US MASH is a staged disease — the commercial pool is not the 86.3M with fatty liver but the 6.7M with F2-F3 fibrosis, and most of them are unstaged.
MASH (metabolic dysfunction-associated steatohepatitis) is the progressive, inflammatory form of fatty liver disease in which fat accumulation drives hepatocyte injury and fibrosis, renamed from NASH under the 2023 multi-society MASLD/MASH consensus. It sits inside a wide funnel: 86.3 million US adults have MASLD (33.7%), 14.9 million have MASH (5.8%), and 6.7 million have MASH with clinically significant (F2-or-worse) fibrosis — the FDA label-eligible pool, projected to rise to 11.7 million by 2050 (JAMA Network Open, PMID 39821400). Fibrosis stage, not steatosis, is what tracks with liver-related outcomes, so the treatable population is defined by staging.
Fibrosis is graded on the NASH-CRN (Kleiner) system from F0 to F4. The treatable band is F2 (significant) to F3 (advanced, bridging) fibrosis without cirrhosis; compensated cirrhosis (F4) is excluded from both approved agents and is the segment the FGF21 pipeline is chasing. Clinically, the priority target is "at-risk MASH" — a MASLD activity score (NAS) ≥4 with fibrosis ≥F2, the phenotype with the highest progression risk (AASLD Practice Guidance).
The commercial constraint is diagnosis, not disease size. Most of the 6.7M eligible pool is undiagnosed or unstaged in primary care. The AASLD noninvasive pathway is a sequential filter: FIB-4 <1.3 rules out advanced fibrosis, >2.67 rules it in, and the 1.3-2.67 indeterminate band is referred to vibration-controlled elastography (VCTE / FibroScan ≈8 kPa for F2, ≈12 kPa for F3) or MR elastography. Where FIB-4 reflex to elastography does not happen, eligible patients never reach a labeled therapy — a funnel-leak that sits upstream of any THR-β-vs-GLP-1 efficacy debate.
MASH fibrosis staging — histology, position in the US funnel, and label eligibility
| Fibrosis stage (NASH-CRN) | Histologic definition | Position in US funnel | Label eligibility |
|---|---|---|---|
| F0 — No fibrosis | Steatohepatitis without scarring | Within the 14.9M MASH pool | Not label-eligible |
| F1 — Mild | Perisinusoidal or periportal fibrosis | Within the 14.9M MASH pool | Not label-eligible |
| F2 — Significant | Perisinusoidal and portal/periportal fibrosis | Part of the 6.7M F2-F3 pool | Rezdiffra + Wegovy labeled |
| F3 — Advanced | Bridging fibrosis | Part of the 6.7M F2-F3 pool | Rezdiffra + Wegovy labeled |
| F4 — Cirrhosis | Compensated cirrhosis | Beyond the label boundary | Excluded from both; FGF21 pipeline (efruxifermin, pegozafermin) target |
Sources: NASH-CRN (Kleiner) fibrosis staging; AASLD Practice Guidance on noninvasive assessment of MASLD; US burden — JAMA Network Open 2025 (PMID 39821400); FDA labels for Rezdiffra (resmetirom, NDA217785) and Wegovy (semaglutide 2.4 mg). At-risk MASH defined as NAS ≥4 with fibrosis ≥F2.
What this assessment answers
Every section answers a named commercial question your team is asking, scoped to your asset.
Delivers
- The US funnel: MASLD 86.3M → MASH 14.9M → F2-F3 6.7M, the diagnosed-vs-undiagnosed split, and the 11.7M-by-2050 projection (PMID 39821400)
Delivers
- The AASLD noninvasive pathway: FIB-4 thresholds (1.3 / 2.67), VCTE and MR elastography cutoffs, the at-risk MASH definition (NAS ≥4, fibrosis ≥F2), and where staging leaks
Delivers
- NASH-CRN F0-F4 staging mapped to label eligibility: the F2-F3 treatable band vs compensated cirrhosis (F4), the segment neither Rezdiffra nor Wegovy covers and the FGF21 pipeline is chasing
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Commission This AssessmentWhat's inside
- MASH was renamed from NASH under the 2023 multi-society MASLD/MASH consensus, redefining the disease as fat-driven hepatocyte injury and fibrosis
- The US funnel runs from 86.3 million adults with MASLD (33.7%) to 14.9 million with MASH (5.8%) to 6.7 million with clinically significant F2-or-worse fibrosis
- The 6.7 million F2-F3 label-eligible pool is projected to rise to 11.7 million by 2050 (JAMA Network Open, PMID 39821400)
- Fibrosis stage, not steatosis, tracks with liver-related outcomes, which is why the treatable population is defined by staging rather than raw MASLD prevalence
- Fibrosis is graded F0 to F4 on the NASH-CRN (Kleiner) system; the treatable band is F2 (significant) to F3 (advanced, bridging) without cirrhosis
- At-risk MASH, a MASLD activity score of 4 or higher with fibrosis of F2 or worse, is the priority clinical target with the highest progression risk
- FIB-4 below 1.3 rules out advanced fibrosis and above 2.67 rules it in, with the indeterminate 1.3–2.67 band referred to elastography
- Vibration-controlled elastography cutoffs of roughly 8 kPa for F2 and 12 kPa for F3, or MR elastography, resolve the FIB-4 indeterminate zone
- Most of the 6.7 million F2-F3 eligible pool is undiagnosed or unstaged in primary care, not yet identified by the noninvasive pathway
- Where FIB-4 reflex testing to elastography does not happen in primary care, eligible patients never reach a labeled therapy, a funnel leak upstream of any efficacy debate
- Compensated cirrhosis (F4) is excluded from both Rezdiffra and Wegovy's labels, drawing a hard boundary at noncirrhotic fibrosis
- The FGF21 pipeline, efruxifermin and pegozafermin, is chasing the compensated-cirrhosis segment that sits beyond the current label boundary
Included with every brief
How AXLRx builds this assessment
Prepared by MoatRx analysts.
MASH disease landscape is built from primary epidemiological sources, peer-reviewed clinical literature, society guidance, and FDA labels. Epidemiological estimates are triangulated across sources; all figures carry source citations.
Key sources: US burden model, JAMA Network Open 2025 (PMID 39821400); AASLD Practice Guidance on the noninvasive assessment of MASLD; NASH-CRN (Kleiner) fibrosis staging; MAESTRO-NASH (PMID 38324483) and ESSENCE (PMID 40305708) for the F2-F3 treatable-band evidence; FDA labels for Rezdiffra (NDA217785) and Wegovy.
- US burden — MASLD 86.3M adults (33.7%, 2020); MASH 14.9M (5.8%); MASH + ≥F2 fibrosis (label-eligible) 6.7M, rising to 11.7M by 2050 (JAMA Network Open, PMID 39821400).
- Label boundary — Rezdiffra (resmetirom, NDA217785) and Wegovy (semaglutide 2.4 mg) are both approved for noncirrhotic MASH with F2-F3 fibrosis; compensated cirrhosis (F4) is excluded from both labels (Drugs@FDA; FDA labels).
- At-risk MASH — defined as MASLD activity score (NAS) ≥4 with fibrosis stage ≥F2; these patients carry the highest progression risk (AASLD Practice Guidance).
- Noninvasive staging — AASLD pathway: FIB-4 <1.3 rules out advanced fibrosis, >2.67 rules it in, 1.3-2.67 is indeterminate and referred to VCTE (FibroScan ≈8 kPa for F2, ≈12 kPa for F3) or MR elastography (AASLD Practice Guidance).
- Fibrosis staging system — NASH-CRN (Kleiner) F0-F4; significant fibrosis (F2) and bridging fibrosis (F3) define the treatable band, cirrhosis (F4) the excluded stage (NASH Clinical Research Network staging).
- Staging evidence — resmetirom achieved ≥1-stage fibrosis improvement 24.2%/25.9% (80/100 mg) vs 14.2% placebo in MAESTRO-NASH (Harrison et al., NEJM 2024, PMID 38324483; NCT03900429).
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