What this calculator does
FIB-4 estimates the likelihood of advanced liver fibrosis from four things already on routine bloods: age, two liver enzymes and the platelet count. No imaging or biopsy is needed.
Two thresholds divide the result. Below 1.45 advanced fibrosis is unlikely, above 3.25 it is more likely, and the wide band between is indeterminate, which is where a substantial share of patients land.
The formula
Age multiplied by AST is divided by the platelet count multiplied by the square root of ALT. Platelets fall in advanced liver disease, which is why they sit in the denominator.
| Term | Meaning |
|---|---|
| FIB-4 | A four-variable fibrosis index from routine bloods. |
| Advanced fibrosis | Significant scarring of the liver, the outcome the score screens for. |
| Indeterminate zone | Between 1.45 and 3.25, where the score does not resolve the question. |
The inputs explained
| Field | What to enter |
|---|---|
| Age (years) | Age in years. The score was validated mainly in adults aged 35 to 65. |
| AST (U/L) | AST in U/L. |
| ALT (U/L) | ALT in U/L, which enters under a square root. |
| Platelet count (×10⁹/L) | Platelet count in ×10⁹/L. A falling count is itself a sign of advanced disease. |
When to use it
Screening for fatty liver disease
FIB-4 is widely used as the first step in non-invasive fibrosis pathways.
Deciding who needs specialist referral
A low score can safely avoid referral; a high one supports it.
Using bloods you already have
All four inputs appear on routine testing, so the score costs nothing extra.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does AST move the score?
Three AST levels against the same other values.
| AST | FIB-4 index | Reading |
|---|---|---|
| 30 U/L | 1.10 | Below 1.45: advanced fibrosis is unlikely |
| 60 U/L | 2.19 | 1.45 - 3.25: indeterminate, further testing is often used |
| 120 U/L | 4.38 | Above 3.25: advanced fibrosis is more likely |
Questions
Why does the platelet count matter?
Because advanced liver scarring raises pressure in the portal circulation, which enlarges the spleen and traps platelets. A falling platelet count is therefore an indirect sign of advanced disease, and it carries a lot of weight in the score.
What happens in the indeterminate zone?
Further non-invasive testing, commonly transient elastography or a proprietary blood panel. A substantial proportion of patients fall in this band, which is the score's main practical limitation.
Does it work at all ages?
Less well at the extremes. Because age is in the numerator, the score tends to overestimate in older adults and underestimate in younger ones, and some pathways use age-adjusted thresholds above 65.
Can it replace a biopsy?
Not entirely, though it has substantially reduced how often biopsy is needed. It is a screening tool that identifies who warrants further assessment, not a measurement of liver architecture.
For the enzyme ratio from the same panel, see the AST-to-ALT ratio calculator. For protein balance in blood, see the albumin-to-globulin ratio calculator.