Medical AIFIB-4 liver fibrosis calculator
A non-invasive index combining age, AST, ALT, and platelet count to estimate the likelihood of advanced hepatic fibrosis, widely used as the first step in triaging chronic liver disease before elastography or biopsy.
How to measure each input
- Age
- Use the patient's current age in years. FIB-4 reliability degrades at the extremes (see pitfalls).
- AST and ALT
- Use the laboratory values in U/L from the same panel. Avoid mixing results drawn weeks apart, as transaminases fluctuate.
- Platelet count
- Enter in 10^9/L (equivalently 1000/microL). A platelet count of 180,000/microL is entered as 180.
Interpretation
| Band | Meaning |
|---|---|
| < 1.30, low | Advanced fibrosis is unlikely. High negative predictive value; further fibrosis testing is generally not needed in the absence of other concern. |
| 1.30 to 2.67, indeterminate | Indeterminate zone. Proceed to a second-line assessment such as transient elastography (liver stiffness) or a specialist serum panel. |
| < 2.00, low (age > 65) | Low-risk result using the commonly recommended higher lower cut-off for patients older than 65, where the standard 1.30 threshold overcalls fibrosis. |
| 2.00 to 2.67, indeterminate (age > 65) | Indeterminate result in an older adult. Use second-line assessment such as transient elastography or an enhanced liver fibrosis test rather than treating FIB-4 alone as diagnostic. |
| > 2.67, high | Advanced fibrosis (F3 to F4) is likely. Refer for hepatology assessment and consider elastography or biopsy to confirm and stage. |
Pitfalls, exclusions and caveats
- FIB-4 is unreliable below age 35 and above age 65: in older adults it over-calls fibrosis, so some guidance raises the upper cut-off to about 2.0 in those over 65.
- It estimates fibrosis probability, not aetiology. It does not distinguish MASLD, alcohol-related, viral, or autoimmune liver disease.
- Acute hepatitis, severe necroinflammation, or any acute illness that shifts AST/ALT or platelets will distort the result.
- Conditions that independently lower platelets (hypersplenism, ITP, marrow disease) can falsely raise the score.
- An indeterminate result is common and should trigger a second-line test rather than reassurance or biopsy by default.
FIB-4 = (Age x AST) / (Platelets x sqrt(ALT)), with AST and ALT in U/L and platelets in 10^9/L.The FIB-4 index was described by Sterling et al. (2006). This implementation is an educational tool and is not affiliated with the original authors or any guideline body.
Frequently asked
What is a normal FIB-4 score?
Below 1.30 advanced fibrosis is unlikely and is treated as a reassuring result in most adults. Between 1.30 and 2.67 is indeterminate and warrants further testing; above 2.67 suggests advanced fibrosis.
Why is FIB-4 unreliable in older patients?
Age is in the numerator, so the score rises with age independently of liver disease. Above 65 it over-calls fibrosis, and some guidance raises the lower cut-off to roughly 2.0 to preserve specificity.
What should I do with an indeterminate FIB-4?
Move to a second-line test rather than stopping. Transient elastography (liver stiffness measurement) or a validated serum panel refines the estimate before considering biopsy.
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