Nafld Fibrosis Score (Nfs) Calculator

What Is the NAFLD Fibrosis Score?

The NAFLD Fibrosis Score (NFS) is a validated, non-invasive clinical tool used to estimate the likelihood of advanced hepatic fibrosis (stages F3–F4) in patients diagnosed with non-alcoholic fatty liver disease (NAFLD). Developed by Angulo et al. and published in Hepatology (2007), the score uses six readily available laboratory and clinical parameters to stratify fibrosis risk without requiring a liver biopsy.

The NFS Formula

The NAFLD Fibrosis Score is calculated using the following equation:

NFS = −1.675 + (0.037 × Age) + (0.094 × BMI) + (1.13 × IFG/DM) + (0.99 × AST/ALT) − (0.013 × Platelets) − (0.66 × Albumin)

Variable Definitions

• Age: Patient age in years. Older age is independently associated with higher fibrosis stage in NAFLD populations.
• BMI: Body Mass Index in kg/m². Higher BMI reflects the metabolic burden linked to hepatic steatosis and fibrosis progression.
• IFG/DM (Impaired Fasting Glucose or Diabetes): Entered as 1 if the patient has impaired fasting glucose (fasting glucose ≥100 mg/dL) or type 2 diabetes mellitus; 0 otherwise. This binary variable carries the largest single coefficient in the formula (1.13), underscoring the central role of metabolic dysregulation in fibrosis progression.
• AST/ALT Ratio: The ratio of serum aspartate aminotransferase (U/L) to alanine aminotransferase (U/L). A rising AST/ALT ratio correlates with worsening fibrosis and declining hepatocyte mass and function.
• Platelet Count: Measured in ×10⁹/L (equivalent to ×10³/µL). Thrombocytopenia in advanced cirrhosis results from portal hypertension and splenic sequestration, making platelet count an important inverse predictor of fibrosis severity.
• Serum Albumin: Measured in g/dL. Decreasing albumin levels reflect declining synthetic function of the liver as fibrosis advances toward cirrhosis.

Interpreting the Score

The NFS produces a continuous numerical value. Published cutoff thresholds allow clinicians to stratify patients into three risk categories:

• NFS < −1.455: Low probability of advanced fibrosis (F0–F2). The negative predictive value (NPV) at this cutoff is approximately 93%, supporting safe deferral of liver biopsy in most patients.
• NFS > 0.676: High probability of advanced fibrosis (F3–F4). The positive predictive value (PPV) at this threshold is approximately 90%, supporting referral for hepatology evaluation or confirmatory testing such as transient elastography (FibroScan).
• NFS between −1.455 and 0.676: Indeterminate zone. Patients in this range require additional workup, which may include FibroScan, the Enhanced Liver Fibrosis (ELF) panel, or percutaneous liver biopsy.

Worked Example

Consider a 52-year-old patient with BMI 31.4 kg/m², type 2 diabetes (IFG/DM = 1), AST 54 U/L, ALT 38 U/L, platelet count 189 ×10⁹/L, and serum albumin 3.9 g/dL:

NFS = −1.675 + (0.037 × 52) + (0.094 × 31.4) + (1.13 × 1) + (0.99 × 54/38) − (0.013 × 189) − (0.66 × 3.9)

NFS = −1.675 + 1.924 + 2.952 + 1.130 + 1.407 − 2.457 − 2.574 = 0.707

This result exceeds the 0.676 upper cutoff, placing the patient in the high-probability advanced fibrosis category and warranting specialist referral and confirmatory testing.

Clinical Use Cases

• Primary care screening of patients with incidentally detected hepatic steatosis on abdominal ultrasound or CT imaging.
• Endocrinology and diabetology clinics, where NAFLD prevalence among type 2 diabetes patients can reach 70–80%.
• Serial monitoring to track fibrosis progression or regression in response to lifestyle interventions or pharmacotherapy.
• Pre-screening triage before ordering more expensive non-invasive tests (e.g., transient elastography) or invasive liver biopsy.

Methodology and Sources

The NFS was originally derived and validated in a multicenter cohort of 733 NAFLD patients, achieving an AUROC of 0.88 for detection of advanced fibrosis. The formula has since been replicated across diverse ethnic populations and metabolic phenotypes. For further reading, consult the PMC review examining NFS performance in lean NAFLD patients, the VA/DoD Clinical Practice Guideline for Type 2 Diabetes Mellitus — which recommends NFS as part of structured NAFLD risk assessment — and the University of Washington Hepatitis C Online clinical calculators resource for broader non-invasive fibrosis scoring context.

Limitations

The NFS performs less reliably in lean NAFLD patients (BMI <25 kg/m²), in patients with concurrent liver disease such as alcoholic hepatitis or viral hepatitis, and in populations with extreme values of any single input variable. Clinicians should interpret results alongside full clinical context and, when scores fall in the indeterminate range, proceed to advanced confirmatory testing rather than relying on the NFS alone.