Plasma fibrinogen levels in women with infertility undergoing surgical evaluation for endometriosis: a retrospective cross-sectional study

Background Endometriosis is a chronic inflammatory disease linked to infertility. Coagulation-related processes may contribute to lesion persistence and pelvic remodeling, but the relevance of coagulation parameters in women with infertility undergoing surgical evaluation remains unclear. This study examined the association of plasma fibrinogen (FIB) with the presence and stage of endometriosis. Methods This retrospective cross-sectional study included 389 women with infertility undergoing combined hysteroscopy and laparoscopy. Endometriosis was surgically confirmed and staged using the revised American Society for Reproductive Medicine classification. Plasma FIB was analyzed continuously and by quartiles. Multivariable logistic regression estimated odds ratios (ORs) and 95% confidence intervals (CIs) for overall and stage-specific endometriosis after adjustment for age, body mass index, education level, dysmenorrhea, and parity. Spline analyses explored potential nonlinearity. Activated partial thromboplastin time (APTT), prothrombin time (PT), thrombin time (TT), and international normalized ratio (INR) were exploratory comparators. Sensitivity analyses considered reproductive, inflammatory, and coagulation-related variables. Results Among 389 women, 259 had endometriosis, including 186 with stage I–II and 73 with stage III–IV disease. Continuous FIB was not significantly associated with overall endometriosis (OR, 1.20; 95% CI, 0.76–1.91; P = 0.433). Compared with the first quartile, the third FIB quartile was associated with higher odds of overall endometriosis (OR, 2.37; 95% CI, 1.20–4.66; P = 0.012), with the largest estimate for stage III–IV disease (OR, 4.03; 95% CI, 1.56–10.43; P = 0.004). Q4 did not show a further increase, and trend tests were not significant. The adjusted spline increased across the lower-to-middle FIB range and then plateaued. The overall Q3 estimate remained elevated across sensitivity analyses, including the exploratory expanded model (OR, 2.27; 95% CI, 1.09–4.75). APTT, PT, TT, and INR showed no consistent associations. Conclusions Plasma FIB did not show a simple linear association with endometriosis. A higher estimate was observed in the intermediate FIB range and persisted across several adjusted analyses, particularly in stage III–IV disease. This quartile-specific pattern remains exploratory and does not define a clinical cutoff. Independent cohorts are needed to determine whether this pattern is reproducible and clinically informative.