NAFLD activity score (NAS)34
| Histological feature | Score | Category definition |
|---|---|---|
| Steatosis | 0 | <5% |
| 1 | 5–33% | |
| 2 | 34–66% | |
| 3 | >66% | |
| Plus | ||
| Hepatocyte ballooning | 0 | None |
| 1 | Few | |
| 2 | Many | |
| Plus | ||
| Inflammation | 0 | None |
| 1 | 1–2 foci per ×20 field | |
| 2 | 2–4 foci per ×20 field | |
| 3 | >4 foci per ×20 field | |
| NAS total 0–8 | ||
| Fibrosis | 0 | No fibrosis |
| 1a | Zone 3 mild perisinusoidal fibrosis | |
| 1b | Zone 3 moderate perisinusoidal fibrosis | |
| 1c | Periportal/portal fibrosis only | |
| 2 | Zone 3+periportal/portal fibrosis | |
| 3 | Bridging fibrosis | |
| 4 | Cirrhosis | |
| Fibrosis score 0–4 | ||
A score of ≥5 with steatosis and hepatocyte ballooning is generally considered diagnostic of non-alcoholic steatohepatitis (NASH), but patients can still have NASH with lower NAS scores if steatosis and hepatocyte ballooning are present.
NAFLD, non-alcoholic fatty liver disease.