (+) Right ovary 17 Dog Mixed 1.1 Estrous signs Right OR (+) Ventral Midline Right (-) Right ovary w/ CL and follicles (+) 18 Dog Mixed 9.7 Estrous signs Proestrus/ Diestrus 31.4 + Left OR (+) 19 Dog Pug 5.9 Estrous signs Diestrus Large mid- abdominal mass (-) Ventral Midline Left (+) Left ovary w/ cyst (+) 20 Dog Labrador retriever 6.4 Vomiting, diarrhea 5.1 Right OR (+) Ventral Midline Right (+) Right ovary w/ CL and paraovarian cyst (+) 21 Dog Mixed 3.7 Estrous signs Proestrus/ Diestrus 3.5 + Right OR w/ CL (+) Ventral midline Right (+) Right OR w/ CL (+) 22 Dog German Short- Haired Pointer 4.1 Estrous signs Proestrus/ Diestrus 0.78 Inc Right OR (+) Ventral Midline Right (+) Right OR w/ multiple CL (+) 23 Dog Westie 3.6 Estrous signs Right OR and oviduct (+) Ventral Midline Right (+) Entire ovary, uterine tube (+) 24 Dog Brussels Griffon 5.7 Estrous signs Proestrus/ Diestrus 0.34 Uterine stump (-) Ventral Midline Both (+) Left granulosa cell hyperplasia and right hydrosalpinx (+) 25 Dog Labrador retriever 1.4 Estrous signs Diestrus 0.44 OR (unknown location) (+) Laparoscopy Right (+) Endometrial hyperplasia and focal hydrosalpinx (-) 26 Dog Mixed 2.3 Estrous signs Estrus 0.2 - Laparoscopy Right (+) Right ovarian remnant and hydrosalpinx (+) 27 Dog Kerry Blue Terrier 1.4 Estrous signs Diestrus 0.57 + Right OR (+) Laparoscopy Right (+) Ovarian remnant and hydrosalpinx (+) Clinical Theriogenology 2022; 14: 374 Case Species Breed Age at presentation (years) Presenting complaint Vaginal cytology* P4 † (ng/ ml) AMH LH Ultrasonography findings Surgical approach OR at Surgery Surgery/histology findings 28 Dog Mixed 3.0 Estrous signs Estrus Right ovary (+) Laparoscopy Right (+) OR w/ CL (+) 29 Dog Mixed 2.0 Pseudo- pregnancies Bilateral ovaries and cyst-like oviductal structures (+) Laparoscopy Both (+) Left and right ovarian remnant, cystic remnant of uterine horn (+) 30 Dog Soft- coated Wheaten Terrier 1.0 Estrous signs Diestrus 20.0 + Right ovary (+) Ventral Midline Right (+) Ovotestis w/ CL (+) 31 Dog Labrador retriever 1.6 Estrous signs 0.57 Left OR (+) Laparoscopy Left (+) Granulosa cell hyperplasia, CL, uterine tube (+) 32 Cat DSH 10.3 Mammary tumors; cyclicity Ventral Midline Left (+) OR (follicles and CL); mammary gland adenocarcinoma and ductal carcinoma (+) 33 Cat Himalayan 10.7 Estrous signs Large abdominal mass suspect of ovarian tumor (+) Ventral Midline Both (+) Granulosa cell tumor (+) 34 Cat DSH 1.8 Estrous signs Estrus Ventral Midline Right (+) Follicles, granulosa cells (+) 35 Cat DLH 3.4 Estrous signs Ventral Midline Left (+) Left cystic ovary w/ luteinization (+) 36 Cat DSH 1.5 Estrous signs 7.34** Left ovary (+) Ventral Midline Left (+) Left ovary w/ CL and follicles (+) 37 Cat DSH 0.8 Estrous signs Uterine stump (-) Ventral Midline Left (+) Left ovary w/ follicles (+) 38 Cat DSH 2.4 Estrous signs Left ovary w/ follicles or CL (+) Ventral Midline Left (+) Left ovary w/ follicles (+) 39 Cat DLH 1.0 Estrous signs Ventral Midline Left (+) Left entire ovary 40 Cat DSH 0.7 Estrous signs Left OR w/ follicles (+) Ventral Midline Left (+) Left ovary, uterine tube (+) 41 Cat DSH 1.0 Estrous signs Estrus 0.93** No OR (-) Laparoscopy Left (+) Left ovary w/ CL and follicle (+) 42 Cat DSH 3.5 Estrous signs Left ovary (+) Laparoscopy Left (+) Left granulosa cell hyperplasia (+) 43 Cat DSH 1.1 Estrous signs Estrus 4.87** + Left OR (+) Laparoscopy Left (+) Entire normal ovary (+) 44 Cat DSH 1.2 Estrous signs Estrus 7.58** Laparoscopy Left (+) Normal ovarian tissue (+) 45 Cat DSH 1.1 Estrous signs Estrus 6.23** Laparoscopy Right (+) Normal ovarian tissue, oviduct (+) 46 Cat DSH 4.8 Estrous signs Estrus 10.09** Ventral Midline Right (+) Normal ovarian tissue (+) 47 Cat Bengal 1.0 Estrous signs Estrus + Left ovary w/ cysts (+) Laparoscopy Left (+) Normal ovarian tissue (+) 48 Cat DSH 1.5 Estrous signs Estrus + Left OR (+) Laparoscopy Left (+) Ovary w/ CL and cystic rete ovarii (+) Positive results 12 16 4 4 25 44 39 Clinical Theriogenology 2022; 14: 375 Case Species Breed Age at presentation (years) Presenting complaint Vaginal cytology* P4 † (ng/ ml) AMH LH Ultrasonography findings Surgical approach OR at Surgery Surgery/histology findings Negative results 12 6 2 1 8 1 1 Total tested 24 22 6 5 33 45 41 Percentage of correctly identified via histopathology as final diagnos 61.9% 83.3% 66.6% 71.4% 95% estrous signs = vulva enlargement + vaginal bleeding in canines; rolling and vocalizing in felines P4 = progesterone; OR = ovarian remnant; CL = corpora lutea; Inc = inconclusive *vaginal cytology only marked positive if in estrus †progesterone concentrations positive if > 0.5 ng/ml ‡Interpretation of hCG stimulation: progesterone - baseline: 0.38; posthCG (1 hour): 0.18; posthCG (5 hours): 0.31; estradiol - baseline: 37.31; posthCG (1 hour): 65.78; posthCG2 (5 hours): 31.75 surgery/histology report: (+) sign identifies cases which had histopathology performed and therefore description from pathology, otherwise description from surgery finding §histology report was lost **serum progesterone samples measured after hCG or GnRH stimulation Transabdominal ultrasonography was performed by a boar-certified radiologist or a radiology resident in 33/48 an- imals (68.8%). An ovarian remnant was diagnosed ultraso- nographically in 25/33 animals (75.8%). Thirty-one animals that had transabdominal ultrasonography underwent an ex- ploratory laparotomy to remove the remnant. There was no difference between species or size of dog in the probability of identifying an ORS via transabdominal ultrasonography. Ultrasonography identified the side on which the remnant was located in 23/25 cases (92.0%). The Kappa coefficient of agreement of remnant side on transabdominal ultrasonogra- phy with the actual side determined at surgery was very strong at 0.7 (0.7, 1) (lower 95% CI, upper 95% CI) (p < 0.0001). Forty-five animals underwent an exploratory laparotomy to remove the OR (93.8%). A ventral midline approach was performed on 31/45 of the animals (68.9%); 14/45 animals underwent a laparoscopic procedure (31.1%). Samples were submitted for histopathology in 41/45 (91.1%) of the ani- mals. An OR was identified in 39/41 (95.1%) of the samples submitted for histopathology; the histopathology report for 1 of the 2 unconfirmed samples could not be located and the other histopathology result was consistent with uterine and oviduct tissue. Two of the 39 ovarian tissue samples were gran- ulosa cell tumors. Both samples belonged to dogs, and the time from OHE to clinical signs for each was 9.4 and 12.4 years, respectively. Example of an OR in a dog is provided (Fig- ure); OR was on the right side of the abdomen (identified via transabdominal ultrasonography and confirmed via histopa- thology). Figure. Ultrasonogram of suspected right gonad (craniocau- dal view); note: hypoechoic gonad with multifocal anecho- ic follicles, presumed to be OR (A); histopathological (H&E stain) confirmation of the right OR surrounded by a thin layer of adipose tissue, with 2 cystic structures surrounded by ovar- ian stroma and presence of a corpus luteum (arrow) (B), and higher magnification of the corpus luteum demonstrating lu- teal cells (C). Discussion The mean age at presentation for dogs and cats with ORS has been reported to be 2.8 years and the interval between OHE and presentation to be 1.4 years.3 In our study, the median age at presentation was 2.3 years and the interval between OHE and presentation was 0.5 years, with the owners reporting Clinical Theriogenology 2022; 14: 376 signs from immediately after ovariohysterectomy up to 12.4 years later. Data were skewed towards younger animals similar to reports.3,4 Furthermore, in the clinical history, several own- ers reported that their animals returned to estrus from very soon after OHE to several months following OHE, but they did not pursue further diagnostics or treatment until later in the animal’s life. Weight may be associated with ORS as a complication pos- tOHE;4,9,10” however, not all had similar finding. In our study, we did not evaluate animals’ weight as not all of them were available; however, when breed size (small, medium, or large) was evaluated, no difference in the proportion of ORS were present among dog breed sizes. In this regard, some studies have demonstrated that cats are more likely to have ORS,4,9 but others have demonstrated the opposite.3,13 In our study, no difference was detected between the proportions of cats and dogs with ORS. However, the number of cats in our study was lower and therefore the data should be interpreted with caution. When vaginal cytology demonstrated predominantly superfi- cial cells consistent with estrus, not surprisingly all animals were diagnosed on histopathology. Of the 12 cytology that were suspected to be proestrus, diestrus, or undetermined, 7 had serum progesterone concentrations consistent with pres- ence of luteal function (> 0.5 ng/ml) that enabled to con- firm the diagnosis of ORS. Four animals had progesterone concentrations < 0.5 ng/ml, and 1 of these 4 cases had ORS confirmed via AMH whereas the other 3 had confirmation at surgery. It is important to note that in cats, progesterone was measured after ovulation induction. These findings supported the importance of multiple diagnostics, as a single cytology sample may not always be enough to determine the stage of the estrous cycle. However, vaginal cytology remains an easy, cheap and simple test to perform and when combining with progesterone measurements will increase the odds of correct- ly identifying ORS.1 AMH was only measured in 6 animals. The low number of cases is due to the fact that most of the cases included in this study were seen before AMH testing was available,19 and because of the high expense to send samples to referral laboratories for AMH testing. Additionally, some of the cases were less complex and presented with more obvious signs of estrus concurrent with corroborating vaginal cytology and/or confirmation with elevated serum progesterone. That serum estradiol was measured only in 4 animals whereas vagi- nal cytology was performed in 24 cases was most likely due to vaginal cytology being a cheaper, easier, and faster method to diagnose endogenous estrogens with less chances for labora- tory reference range, assay and diurnal variations.17 Clinician preferences, advancement on diagnostic capabilities through- out the years and financial budget could have affected the di- agnostic test performed. In this sample population, no difference in the proportions were noted between right and left OR. This contradicts (right sided remnants were significantly higher than left) certain reports,3,11 suggesting surgical error. However, others have demonstrated that surgical experience of the veterinarian is not associated with increased chance of complication for ORS.4,9 This study has not evaluated surgeon’s experience, as most of the OR that were diagnosed were referrals, and there- fore experience of the veterinarian that performed the orig- inal OHE was unavailable. A study with a larger population of dogs and cats would be required to determine if the side of OR and if weight or size of the animals may influence the risk of ORS. Percentage of correctly identifying an OR via transabdominal ultrasonography was consistent with what was reported.3 Ad- ditionally, of those cases that we were able to identify an ORS via transabdominal ultrasonography, there was a strong agree- ment of the location of the remnant with surgical findings. Likewise, a study that used transabdominal ultrasonography for prelaparoscopy diagnosis of ORS also demonstrated that transabdominal ultrasonography can be quite reliable in iden- tifying the remnant location.11 Species or breed size of dog were not associated with whether or not a remnant was identi- fied in transabdominal ultrasonography. However, our study’s dog population lacked giant breeds that may be more difficult for the ultrasonographer and machine capabilities. The level of experience of the ultrasonographer performing the exam- ination was not evaluated during this study and could be a confounding factor for the successful diagnosis of an ovarian remnant via transabdominal ultrasonography. We would ex- pect more experienced ultrasonographers to have even higher detection rates. Further studies regarding ultrasonographer’s experience (board-certified radiologist versus radiology resi- dent) are warranted with a larger population of animals. Of the 45 exploratory surgeries performed, 1 was negative for histopathology. In this case, a remnant was identified on the midline via transabdominal ultrasonography and grossly at surgery, but histopathology results revealed cystic endometrial hyperplasia and focal hydrosalpinx. Tissues were not submit- ted for histopathology in 4 cases that underwent surgery. In all 4 cases, a remnant was identified grossly at surgery. In 3/4 cases, transabdominal ultrasonography and/or progesterone concentrations were used to confirm the presence of an OR prior to surgery, and the remaining case had no diagnostics performed prior to surgery, but a complete ovary was iden- tified at surgery. Similar to our study, identification of ovari- an remnants at surgery (gold standard) had a high sensitivity compared to histopathology.11,12 Surgical removal of ovarian remnants remains the treatment of choice.1 In our study, vag- inal cytology results, serum progesterone concentrations, and transabdominal ultrasonography findings had the highest chance of correctly identifying a OR prior to exploratory sur- gery, and transabdominal ultrasonography was very effective in locating the remnant side that could help reduce surgical and anesthesia time. Auxiliary diagnostic testing should be recommended to confirm functional ovarian tissue before surgery to reduce the chance of unnecessary surgery. Addi- tional studies are needed to determine which combination of diagnostic tests, in addition to clinical signs, is the most cost effective and accurate method of diagnosing an ovarian remnant prior to pursue an invasive exploratory abdominal surgery. 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