NAFLD (Non-Alcoholic Fatty Liver Disease) Activity Score

NAS Calculator

Steatosis

Lobular Inflammation (foci per 200× field)

Hepatocellular Ballooning

NAS Score: –

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Diagnoses steatohepatitis, based on histology, in NAFLD patients.

Why Use

Grades the activity of MASH, which has been used in clinical trials to monitor treatment response. Provides a standardized scoring system to quantify key histological features of MASH, facilitating communication and consistency in research and clinical practice.

When to Use

Use to evaluate histological severity of MASLD in biopsy samples.

Formula

Addition of the selected points: Variable Points Steatosis (grade): low- to medium-power evaluation of parenchymal involvement by steatosis <5% 0 5-33% 1 34-66% 2 >66% 3 Inflammation (lobular): overall assessment of all inflammatory foci No foci 0 1 focus per 200× field 1 2‐4 foci per 200× field 2 >4 foci per 200× field 3 Liver cell injury (ballooning) None 0 Few balloon cells 1 Many cells/prominent ballooning 2

Pearls / Pitfalls

Created to measure changes in MASLD during clinical trials. Does not include fibrosis staging. Should not be used as a sole diagnostic tool, as threshold values do not always correlate with a definitive diagnosis of MASH. Interobserver variability may impact scoring.

Management

All patients with MASLD or MASH should be: Counseled on weight loss (e.g., diet, exercise). Provided appropriate weight loss treatments, if applicable (e.g., medication, bariatric surgery). Assessed and treated for comorbidities and cardiovascular risk factors, including hyperlipidemia, hypertension, and diabetes. Advised to avoid heavy alcohol consumption and smoking. Vaccinated according to CDC guidance (e.g., hepatitis A and B, influenza, pneumonia). Monitored through liver function tests, CBC, and coagulation studies. For patients with MASH: Vitamin E may be appropriate in nondiabetic patients (avoid in individuals with a personal or strong family history of prostate cancer). Pioglitazone and GLP-1 receptor agonists may be beneficial for patients with diabetes who cannot tolerate metformin or require additional glucose-lowering therapy. Regular monitoring for fibrosis is recommended. If advanced fibrosis is present, consider screening for hepatocellular carcinoma (HCC). For more detailed management recommendations for MASLD and MASH, see your local medical society clinical guidelines, such as those from the AASLD , AACE/AASLD , and AGA .

Advice

Results are not typically used to make a diagnosis, but rather grade the activity of MASH. Changes in score may reflect progression or regression of the disease. Based on the derivation study: Scores of 0–2: Occurred in cases largely considered not diagnostic of MASH. Scores of 3–4: Were evenly divided among cases considered not diagnostic, borderline, or diagnostic for MASH. Scores ≥5: Occurred in cases largely considered diagnostic of MASH. Patients with biopsy-proven MASH should undergo an assessment for fibrosis, such as elastography or serum fibrosis markers.

More Information

Interpretation: NAFLD Activity Score Activity* 0 No activity 1–2 Mild activity 3–5 Moderate activity 6–8 Marked activity *Note: the score correlates with steatohepatitis diagnosis, but should not be used to definitively diagnose MASH.