MAGENT

NAS score: the NAFLD activity score from liver biopsy (Kleiner 2005)

The NAS score, or NAFLD activity score, adds three grades read from a liver biopsy in nonalcoholic fatty liver disease: steatosis (fat), lobular inflammation and hepatocellular ballooning. The total runs from 0 to 8. This calculator adds the grades the pathologist has assigned and returns the total with each part.

Not a medical device. Not a diagnosis or dosing recommendation.

Use this calculator via API

What is the NAFLD activity score?

Kleiner and colleagues set out a histological scoring system for nonalcoholic fatty liver disease. The NAS is the part of it that measures disease activity. Fibrosis is staged separately and isn't part of the NAS.

Magent doesn't read slides. Every grade is supplied by the caller.

The NAS formula

NAS = steatosis (0-3) + lobular inflammation (0-3) + ballooning (0-2)

total 0 to 8

Exact expression the calculator uses:

NAS = steatosis (integer 0-3) + lobular_inflammation (integer 0-3) + ballooning (integer 0-2); Kleiner 2005 caller-assigned; max 8; fibrosis is not in NAS; no NASH diagnostic bands

Inputs

InputGradePaper's description
steatosis0under 5% of cells
15% to 33%
2over 33% to 66%
3over 66%
lobular_inflammation0no foci
1under 2 foci per 200× field
22 to 4 foci per 200× field
3over 4 foci per 200× field
ballooning0none
1few balloon cells
2many or prominent

How the calculation works

  1. Take the three grades as assigned.
  2. Add them. The maximum is 3 + 3 + 2 = 8.

The calculator returns the total and the three parts. It doesn't attach a NASH label to any total.

Worked examples

These use Magent's own expected values for the calculator.

Example 1: every grade at its highest

  1. Steatosis 3, lobular inflammation 3, ballooning 2.
  2. 3 + 3 + 2 = 8.
  3. NAS 8.

Example 2: no steatosis, but inflammation and ballooning at their highest

  1. Steatosis 0, lobular inflammation 3, ballooning 2.
  2. 0 + 3 + 2 = 5.
  3. NAS 5.

Example 3: steatosis grade 2 alone

  1. Steatosis 2, lobular inflammation 0, ballooning 0.
  2. 2 + 0 + 0 = 2.
  3. NAS 2.

History of the NAS

Kleiner and colleagues published the scoring system in Hepatology in 2005. The three components and their grades come from that paper.

When it's used

When a liver biopsy in fatty liver disease is graded and a single activity total is wanted, often in research.

Where it's used

Pathology departments, hepatology clinics and clinical trials.

Who uses it

Pathologists, hepatologists and researchers. The calculator adds the grades.

Limits of the calculation

  • It depends on a biopsy and on the pathologist's grading, which the calculator can't check.
  • A biopsy samples a small piece of liver.
  • The NAS leaves out fibrosis.
  • The calculator doesn't label any total as NASH or not NASH.
  • This page doesn't say what a score means for a particular person.

Clinical decisions belong to a qualified clinician who knows the patient.

  • FIB-4 estimates liver fibrosis from age, AST, ALT and platelets, with no biopsy.

Glossary

  • Steatosis: fat in liver cells.
  • Lobular inflammation: clusters of inflammatory cells within the liver lobule.
  • Ballooning: swollen, injured liver cells.
  • Fibrosis: scarring, staged separately from the NAS.

Common mistakes

  • Adding the fibrosis stage. It isn't part of the NAS.
  • Grading ballooning 0 to 3. Ballooning goes to 2.
  • Reading a total as NASH. The calculator doesn't do that.

Source

Kleiner DE, et al. Design and validation of a histological scoring system for nonalcoholic fatty liver disease. Hepatology. 2005;41(6):1313-1321. DOI: 10.1002/hep.20701. PMID: 15915461

Frequently asked questions

How is the NAS score calculated?

Add the steatosis grade (0 to 3), the lobular inflammation grade (0 to 3) and the ballooning grade (0 to 2).

What is the maximum NAS?

8.

Is fibrosis part of the NAS?

No. It's staged separately.

Does the calculator say whether a biopsy shows NASH?

No. It returns the total and its parts.