This study was designed to explore matrix metalloproteinase-9 (MMP-9), neutrophil gelatinase-associated lipocalin (NGAL) levels and MMP-9/NGAL ratio in women with and without endometriosis diagnosed surgically and/or histopathologically. The correlation between biomarkers and the severity of the disease is analysed. The revised American Fertility Society classification system was used to determine the severity of endometriosis. Serum MMP-9 and Ca125, urine NGAL levels were measured in all participants. Serum MMP-9 levels were significantly higher in the study group (n = 60) compared to controls (n = 31) (15.0 pg/mL (6.0-143.0) vs. 12.0 (4.0-18.0), respectively; p=.002). MMP-9 levels were significantly higher in severe endometriosis compared to mild endometriosis subgroups (p<.001). No significant difference was found between NGAL levels in study and control groups (p>.05). The diagnostic value of MMP-9 and NGAL is not superior than CA-125 for endometriosis. Nevertheless, MMP-9 might be a potential predictive marker for advanced stage of the disease. Impact Statement What is already known on this subject? The gold standard diagnostic test for diagnosis of endometriosis is laparoscopy combined with histopathological confirmation of eutopic endometrial glands and/or stroma. Both invasiveness and possible accompanying complications limit the preference regarding the surgical approach. Among non-invasive markers none has been accepted as gold standard neither for diagnosis nor for determining the severity of the disease. MMPs are extracellular endopeptidases, which have a significant role in degradation and remodelling of extracellular matrix for cellular migration and invasion. Among these, MMP-9 has been shown to be higher in eutopic/ectopic endometrial tissue in women with endometriosis and has been suggested to have a role in pathogenesis of endometriosis by promoting invasion of the endometriotic lesions. NGAL is an acute phase protein, which is involved in a variety of physiological and pathophysiological processes. The molecule has also been revealed to correlate with endometriosis pathophysiology through the epithelial-mesenchymal transition process which is the basis for the onset of endometriosis. But also, NGAL which composes a complex with MMP-9 (MMP-9 and NGAL complex), has been shown to protect MMP-9 from autodegradation in vitro which might be a contributing factor for endometriosis pathophysiology. What the results of this study add? MMP-9 cut-off level for prediction of severe endometriosis is a novel finding obtained from this study with acceptable sensitivity and specificity. On the other hand, NGAL seems to have no significant value either for diagnosis of for determining severity of the disease. After all, MMP-9 might be an easy use acceptable biomarker for endometriosis but further studies on larger populations are needed. What the implications are of these findings for clinical practice and/or further research? MMP might be a potential non-invasive predictive marker for advanced stage disease.

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