NAFLD Fibrosis Score Calculator
The NAFLD Fibrosis Score (NFS) estimates the probability of advanced liver fibrosis (stages F3 or F4) in people with non-alcoholic fatty liver disease. Enter six readily available clinical values and the calculator returns your NFS with a low, intermediate, or high fibrosis probability classification, the full step-by-step calculation, and a plain-English interpretation based on the original 2007 validation study by Angulo et al.
Formula
Worked example
A 50-year-old with BMI 28 kg/m2, no diabetes, AST 45 IU/L, ALT 55 IU/L, platelets 200 x10^9/L, albumin 4.0 g/dL: NFS = -1.675 + (0.037x50) + (0.094x28) + (1.13x0) + (0.99x0.818) - (0.013x200) - (0.66x4.0) = -1.675 + 1.850 + 2.632 + 0 + 0.810 - 2.600 - 2.640 = -1.623, which falls below -1.455, indicating low probability of advanced fibrosis.
What is the NAFLD Fibrosis Score?
The NAFLD Fibrosis Score (NFS) is a non-invasive clinical tool developed to estimate the likelihood of advanced hepatic fibrosis in patients with non-alcoholic fatty liver disease (NAFLD). It was derived and validated by Angulo et al. in 2007 using data from 733 patients across multiple international centres. The score combines six variables available from routine blood tests and a clinical assessment: age, body mass index, the presence of impaired fasting glucose or diabetes, the ratio of AST to ALT, platelet count, and serum albumin. Because liver biopsy carries procedural risk and is expensive, the NFS provides a practical first-line screen to identify patients who are unlikely to have advanced fibrosis (and can therefore avoid biopsy) and those at high risk who need further workup.
How to interpret your score
Scores below -1.455 suggest low probability of advanced fibrosis (histological stages F0-F2) with a negative predictive value of 93%. This means that in 93 out of 100 people with a score in this range, biopsy would confirm the absence of advanced fibrosis. Scores above 0.676 suggest high probability of advanced fibrosis (stages F3-F4) with a positive predictive value of 90%. The zone between these two cutoffs is indeterminate, meaning the NFS alone cannot reliably rule fibrosis in or out, and additional testing such as FibroScan transient elastography, acoustic radiation force impulse imaging, or liver biopsy may be required. The NFS performs best in people with established NAFLD confirmed by imaging or histology; it should not be used as a standalone screening tool in the general population.
Understanding each component of the formula
Age is a positive predictor because fibrosis accumulates over time. BMI contributes positively because obesity drives hepatic steatosis and inflammation, which are precursors to fibrosis. The presence of impaired fasting glucose or type 2 diabetes adds the largest single positive increment (1.13) because insulin resistance is a central driver of NAFLD progression. The AST/ALT ratio rises as fibrosis advances: in early disease ALT typically exceeds AST, but as the liver loses functional mass and portal hypertension develops, AST rises relative to ALT, pushing the ratio above 1.0. Platelet count decreases with advancing fibrosis due to portal hypertension causing sequestration in an enlarged spleen (hypersplenism) and reduced thrombopoietin synthesis. Albumin falls as hepatic synthetic function declines in later-stage disease. Together, the six variables capture the metabolic and structural burden of NAFLD progression.
Limitations and when to use other tests
The NFS was validated in adults with biopsy-confirmed NAFLD and should not be applied to patients with other liver diseases such as viral hepatitis, alcoholic liver disease, or autoimmune hepatitis, where the component relationships differ. The score performs less well at distinguishing between individual intermediate stages (F1 vs F2 vs F3) and is designed only to separate no-advanced-fibrosis from advanced-fibrosis. External factors can shift individual variables independently of fibrosis: strenuous exercise raises AST, severe malnutrition lowers albumin, and immune thrombocytopenia reduces platelets for reasons unrelated to liver disease. In the indeterminate zone, the FIB-4 index (another non-invasive score using age, ALT, AST, and platelets) and transient elastography (FibroScan) are recommended as complementary second-line tests by the AASLD/AGA/ACG joint guidance. Liver biopsy remains the definitive reference standard when non-invasive tests are inconclusive or when management decisions depend on precise staging.
NAFLD Fibrosis Score interpretation
| NFS range | Interpretation | Fibrosis stage | Accuracy |
|---|---|---|---|
| Below -1.455 | Low probability of advanced fibrosis | F0-F2 | 93% NPV |
| -1.455 to 0.676 | Indeterminate - biopsy may be needed | F0-F4 unclear | N/A |
| Above 0.676 | High probability of advanced fibrosis | F3-F4 | 90% PPV |
Cutoffs from Angulo et al. (Hepatology, 2007). NPV = negative predictive value; PPV = positive predictive value.
Frequently asked questions
What does the NAFLD Fibrosis Score measure?
The NAFLD Fibrosis Score estimates the probability that a person with non-alcoholic fatty liver disease has advanced fibrosis (stage F3 or F4 on the METAVIR scale). It does not diagnose NAFLD itself, measure fat content, or detect inflammation. Its purpose is to separate patients unlikely to have significant scarring from those who may need biopsy or specialist referral.
What are the two cutoff values and what do they mean?
The lower cutoff is -1.455. Scores below this value carry a 93% negative predictive value for advanced fibrosis, meaning the result can be used to exclude significant fibrosis in most cases. The upper cutoff is 0.676. Scores above this value carry a 90% positive predictive value, meaning advanced fibrosis is likely and further evaluation is warranted. Scores between the two cutoffs fall in the indeterminate zone where the NFS alone cannot determine fibrosis status.
Can I use this score to diagnose NAFLD?
No. The NFS assumes that NAFLD has already been identified, typically through imaging findings of hepatic steatosis or prior liver biopsy. It answers the narrower question of whether significant liver scarring (advanced fibrosis) is present, not whether NAFLD exists at all. Diagnosis of NAFLD itself relies on imaging, lab patterns, exclusion of other causes, and sometimes biopsy.
Why does having diabetes raise the score?
Diabetes and impaired fasting glucose are coded as 1 in the NFS formula and contribute the largest single increment (1.13 points) because insulin resistance drives the progression from simple steatosis to steatohepatitis and ultimately fibrosis. Patients with NAFLD and diabetes are known to progress more rapidly and have a higher prevalence of advanced fibrosis compared to those without metabolic dysregulation.
Is the NAFLD Fibrosis Score the same as FIB-4?
No. FIB-4 is a different non-invasive score that uses age, ALT, AST, and platelet count in a different formula (age x AST / (platelets x square root of ALT)). Both scores estimate fibrosis probability in NAFLD, but they use different variable weightings. Published guidelines recommend using both together in sequential algorithms to improve accuracy and reduce the indeterminate rate. This calculator computes only the NFS; use our FIB-4 calculator for the companion score.
How accurate is the NAFLD Fibrosis Score?
In the original Angulo et al. validation cohort, the NFS had an AUROC of 0.82 for detecting advanced fibrosis. The low cutoff (-1.455) correctly excluded advanced fibrosis in 93% of patients below it (NPV 93%), and the high cutoff (0.676) correctly identified advanced fibrosis in 90% of patients above it (PPV 90%). About 25-30% of patients fall in the indeterminate zone where the score is inconclusive. Multiple subsequent studies have broadly confirmed these performance characteristics.
Sources
- Angulo P et al. (2007). The NAFLD fibrosis score: a noninvasive system that identifies liver fibrosis in patients with NAFLD. Hepatology, 45(4), 846-854.
- Chalasani N et al. (2018). The diagnosis and management of nonalcoholic fatty liver disease: Practice guidance from the American Association for the Study of Liver Diseases. Hepatology, 67(1), 328-357.