Diagnostic and prognostic performance of the fibrotic nonalcoholic steatohepatitis index in patients with metabolic dysfunction-associated steatotic liver disease

Background and aims Identifying patients with fibrotic metabolic dysfunction-associated steatohepatitis (MASH) is crucial in order to refer them to specialist care as fibrotic MASH represents one of the major inclusion criteria for clinical trials. This study evaluated the diagnostic and prognostic value of the fibrotic nonalcoholic steatohepatitis index (FNI) in the representative US population. Methods The FNI was assessed in National Health and Nutrition Examination Survey (NHANES) 2017–2020 (cross-sectional; n = 6520) for fibrotic MASH detection, using FibroScan-AST score-defined cases. Prognostic value was examined in NHANES III (1988–1994; n = 6166) with mortality follow-up through 2019. Receiver operating characteristic analysis evaluated diagnostic accuracy. Cox models assessed associations between FNI and all-cause, cardiovascular, and diabetes-related mortality. Correlations with the triglyceride-glucose (TyG) index and cardiometabolic risk burden were analyzed. Results FNI showed excellent diagnostic accuracy for fibrotic MASH (area under the curve AUC = 0.937), outperforming FIB-4 (AUC = 0.697). In NHANES III, elevated FNI (≥0.33) was independently associated with higher risk of all-cause hazard ratio = 3.19; 95% confidence interval (CI): 2.56–3.97, cardiovascular (hazard ratio = 6.43; 95% CI: 3.32–12.47), and diabetes-related mortality (hazard ratio = 25.15; 95% CI: 10.56–59.89). FNI correlated positively with TyG ( R = 0.458, P < 0.001) and increased progressively with the number of cardiometabolic risk factors. Conclusion FNI is a robust, noninvasive marker that identifies individuals with fibrotic MASH and stratifies mortality risk. Its integration into clinical and public health practice may improve early detection and risk-guided management of metabolic liver disease.