Design: retrospective case-control study.

Setting: tertiary referral center for endometriosis.

Patients or Participants: women that were hospitalized due to PID between June January 1^st 2021 to September 30^st 2023.

Interventions: Patients with endometriosis were compared to patients without endometriosis. Diagnosis of endometriosis was suspected at the time of admission and confirmed with a subsequent ultrasound by an expert sonographer.

Measurements and Main Results: a total of 132 patients aged between 18-50 were included. 35 women (26.5%) were diagnosed with endometriosis. Mean age in the endometriosis group was 37.2 (SD + 7.4) vs 31.9 (SD + 9.1) in the no endometriosis group. Women with endometriosis were more likely to experience a severe and complicated course of PID with longer duration of hospitalization (6.1 + 4.3 vs 4.3 +2.4 days, p<0.05) and higher rate of antibiotic treatment failure (12/35 (34.2%) vs 18/97 (18.5%)) compared to patients without endometriosis. Readmission rate was higher in the endometriosis group (28.5% vs 8.1%, p<0.05) and they required more frequently surgical intervention (11.4% vs 6.1%). However, it did not reach statistical significance. A positive cervical or vaginal bacterial culture or polymerase chain reaction (PCR) for sexually transmitted disease (C. trachomatis, Mycoplasma or N. gonorrhoeae) was found to be significantly higher (34.9% vs 2.9%, p<0.05) in patients without endometriosis.

Conclusion: according to our results, women with endometriosis are at increased risk for a severe course of PID compared with non-endometriosis patients, requiring more often longer hospitalization, adjustments in antibiotic treatment and urgent surgical intervention. Our results also suggest that PID in women with endometriosis is less likely to be caused by a sexually transmitted disease bacteria, thus, suggesting a different mechanism of pathogenesis.