NAFLD Fibrosis Score

The NAFLD Fibrosis Score (NFS) Calculator is a validated non-invasive clinical prediction model specifically developed for assessing the probability of advanced fibrosis in patients with non-alcoholic fatty liver disease (NAFLD). Developed by Angulo et al. in 2007 using data from 733 biopsy-confirmed NAFLD patients, the NFS uses six readily available clinical and laboratory variables: age, BMI, diabetes status, AST/ALT ratio, platelet count, and albumin. It remains one of the most widely recommended initial screening tools for identifying NAFLD patients with advanced fibrosis who need specialist referral.

The formula is: NFS = -1.675 + 0.037 x Age + 0.094 x BMI + 1.13 x IFG/Diabetes (1 if yes, 0 if no) + 0.99 x AST/ALT ratio - 0.013 x Platelet count - 0.66 x Albumin. Two validated cutoff values create three categories: a low cutoff of -1.455 and a high cutoff of 0.676. Scores below -1.455 indicate low probability of advanced fibrosis with a negative predictive value of approximately 93%. Scores above 0.676 indicate high probability with a positive predictive value of approximately 90%. Intermediate values require further investigation.

The NAFLD Fibrosis Score addresses a critical clinical need. NAFLD affects approximately 25% of the global adult population, and while most patients have simple steatosis with benign prognosis, approximately 20% develop non-alcoholic steatohepatitis (NASH), and 10-20% of NASH patients progress to advanced fibrosis or cirrhosis. Identifying the subset with advanced fibrosis is essential because these patients face substantially increased risks of liver-related complications, hepatocellular carcinoma, cardiovascular events, and overall mortality.

Major hepatology guidelines from AASLD, EASL, and AGA recommend the NFS alongside FIB-4 as first-line non-invasive tests for NAFLD fibrosis screening. The recommended clinical pathway begins with calculating FIB-4 or NFS in primary care. Patients with low scores can be managed in primary care with lifestyle interventions and metabolic risk factor management. Those with high scores should be referred to hepatology. Indeterminate scores warrant further evaluation with liver stiffness measurement (FibroScan or shear wave elastography) before deciding on referral.

The NFS incorporates metabolic factors (BMI, diabetes) that are particularly relevant to NAFLD pathophysiology. Type 2 diabetes is a strong independent predictor of fibrosis progression in NAFLD, likely related to insulin resistance-driven lipotoxicity, inflammation, and stellate cell activation. Higher BMI correlates with greater steatosis burden and inflammatory stimulus. The AST/ALT ratio increases with fibrosis progression as ALT clearance decreases. Thrombocytopenia and hypoalbuminemia reflect advancing cirrhosis.

Limitations include the large indeterminate zone (approximately 25-30% of patients), reduced accuracy in morbidly obese patients and in patients over 65, and inability to reliably distinguish intermediate fibrosis stages (F1 from F2). The NFS also cannot assess steatosis grade or inflammation activity, which may be important for treatment decisions in clinical trials targeting NASH. Despite these limitations, the NFS has been shown to predict long-term liver-related events and overall mortality, validating its clinical utility beyond simple fibrosis staging.