FIB-4 Calculator
Compute the Fibrosis-4 (FIB-4) score from age, AST, ALT and platelet count — with automatic age-adjusted cutoffs (1.30 / 2.00 / 2.67), unit handling, a worked example, and a FIB-4 vs APRI comparison. Based on Sterling et al., 2006.
Calculate your FIB-4 score
The FIB-4 formula
Age (years) × AST (U/L)
Platelet (×10⁹/L) × √ALT (U/L)
Source: Sterling RK et al., Hepatology 2006 — developed to estimate advanced fibrosis in chronic liver disease.
What each component does to the score
Age
FIB-4 rises with age even without disease. That is exactly why the low-risk cutoff is raised to 2.00 at age 65+ — to stop age alone from pushing healthy older adults into the indeterminate zone.
AST
Aspartate aminotransferase leaks from injured liver cells. Higher AST pushes the score up linearly — it is the strongest lab driver of a high FIB-4.
ALT
Alanine aminotransferase is square-rooted, deliberately dampening its influence. A normal ALT does not rule out fibrosis — scar tissue does not leak ALT.
Platelets
Advanced fibrosis can lower platelets (splenic sequestration from portal hypertension). Lower platelets raise the score; the formula expects ×10⁹/L, numerically equal to ×10³/µL.
Interpretation cutoffs — applied automatically
This calculator switches the low-risk cutoff based on the age you enter. Every result is labeled a screening aid, not a diagnosis.
| Age | Lower-risk range | Indeterminate range | Higher-risk range |
|---|---|---|---|
| Under 65 | < 1.30 — advanced fibrosis less likely | 1.30 – 2.67 — cannot rule in or out | > 2.67 — higher likelihood, clinician follow-up |
| 65 and older | < 2.00 — advanced fibrosis less likely | 2.00 – 2.67 — cannot rule in or out | > 2.67 — higher likelihood, clinician follow-up |
Boundary values (1.30, 2.00, 2.67) fall in the indeterminate band. Wording is intentionally non-diagnostic: a result suggests a risk range — it never states that someone has fibrosis.
Worked example, step by step
Lab slip: Age 50 · AST 40 U/L · ALT 36 U/L · Platelets 200 ×10⁹/L
- Numerator: 50 × 40 = 2,000
- Square-root the ALT: √36 = 6
- Denominator: 200 × 6 = 1,200
- FIB-4 = 2,000 ÷ 1,200 = 1.667
- Age 50 (< 65) → low-risk cutoff 1.30. 1.667 sits between 1.30 and 2.67.
FIB-4 vs APRI — same family, different tool
FIB-4 is often compared with APRI (AST-to-Platelet Ratio Index). They answer the same screening question with different inputs:
| Feature | FIB-4 (this tool) | APRI |
|---|---|---|
| Inputs | Age, AST, ALT, platelets | AST, platelets (+ lab's AST upper limit of normal) |
| Lower-risk zone | < 1.30 (< 2.00 at 65+) | < 0.5 |
| Indeterminate zone | 1.30–2.67 (2.00–2.67 at 65+) | 0.5 – 1.5 |
| Higher-risk zone | > 2.67 | > 1.5 |
| Age-adjusted | Yes — cutoff rises at 65 | No age component |
| Needs lab ULN | No | Yes (AST upper limit of normal) |
Want the APRI version? Try our dedicated APRI calculator — it uses the AST/ULN-based formula with its own three-zone gauge.
Scope boundary — what this tool is and isn't
FIB-4 screens for the likelihood of advanced liver fibrosis (Sterling et al. 2006). It does not measure cirrhosis severity and does not predict short-term outcomes. Related but different questions belong to different tools:
- APRI — another fibrosis screening aid with a different formula (AST/ULN-based). → APRI calculator
- Child-Pugh — stages severity in already-diagnosed cirrhosis (bilirubin, albumin, INR, ascites, encephalopathy). Different question from screening. → Child-Pugh calculator
- Maddrey's discriminant function — predicts short-term mortality in acute alcoholic hepatitis, not fibrosis. Different disease, different formula. → Discriminant function calculator
If you are unsure which score applies, a clinician — not an online calculator — should decide.
Limitations to know
- Acute illness distorts the score — interpret only on stable outpatient labs.
- Under age 35 the score is less validated; the age-adjusted 2.00 cutoff applies at 65+.
- Normal ALT does not exclude fibrosis — scar tissue doesn't leak enzymes.
- Performance varies by liver disease etiology (MASLD, viral hepatitis, alcohol-related).
- Muscle injury, strenuous exercise, alcohol, and some medications can spike AST/ALT temporarily.
Frequently asked questions
What is the FIB-4 score?
FIB-4 (Fibrosis-4) is a simple index developed by Sterling et al. in 2006 that estimates the likelihood of advanced liver fibrosis from four routine values: age, AST, ALT, and platelet count. It is used as a first-line, non-invasive screening aid — it is not a diagnosis, and liver assessment requires a clinician.
How is the FIB-4 score calculated?
FIB-4 = (Age in years × AST in U/L) ÷ (Platelet count in ×10⁹/L × √ALT in U/L). The ALT value is square-rooted, so it influences the result less than AST or platelets. A higher score suggests a higher risk range for advanced fibrosis.
What is a normal or low-risk FIB-4 score?
A score below 1.30 is considered the low-risk range for people under 65, and below 2.00 for people aged 65 and older. In these ranges advanced fibrosis is considered less likely, but the score remains a screening aid — only a clinician can interpret it in context.
What does an indeterminate FIB-4 score mean?
An indeterminate score — 1.30 to 2.67 for people under 65, or 2.00 to 2.67 for people 65 and older — means the score can neither rule in nor rule out advanced fibrosis. Clinicians usually follow up with additional assessment or referral. It is not a diagnosis of anything.
What does a FIB-4 score above 2.67 mean?
A score above 2.67 falls in the higher-risk range, suggesting a higher likelihood of advanced fibrosis. It does not diagnose fibrosis or cirrhosis — confirmation requires clinical evaluation, and a clinician should be consulted.
Why does the low-risk cutoff change at age 65?
FIB-4 rises with age even in healthy people because age is in the numerator, so older adults were frequently pushed into the indeterminate range without having fibrosis. Using a higher low-risk cutoff of 2.00 for people aged 65 and older reduces these age-driven false alarms.
What platelet units does the FIB-4 formula use?
The formula expects platelets in ×10⁹/L. The commonly reported unit ×10³/µL is numerically identical (200 ×10³/µL = 200 ×10⁹/L), so you can enter the number as printed on the lab report — no conversion math is needed.
What is the difference between FIB-4 and APRI?
Both are non-invasive fibrosis screening aids but use different inputs: FIB-4 uses age, AST, ALT and platelets with cutoffs 1.30 / 2.00 / 2.67, while APRI uses only AST and platelets (plus the lab's AST upper limit of normal) with zones below 0.5, 0.5–1.5 and above 1.5. Neither diagnoses fibrosis on its own.
Can the FIB-4 score diagnose liver disease?
No. FIB-4 is a screening aid, not a diagnostic test. A low score makes advanced fibrosis less likely, and a high score suggests further evaluation — but diagnosis of liver disease or cirrhosis requires a clinician, and may involve imaging, elastography, or biopsy.
Who should be cautious interpreting a FIB-4 score?
FIB-4 is less reliable in acutely ill patients, in people under 35 (less validated), and whenever AST or ALT are temporarily spiked by non-liver causes. It was validated in specific chronic liver disease populations, so results should always be interpreted by a clinician familiar with the patient's history.
How often should a FIB-4 score be rechecked?
There is no single schedule: guidelines commonly suggest repeating FIB-4 every 1–3 years in people being monitored for fatty liver disease, more often (about yearly) with diabetes or multiple risk factors. The right interval is a decision for the patient's clinician.
What can raise or lower a FIB-4 score?
Older age, higher AST, and lower platelets raise the score; higher ALT slightly lowers it because it sits under a square root in the denominator. Temporary AST/ALT spikes (for example from muscle injury, alcohol, or medications) can distort the score, so clinicians usually repeat abnormal labs before acting.
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