Percutaneous Liver Biopsy & Slide Preparation
A thin needle core of liver tissue is fixed, sectioned, and stained for review.
- 1.5–2.5 cm: Standard core length (percutaneous needle sample)
- ≥ 11: Portal tracts needed (for adequate grading)
- 2005: NAS system published (Kleiner et al., NASH CRN)
- 0–8: NAS score range (sum of three components)
Percutaneous liver biopsy
A thin needle core extracts a liver tissue sample.
Histology slide preparation
Sample is fixed, sectioned thin, and H&E stained.
Microscopic field examination
Pathologist scans multiple fields at 100–200x magnification.
Steatosis Grading — Fat-Laden Hepatocytes
Pathologists estimate the percentage of hepatocytes containing macrovesicular fat.
- < 5%: Grade 0 (no significant steatosis)
- 5–33%: Grade 1 (mild fat accumulation)
- 34–66%: Grade 2 (moderate fat accumulation)
- > 66%: Grade 3 (severe, panlobular fat)
Macrovesicular fat droplets
A single large lipid vacuole displaces the nucleus.
Field-based percentage estimate
Pathologist visually estimates the affected hepatocyte percentage.
Zonal distribution pattern
Fat typically starts in perivenular zone three.
Lobular Inflammation Grading — Inflammatory Foci
Clusters of immune cells scattered through the lobule signal active injury.
- 0 foci: Score 0 (no inflammatory foci)
- < 2 foci: Score 1 (per 200x field)
- 2–4 foci: Score 2 (per 200x field)
- > 4 foci: Score 3 (per 200x field)
Inflammatory foci definition
Clusters of immune cells form within the liver lobule.
Mixed cell infiltrate
Lymphocytes, macrophages, and neutrophils cluster together.
Field counting method
Foci are counted across several fields, then averaged.
Hepatocyte Ballooning — Cytoskeletal Collapse
Enlarged, cleared hepatocytes mark the strongest single predictor of MASH.
- None: Score 0 (no ballooned cells)
- Few: Score 1 (occasional ballooned hepatocytes)
- Many: Score 2 (numerous ballooned hepatocytes)
- CK18 loss: Key marker (keratin cytoskeleton collapse)
Ballooning morphology
Enlarged hepatocytes show clumped, cleared cytoplasm.
Cytoskeletal collapse
Keratin 8/18 filaments collapse, causing cell swelling.
Mallory-Denk bodies
Some ballooned cells contain eosinophilic protein clumps.
NAS Composite Score — Summing the Components
Steatosis, inflammation, and ballooning combine into one composite NAS score.
- S + I + B: NAS formula (steatosis + inflammation + ballooning)
- Likely MASH: NAS ≥ 5 (active steatohepatitis)
- MASH unlikely: NAS < 3 (simple steatosis pattern)
- Borderline: NAS 3–4 (indeterminate zone)
Composite score calculation
Add all three component scores together.
Clinical interpretation thresholds
Higher NAS correlates with more active liver injury.
Fibrosis staged separately
Fibrosis is scored independently, outside the NAS.