FIB-4 Calculator
The FIB-4 (Fibrosis-4) index is a simple, non-invasive score that estimates the chance of advanced liver fibrosis from four routine values: your age, AST, ALT and platelet count. Enter the four numbers to get your FIB-4 score with its interpretation band, plus the matching APRI score and AST/ALT ratio. You can switch platelet units and turn on age-adjusted cutoffs for adults 65 and over.
Formula
Worked example
A 50-year-old with AST 40 U/L, ALT 30 U/L and platelets 200 (10^9/L): numerator = 50 × 40 = 2000; denominator = 200 × √30 = 200 × 5.477 = 1095.4; FIB-4 = 2000 ÷ 1095.4 = 1.83, an indeterminate result. The APRI works out to (40 ÷ 40) ÷ 200 × 100 = 0.50.
What the FIB-4 index measures
FIB-4 is a non-invasive way to estimate how much scarring (fibrosis) has built up in the liver, without a biopsy. It combines four values that are usually already in your records: your age, two liver enzymes (AST and ALT) and your platelet count. Rising AST relative to ALT, a falling platelet count and older age all push the score up, because they tend to accompany worsening fibrosis. The index was first developed by Sterling and colleagues in people with HIV and hepatitis C co-infection, and it is now widely used to screen for advanced fibrosis in conditions such as hepatitis B, hepatitis C and metabolic (fatty) liver disease.
How to read your score
There are two standard cutoffs. A FIB-4 below 1.45 makes advanced fibrosis unlikely: at this threshold the test correctly rules out advanced disease about 90% of the time. A FIB-4 above 3.25 points to a high probability of advanced fibrosis, with a specificity of roughly 97%. Scores between the two cutoffs fall in an indeterminate grey zone, where the index alone cannot decide, and a second test such as transient elastography (FibroScan), an Enhanced Liver Fibrosis (ELF) blood test or specialist review is typically used. The score is a probability estimate for a population, not a definite diagnosis for one person.
Age-adjusted cutoffs for adults 65 and over
FIB-4 rises naturally with age, so in older adults the standard 1.45 lower cutoff over-calls fibrosis and sends too many people for unnecessary further testing. Both the American Gastroenterological Association (AGA) and the EASL-EASD-EASO MASLD guidelines recommend raising the lower cutoff to 2.0 in adults aged 65 and over, while keeping the 3.25 upper cutoff the same. With the age-adjusted toggle on, this calculator applies the 2.0 lower cutoff automatically once you enter an age of 65 or more, and the result and steps show which cutoff was used. The cutoff for adults under 65 is unchanged at 1.45.
APRI score and AST/ALT ratio
Alongside FIB-4 the calculator reports two related blood-based markers from the same panel. The AST to Platelet Ratio Index (APRI) is computed as (AST divided by its lab upper limit of normal, usually 40 U/L) divided by the platelet count in 10^9/L, multiplied by 100. An APRI of 1.0 or higher raises concern for significant fibrosis or cirrhosis, while a value below 0.5 argues against it. The AST/ALT ratio (the De Ritis ratio) is simply AST divided by ALT; a ratio above 1, especially above 2, can suggest alcohol-related liver disease or more advanced scarring. These extra outputs let you cross-check the FIB-4 result without a separate calculator. Open advanced options to change the AST upper limit of normal if your lab uses a value other than 40.
Limitations and accuracy
FIB-4 performs best in adults between about 35 and 65. In people over 65 the standard cutoffs can over-call fibrosis because the score rises naturally with age, which is why the age-adjusted 2.0 cutoff exists. The index is unreliable during acute hepatitis or other acute liver injury, when AST and ALT are temporarily very high, and it can be skewed by conditions that change platelet counts independently of the liver. Results are estimates and any clinical decision should be made with a qualified healthcare professional who can see your full picture.
FIB-4 interpretation bands
| FIB-4 score | Interpretation | What it suggests |
|---|---|---|
| Below 1.45 (or 2.0 if 65+) | Low probability | Advanced fibrosis unlikely (NPV ~90-95%) |
| Lower cutoff to 3.25 | Indeterminate | Further assessment, e.g. elastography, often needed |
| Above 3.25 | High probability | Advanced fibrosis likely (PPV ~65%, specificity ~97%) |
Standard Sterling cutoffs. A FIB-4 below the lower cutoff has a high negative predictive value for excluding advanced fibrosis; above 3.25 has a high positive predictive value. Values in between are indeterminate and usually prompt further testing. In adults 65 and over the lower cutoff is raised to 2.0.
Frequently asked questions
What units should I use for platelets?
Use the unit selector to match your lab report. The SI value in 10^9 per litre (a typical normal range is roughly 150 to 400) is numerically the same as thousands per microlitre, so 200,000/µL is entered as 200 in 10^9/L. If your report gives the full count per mm^3 or per µL (for example 200,000), pick that unit and the calculator converts it for you.
Why does the cutoff change when I am 65 or older?
FIB-4 increases with age, so the original 1.45 lower cutoff flags too many older adults as possibly having fibrosis. AGA and EASL guidance therefore raises the lower cutoff to 2.0 for people aged 65 and over. With the age-adjusted toggle on, this calculator uses 2.0 automatically at 65+; the upper 3.25 cutoff stays the same. You can switch the toggle off to use the standard 1.45 for everyone.
What is the APRI score shown next to FIB-4?
APRI is the AST to Platelet Ratio Index, another non-invasive fibrosis marker from the same blood panel. It is calculated as (AST divided by the AST upper limit of normal) divided by the platelet count in 10^9/L, times 100. An APRI of 1.0 or more suggests significant fibrosis or cirrhosis; below 0.5 argues against it. It is shown so you can cross-check the FIB-4 reading.
Is a high FIB-4 a diagnosis of cirrhosis?
No. A FIB-4 above 3.25 indicates a high probability of advanced fibrosis and warrants further evaluation, but it is not a diagnosis. Confirmatory tests such as elastography or, occasionally, a liver biopsy are needed to stage fibrosis precisely.
Does FIB-4 work for fatty liver disease?
Yes. FIB-4 is widely recommended as a first-line, non-invasive screen for advanced fibrosis in metabolic (non-alcoholic) fatty liver disease, typically using the 1.45 and 3.25 cutoffs (or 2.0 and 3.25 in adults 65 and over) to decide who needs specialist referral.