FIB-4 Index (Fibrosis-4)
The Fibrosis-4 (FIB-4) Index is a validated, non-invasive score that estimates the likelihood of advanced liver fibrosis from four routine inputs: age, AST, ALT and platelet count (FIB-4 = [age x AST] / [platelets x square-root of ALT]). It is the guideline-recommended first-line tool to risk-stratify metabolic dysfunction-associated steatotic liver disease (MASLD), which is highly prevalent in metabolically at-risk adults.
What It Measures
A calculated estimate of advanced hepatic fibrosis risk derived from age, AST, ALT and platelet count.
Optimal vs. Lab Range
Target range for optimal health and longevity based on research.
Population-based reference range from standard labs.
Why It Matters
MASLD (formerly NAFLD) is common in insulin-resistant and overweight adults and is a major, often silent contributor to liver-related and cardiometabolic risk. The 2023 AASLD Practice Guidance recommends FIB-4 as the first-line, non-invasive test to identify who needs further fibrosis assessment. FIB-4 is inexpensive, uses labs already drawn in a longevity panel, and reliably rules out advanced fibrosis at low values: a threshold <1.3 is a reliable, pragmatic rule-out for advanced (stage >=F3) fibrosis, while >=2.67 identifies higher risk warranting secondary testing (e.g., elastography/VCTE or ELF). FIB-4 is less accurate under age 35, and for adults aged >=65 a higher rule-out threshold (~2.0) is used. It is a triage tool, not a diagnosis of fibrosis.
When Low
FIB-4 <1.3 (or <2.0 if age >=65): low probability of advanced fibrosis; routine monitoring and metabolic optimization.
When Optimal
Low FIB-4 (<1.3; <2.0 if aged >=65) reliably rules out advanced fibrosis in most people.
When High
FIB-4 >=2.67 indicates higher risk of advanced fibrosis and warrants secondary non-invasive testing (elastography/VCTE or ELF) and specialist evaluation; 1.3-2.67 is indeterminate and prompts additional assessment.
How to Optimize
FIB-4 improves when the metabolic drivers of fatty liver are addressed: weight loss (notably >=7-10% of body weight), reducing refined carbohydrate, sugar and alcohol intake, regular exercise, and optimizing glucose and lipids; GLP-1/incretin and other therapies are considered with a physician where indicated. Because FIB-4 depends on AST, ALT and platelets, interpret it alongside those underlying labs and over time. This content is educational and not a substitute for medical advice.
Key Interventions & Linked Compounds
Strong evidenceKey Interventions
Curated from clinical literature. Individual results vary; consult a qualified clinician before changing a protocol.
Ordering Notes
Specimen
Calculated from age, AST, ALT and platelet count (routine labs)
Patient prep
No fasting required. Maintain usual diet and medications unless advised.
Recommended cadence
Annually in metabolically at-risk adults; sooner if elevated or with changing metabolic status
Reporting unit
index (unitless)
Pre-analytic notes
Standardize draw time of day, hydration, and recent exercise. Note any acute illness, supplements, or hormonal therapies on the requisition.
Recommended Follow-Up Actions
- 1
Confirm the result is reliable
Repeat abnormal values before acting — preferably from the same lab, same time of day, and under consistent prep. Rule out acute illness, recent intense exercise, or medication effects that can shift FIB-4 Index (Fibrosis-4).
- 2
Compare against optimal, not just lab range
Use the optimal window above as the target. Lab "normal" is built from the general population and often misses early dysfunction.
- 3
Pair with related markers
Order a complementary panel covering metabolic, inflammatory, and hormonal context to interpret this marker properly.
- 4
Discuss interventions with a clinician
Use the interpretation guidance above to frame the conversation. Bring trends, not just one datapoint, to your visit.
- 5
Re-test on a defined cadence
If a change is implemented, re-measure in 8–12 weeks for fast-moving markers, or 3–6 months for slower ones, to confirm response before escalating.
Testing Information
Recommended Frequency
Annually in metabolically at-risk adults; sooner if elevated or with changing metabolic status
Fasting Requirement
No fasting required
Available Test Types
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