FIB-4 Fibrosis Index
FIB-4 (Fibrosis-4 Index) is a non-invasive score that uses four simple values to estimate the likelihood of liver fibrosis — the scarring of tissue in NAFLD and other chronic liver diseases. Its key feature is that the thresholds depend on age: what is already worrying at 50 may still be normal at 70.
What FIB-4 is and why it matters
Fibrosis is the scarring of the liver: healthy tissue is gradually replaced by connective tissue, and the organ works less well. Fibrosis is confirmed by biopsy or elastography, but these aren't available everywhere and aren't always needed at the outset. FIB-4 (Fibrosis-4 Index) is a calculated index that uses routine blood tests and age to estimate how likely significant fibrosis is. The score was originally developed in patients co-infected with HIV/hepatitis C and validated against biopsy fibrosis stages (Ishak); today it is widely used in NAFLD.
NAFLD and liver fibrosis
Non-alcoholic (metabolic dysfunction–associated) fatty liver disease (NAFLD) is a chronic liver disease linked to metabolic dysfunction, in which macrovesicular steatosis is found in more than 5% of hepatocytes.
Liver fibrosis is the gradual replacement of healthy liver tissue by connective tissue, which disrupts the organ's structure and function. Where steatosis indices answer the question “is there fat,” FIB-4 answers the next one: “has the process turned into scarring.” The end stage of fibrosis is cirrhosis.
What's needed for the calculation
FIB-4 is calculated from just four values — one about you and three from routine lab tests:
- Age, years. It enters the calculation directly and, more importantly, determines the interpretation thresholds (see below).
- AST, U/L. A liver enzyme; it rises when liver cells are damaged.
- ALT, U/L. A second enzyme; together with AST it reflects how active the process in the liver is.
- Platelet count, ×10⁹/L. As fibrosis progresses their number usually falls — which is why they sit in the denominator of the index.
The index combines these values into a single number using a validated model. The calculator below performs the calculation with age-specific thresholds — sparing you manual-calculation errors and misinterpretation.
How to take the tests
ALT, AST and platelets are routine tests: the enzymes come from a blood chemistry panel, platelets from a complete (clinical) blood count.
ALT and AST
Take them on an empty stomach. The day before, avoid intense workouts and alcohol — they temporarily raise the enzymes and will distort the index.
Platelets
Take the value from the complete blood count in units of ×10⁹/L (this is usually the “platelet count” on the report).
FIB-4 Calculator
Enter your age, ALT, AST and platelet count — the calculator will compute the FIB-4 index, select the thresholds for your age, determine the zone and stage, and give recommendations. The calculation runs on the server.
The thresholds depend on age (Shah 2009 / McPherson 2017): the lower “rule-out” bound is 1.3 for ages 36–64 and 2.0 for 65+; then 2.67 and 3.25.
How to read the result
FIB-4 has no single universal threshold — they shift with age. Note the “1.3–2.0” row: what is already a grey zone at ages 36–64 is still considered normal for 65+.
| FIB-4 | Ages 36–64 | 65+ years |
|---|---|---|
| < 1.3 | Fibrosis ruled out | Fibrosis ruled out |
| 1.3 – 2.0 | Grey zone | Fibrosis ruled out |
| 2.0 – 2.67 | Grey zone | Grey zone |
| 2.67 – 3.25 | Probable advanced | Probable advanced |
| > 3.25 | Severe (F4–F6) | Severe (F4–F6) |
For ages under 36, FIB-4 is less accurate — instead of assigning a zone, the calculator will suggest an alternative assessment (elastography/ELF).
Advanced fibrosis is unlikely. Monitoring and correction of risk factors.
No definite conclusion is possible — further testing is needed (FibroScan/ELF).
METAVIR F3–F4: referral to a specialist and imaging confirmation.
Ishak F4–F6: high likelihood; confirmation and a management plan are essential.
What each zone means
Fibrosis ruled out
A low FIB-4 (Ishak F0–F1) rules out advanced fibrosis with high reliability. Recommendations: monitoring, correction of risk factors and recalculation as indicated — for example, every 1–2 years with metabolic risk factors.
Grey zone
An intermediate result: the index can neither confidently rule out nor confirm fibrosis. Further testing is required — FibroScan/VCTE (LSM, CAP) or ELF; if the results are discordant, NFS or Hepamet is considered.
Probable advanced fibrosis (2.67–3.25)
METAVIR F3–F4 — probable advanced fibrosis (McPherson, 2017): referral to a specialist is indicated, along with imaging confirmation (elastography ± MR elastography) and a management plan.
Severe fibrosis (> 3.25)
A high FIB-4 corresponds to Ishak F4–F6. Recommendations: confirm the stage with imaging, assess complications and cancer risks; if needed, use prognostic scores (MELD-Na, Child–Pugh, ALBI).
Why the thresholds depend on age
With age, enzymes and platelets naturally change, and the “raw” FIB-4 tends to overestimate risk in older people. So that the test doesn't send healthy people over 65 for unnecessary work-ups, the lower bound was raised from 1.3 to 2.0 for them. Conversely, under 36 the scale is less reliable, so FIB-4 is used with caution at a young age, and if in doubt, elastography or ELF is used straight away.
Our calculator selects the right thresholds automatically: you enter your age, and the scale and interpretation already account for your age group.
When FIB-4 is not suitable
- Acute hepatitis or a flare. A sharp spike in enzymes inflates the index — FIB-4 is not used during this period.
- Age under 36. Accuracy is lower; the result is interpreted with caution, and if in doubt — elastography or ELF.
- Conditions that alter platelets. Blood disorders, certain medications and pregnancy can shift the result.
- Borderline values. Numbers right at a threshold should be interpreted together with a physician and other data.
Its place in the section and other indices
FIB-4 is about fibrosis — that is, the next stage after fat accumulation. So it logically complements the steatosis indices: FLI, HSI and NAFLD-LFS assess whether there is fat in the liver, while FIB-4 assesses whether the process has turned into scarring. The “steatosis → fibrosis” pairing is convenient to track together, and the conclusion is always interpreted by a physician.
Limitations and important caveats
- It is not a diagnosis. FIB-4 estimates the likelihood of fibrosis, which is confirmed by elastography, ELF or biopsy.
- It is screening, not staging. The exact METAVIR/Ishak stage is established by imaging.
- It depends on test quality. Errors in ALT, AST, platelets or age shift the result.
- The grey zone is a common outcome. It is not an error but a signal that additional methods are needed.
- It does not replace an examination. The result is interpreted by a physician, taking your history and comorbidities into account.
Frequently asked questions
Why is my result interpreted differently from an acquaintance's with the same number?
Because FIB-4 thresholds depend on age. The same value, for example 1.6, falls into the grey zone at 50, but into the “fibrosis ruled out” zone at 70.
Where do I find the platelet count?
In a complete (clinical) blood count — it's a standard value. Enter it in units of ×10⁹/L, as on the report.
I'm under 36 — can I trust FIB-4?
At this age accuracy is lower, so the calculator doesn't assign the result to a zone but suggests an alternative assessment — elastography (FibroScan/VCTE) or ELF. Discuss this with your physician.
Does a high FIB-4 already mean cirrhosis?
No. A high result only increases the likelihood of severe fibrosis and requires confirmation by imaging methods. The diagnosis is made by a physician.
What should I do with the result?
Show it to your physician along with your test results. For a low result — monitoring and work on risk factors; for a grey or high one — elastography/ELF and a work-up plan.
