This is the first study investigating the association between CE and ART outcomes regardless of the existence of RIF. Unexpectedly, we found that many patients undergoing ART before the diagnosis of RIF already had CE. For these patients, antibiotic treatment of CE certainly reduced the number of CD138-positive cells, serving as a possible predictor of subsequent fertility outcomes. These factors suggest that the diagnosis and treatment of CE may be considered not only for patients diagnosed with RIF but also for all patients planning to undergo ART. In addition, our study suggests the efficacy of evaluating CD138-positive cell count in the diagnosis and treatment of CE. We believe that the indications for the diagnosis and treatment of CE could be expanded beyond the currently existing ones.

CE is a chronic inflammation condition of the endometrium caused by various factors. Previous reports have included history of intrauterine device (IUD) insertion [17, 18], multiple pregnancies or abnormal uterine bleeding [17], and bacterial vaginosis, endometrial polyps, or endometriosis [19,20,21,22] as causes.

In this study, we evaluated whether background characteristics of patients, including history of intrauterine manipulation (artificial insemination or ET), affected the occurrence of CE; however, we could not find any specific correlation. Although significant differences were noted for myoma and endometriosis, these factors need not be necessarily considered, as the number of patients in this study were very small.

Generally, diagnosis and treatment of CE should be performed on patients with RIF; however, we consider that all patients who are planning to undergo ART should be indicated. Currently, CE has been reported in approximately 2.8%–56.8% of infertile patients [12,13,14], approximately 14%–67.5% of RIF patients [3,4,5,6,7,8], and approximately 9.3%–67.6% of patients with repeated miscarriages [23, 24]. This variation among reports might be attributed to the different examination methods and diagnostic criteria. In this study, we defined CE positivity as “CD138-positive cells ≥ 1/10 HPFs,” with reference to a previous study [25], and the positivity rate was as high as 83.7% for patients planning to undergo ART with or without RIF. Although a relatively strict definition, such as the one mentioned above, may have influenced this high rate, it was suggested that a significant proportion of patients planning ART could already have CE. Meanwhile, the number of CD138-positive cells was significantly decreased when antimicrobial agents were administered to patients with CE in this study. A high number of CD138-positive cells in the endometrium is thought to have a negative effect on the pregnancy and ongoing pregnancy rates in ART [26,27,28]. Hence, decreasing CD138 positive cells with antimicrobial therapy for such CE patients may be beneficial for pregnancy. In practice, no significant differences were noted in the ongoing pregnancy rates between the CE-negative group and the groups after CE treatment. It is possible that CE treatment reduced the number of CD138-positive cells, resulting in improved and comparable pregnancy outcomes in the CE-positive patients. However, the low pregnancy rate in the CE second-line group suggests that the treatment effect may not be sufficient in severe cases; therefore, early diagnosis of CE and first-line treatment may be beneficial before it becomes severe. In the ongoing pregnancy or live birth group, CD138-positive cell counts were significantly lower at the second biopsy (Fig. 3), suggesting that the reduction in the number of CD138 cells caused by first-line antibiotic treatment could be important. Taken together, it may be useful to consider patients with diagnosis of CE and antibiotic treatment before the planned ART stage.

Our results also suggest the efficacy of evaluating the number of CD138-positive cells in the endometrium in the diagnosis of CE. To the best of our knowledge, no study has reported on pathological evaluation before and after antimicrobial therapy. Currently, CE is diagnosed through hysteroscopy, pathological examination, bacteriological testing, or a combination of two of these methods, with many medical facilities generally using both hysteroscopy and pathological examinations, given their simplicity [8, 16, 21]. During hysteroscopy, “strawberry” aspect, focal hyperemia, hemorrhagic spots, diffuse micropolyps, and stromal edema have been reported as findings that predict CE [29]. Using hysteroscopy, the diagnosis can be made immediately upon examination. However, even in cases where hysteroscopy shows no abnormal findings, pathological examinations often reveal CE. A review by Fani et al. [8] stated that hysteroscopy was not suitable as a first choice for CE diagnosis because of the high risk of bias. Similarly, in our study, the findings of hysteroscopy were often viewed differently by each physician. The positive predictive value of CE through hysteroscopy to pathological examination was 0.886 (95% confidence interval 0.754–0.962), whereas the negative predictive value was only 0.222 (95% confidence interval 0.101–0.392) (data not shown). Therefore, it was considered that hysteroscopy should be in the category of adjunctive diagnosis, as it could not provide reproducible results. Meanwhile, CD138 immunostaining specific for plasma cells has been commonly used for pathological diagnosis of CE. A higher number of CD138-positive cells was associated with reduced subsequent fertility outcomes, and the number of CD138-positive cells was considered to be able to assess the disease status of CE [13, 14, 23]. Although pathological examination took longer than hysteroscopy, it had the advantage of greater objectivity and less bias [8]. Our study also suggested that the number of CD138-positive cells could be a predictor of pregnancy outcome through evaluating CD138-positive cells before and after treatment.

The standard treatment for CE comprises administration of antimicrobial agents, but no general regimen has been established to date. Kitaya et al. reported that 92.3% of patients with CE improved upon treatment with doxycycline (200 mg/day for 14 days), while the remaining patients received ciprofloxacin (400 mg/day for 14 days) and metronidazole (500 mg/day for 14 days), and reported an overall response rate of 99.1% [16]. We referred to this as the treatment protocol at our facility. In other studies, a combination of doxycycline, ciprofloxacin, and metronidazole [5] or ofloxacin and metronidazole [9] has been reported to be effective for the treatment of CE. Cicinelli et al. treated CE with a systematic antibiotic regimen according to the microbial profile of the endometrium [10]. As described above, various studies have reported on antimicrobial therapy for CE; however, previous reports have only determined whether CE was cured and have not discussed the reduction in the number of CD138-positive cells after antibiotic treatment. In this study, we found for the first time that the number of CD138-positive cells was significantly reduced after antimicrobial therapy, which can be useful as an indicator for evaluating the efficacy of antimicrobial treatment.

This study has several limitations. We could not evaluate the number of CD138-positive cells after second-line antimicrobial treatment pathologically. Therefore, the CE of some patients in the treatment group may not yet be cured. In such cases, further treatment of CE may need to be considered. Second, because this study was a retrospective study, and all patients who had CE were treated, direct comparisons between untreated and treated cases were not possible. Further studies with prospective designs are needed to evaluate the accurate effect of CE treatment. However, the usefulness of the pathological examination of CE and the therapeutic effect were at least demonstrated through our study.

CE was found to be more common than that expected in patients planning for ART. The findings suggest that antimicrobial therapy in these patients may have benefited pregnancy outcomes. Further, the diagnosis and treatment of CE should be considered not only for patients with RIF but also for all patients planning to undergo ART like as screening test. In addition, the evaluation of CD138-positive cell count was found to be objective as well as useful as an indicator of treatment. Therefore, pathological examination could be the preferred assessment for diagnosis of CE. Furthermore, many CE-positive patients are said to have an abnormal pattern in endometrial receptivity analysis (ERA) [30]. Taken together, early detection of CE is expected to be a novel key investigation in future ART treatment strategies. Further research is needed on these relationships, including the availability and efficacy of treatment of CE and endometrial receptivity. We plan to conduct these additional studies of early CE treatment and other adjunctive therapies.

In conclusion, for all patients planning to undergo ART, examination for diagnosis of CE should be considered, regardless of existence of RIF, and pathological CD138-positive cell counts could be useful as a method for diagnosis of CE and treatment decision-making. In addition, antimicrobial agents could be useful in the treatment of CE, contributing to better pregnancy outcomes.