Decoding the Endometriosis-Associated Infertility Microenvironment: A Review of FTIR and Raman Spectroscopic Insights into Follicular Fluid

<b>Background</b>: Endometriosis is a major cause of female infertility. It significantly impacts oocyte quality and embryonic development. The condition's pathophysiological mechanisms are multifactorial. However, they are believed to be reflected in the biochemical composition of follicular fluid (FF). FF is the immediate microenvironment of the developing oocyte hence its relevance. Conventional analytical methods provide only a limited view of this complex biofluid. This underlies the need for holistic profiling techniques. <b>Objective</b>: This narrative review synthesizes current knowledge on the potential of Fourier-Transform Infrared (FTIR) and Raman spectroscopy. The two are scrutinized as label-free, non-destructive tools for analyzing FF in the context of endometriosis. As such, the aim is to bridge the understanding of the disease's impact on the follicular niche with the analytical power of these spectroscopic techniques, ultimately highlighting a critical research gap, while critically evaluating the translational pathway required to bring these techniques from research laboratories into routine clinical IVF practice. This includes assessment of practical feasibility, cost-effectiveness, turnaround time, standardization requirements, and comparison with existing clinical biomarkers. <b>Methods</b>: We outline the fundamental principles of FTIR and Raman spectroscopy and their complementary strengths. The review then consolidates evidence from proteomic and metabolomic studies demonstrating FF alterations in endometriosis. We also showcase the successful application of vibrational spectroscopy in other reproductive diagnostics. This synthesis is vital to identifying a specific unmet need in the field. <b>Conclusions</b>: Despite the known importance of FF and the proven capability of FTIR and Raman spectroscopy in related areas, there is a striking lack of studies applying these techniques directly to the FF of women with endometriosis. This review concludes by framing this void as a pivotal research opportunity. In doing so, it presents a direct rationale and methodological framework for a future study designed to characterize the unique spectral fingerprints of endometriosis in FF, with the goal of uncovering novel biomarkers and pathophysiological insights.