Prevalence of Small Intestinal Bacterial Overgrowth (SIBO) in Women With Endometriosis

Endometriosis is a common inflammatory disease affecting approximately 10% of all premenopausal women. It is characterized by the presence of ectopic endometrial tissue at extrauterine sites. Endometriosis causes severe pain episodes and may lead to infertility.

There are different types of endometriosis, including superficial peritoneal endometriosis, ovarian endometriosis, and deep infiltrating endometriosis (DIE). Superficial peritoneal endometriosis refers to lesions located on the peritoneal surface and is considered the most common subtype. Ovarian endometriosis is associated with cystic lesions within the ovaries, commonly referred to as endometriomas. Deep infiltrating endometriosis (DIE) describes lesions penetrating more than 5 mm beneath the peritoneal surface and often affecting surrounding structures and organs.

Endometriosis not only causes severe cyclical and chronic pain but also affects quality of life across nearly all dimensions of daily living. Many affected individuals additionally suffer from fatigue, infertility, psychological distress, and pronounced gastrointestinal symptoms. These are often nonspecific and may include digestive complaints, abdominal cramping to severe abdominal pain, bloating, and alternating diarrhea and constipation. The underlying cause of these symptoms often remains unclear, which means they are frequently inadequately treated or not treated at all.

These factors highlight the urgent need for a better understanding of the disease mechanisms-particularly the frequently observed gastrointestinal manifestations.

SIBO (Small Intestinal Bacterial Overgrowth) is a condition characterized by gut dysbiosis, with an increased number and/or presence of colonic bacteria in the small intestine. Typical symptoms include bloating, diarrhea and gas formation, which may overlap with other gastrointestinal diseases. The intestinal micro¬biome compromises a dynamic, highly complex ecosystem with a very high diversity. The bacterial density varies greatly along the gastrointestinal tract. While the colon can contain up to 1012 bacteria per gram of stool, the proxi¬mal small intestine near the stomach is almost sterile. The sterility is due to antimicrobial mechanisms such as gastric acid, pancreatic enzymes, bile and defensins. The microbiome has proven role in numerous diseases, including inflammatory bowel diseases as well as Crohn's disease and Ulcerative Colitis, metabolic diseases and neuropsychiatric disorders. The above mentioned sterility is disturbed in patients with SIBO. Typical symptoms of SIBO include bloating, diarrhea and gas formation, which may overlap with other gastrointestinal diseases. To the best of the authors knowledge, only one case-control study has investigated the correlation between these two conditions (Philippe Halfon et al., International Journal of Gynecology and Obstetrics, 2025). This study found that 91.9% of women with endometriosis who reported digestive symptoms tested positive in a lactulose breath test (BT) for SIBO, Intestinal Methanogen Overgrowth (IMO), or both. Although research directly linking endometriosis and SIBO is scarce, there are studies exploring associations between other dysbiotic diseases and endometriosis. A Danish cohort study, for example, reported a 50% increase in the risk of IBD in women with endometriosis compared to controls (Francesca Chiaffarino et al., European Journal of Obstetrics and Gynecology and Reproductive Biology, 2020). Furthermore, a meta-analysis reported that approximately half of the patients diagnosed with SIBO through a lactulose breath test, and about one fifth of those diagnosed using a glucose breath test were also diagnosed with IBD (Noemi Salmeri et al., Microorganisms, 2023). These findings highlight the importance of further investigating a potential correlation between SIBO and endometriosis, particularly since the majority of women with endometriosis report significant gastrointestinal symp¬toms that overlap with those described by SIBO patients.

The gold standard of reliable diagnostic methods for both, endometriosis and SIBO remain invasive. Regarding SIBO, non-invasive tests as the lactulose breath test are now being used but have not yet replaced the invasive diagnostic methods. Concerning Endometriosis, a paradigm shift has taken place in the diagnostic approach in recent years. This development has been driven by improved non-invasive diagnostic accuracy, particularly through expert transvaginal ultrasound, as well as by the high clinical suggestiveness of a carefully taken medical history. In addition, the invasive nature of surgery-especially in young women-has contributed to a more conserva-tive diagnostic strategy. According to current guidelines, empirical treatment may already be initiated on the basis of a clinical suspicion of endometriosis, and diagnostic laparoscopy is no longer recommended solely for the purpose of establishing the diagnosis. However, for research purposes and for a definitive confirmation of the diagnosis, histological verification obtained during surgery remains the gold standard.

A novel non-invasive testing of endometriosis has recently been proposed to be performed with a reliable saliva test (Ziwig Endotest®). A saliva sample is used to extract salivary microRNA (miRNA) to identify phenotypic profiles characteristic of endometriosis, identified by NGS and modeled by AI. MiRNAs are small, non coding RNA molecules involved in the regulation of gene expression and are known to reflect disease-specific molecular patterns. In patients with endometriosis, specific miRNAs have been identi-fied that allow to differentiate between affected and non-affected individuals.

The diagnostic performance of salivary microRNA profiling for endometriosis has been investigated in several consecutive studies. An initial prospective proof-of-concept study first demonstrated the potential of a saliva-based microRNA signature to discriminate between patients with and without endo-metriosis, reporting a sensitivity of 96.7% and a specificity of 100% (Bendi-fallah et al., J Clin Med, 2022). This was followed by a prospective external validation study, in which the test showed a sensitivity of 96.2% and a speci-ficity of 95.1% (Bendifallah et al., NEJM Evidence, 2024). More recently, find-ings from a large prospective multicenter cohort study further supported these results, with a reported sensitivity of 97.3% and specificity of 94.1% (Bendi¬fallah et al., NEJM Evidence, 2025).

In the proposed study, we aim to analyze the prevalence of SIBO in patients with a confirmed diagnosis of endometriosis-either histologically verified or identified through a positive Ziwig Endotest®. SIBO testing will be conducted at the clinic for gastroenterology, Bern using standardized diagnostic proce-dures, including the lactulose breath test, gastroduodenoscopy with collection of luminal aspirate, mucus and mucosal biopsies, and complementary stool analyses. The diagnostic gold standard for SIBO is the microbiological analysis of small intestinal luminal aspirate. SIBO is considered confirmed when bacterial concentrations exceeding 103. As an additional non-invasive diagnostic method, a hydrogen (H2) breath test using lactulose may be performed. Although this method has a relatively limited diagnostic accuracy (reported sensitivity around 42%, specificity around 71%), it represents a useful complementary tool for the detection of SIBO, particularly due to its non-invasive nature (Losurdo et al., Journal of Neurogastroenterology and Motility. 2020.) Prior to the test, the patient must fast for at least 12 hours. After a baseline breath sample will be collected, the patient will drink a lactu¬lose solution. Then, breath samples will be collected at regular intervals and analyzed for hydrogen and methane concentrations, which indicate bacterial fermentation occurring in the small intestine. Diagnostic criteria used to indicate SIBO by Lactulose-breath test is an increase in hydrogen (H2) of > 20 ppm above baseline within 90 minutes after ingestion (or methan levels > 10 ppm at any time during the test).

The objective of the study is to assess the prevalence of SIBO within this patient population and to systematically explore potential associations between the two conditions.