FIB-4 Index Calculator
FIB-4 score from age, AST, ALT, and platelets with age-specific fibrosis risk bands.
What it does
FIB-4 is a noninvasive index used to estimate the likelihood of advanced liver fibrosis. It uses four routine numbers: age, aspartate aminotransferase, alanine aminotransferase, and platelet count.
The score is most often used as a first step in people with metabolic dysfunction-associated steatotic liver disease, chronic hepatitis B or C, or other chronic liver conditions. A low result can rule out advanced fibrosis with reasonable confidence, while a high result calls for specialist assessment.
The age-specific lower cutoff matters. For adults older than 65, the low-risk cutoff rises from 1.30 to 2.00 because age is part of the formula and older adults tend to have higher scores without more fibrosis.
The defaults show an age of 58, AST of 48 U/L, ALT of 40 U/L, and platelets of 180 x 10^9/L. The resulting FIB-4 is 2.45, which sits in the indeterminate band and usually leads to elastography or another second-line test.
How to use
- Enter the patient's age at the time of the blood draw.
- Enter AST and ALT in U/L from the same laboratory panel.
- Enter the platelet count in x 10^9/L; multiply a count reported per microliter by 0.001.
- Press Calculate to see the index and the age-specific lower cutoff.
- Compare the result with the low, indeterminate, and high bands.
- Use the trend and the clinical picture; do not treat one number as a diagnosis.
Formula
- age
- Age in years at the time of testing.
- AST
- Aspartate aminotransferase in U/L.
- ALT
- Alanine aminotransferase in U/L.
- platelets
- Platelet count in x 10^9/L.
The formula multiplies age by AST, then divides by platelet count times the square root of ALT. Use platelets in x 10^9/L, not as a raw count per microliter. The lower cutoff is 1.30 for ages 18 to 65 and 2.00 for ages above 65; the upper cutoff remains 2.67. FIB-4 performs poorly in people younger than 35 and should not be used alone in acutely ill patients.
Worked example
Example: age 58, AST 48, ALT 40, platelets 180
Defaults: age 58 years, AST 48 U/L, ALT 40 U/L, platelet count 180 x 10^9/L.
- Multiply age by AST: 58 x 48 = 2,784.
- Take the square root of ALT: sqrt(40) = 6.3246.
- Multiply platelets by that root: 180 x 6.3246 = 1,138.42.
- Divide: 2,784 / 1,138.42 = 2.4455.
- Round to two decimals: FIB-4 = 2.45.
- The age-specific lower cutoff is 1.30 and the upper cutoff is 2.67, so 2.45 is indeterminate.
- The tool reports 2.45, an indeterminate band, and a suggestion to use elastography, ELF, or specialist assessment.
Reference table
| FIB-4 result | Age 65 or younger | Age older than 65 |
|---|---|---|
| Low risk | Below 1.30 | Below 2.00 |
| Indeterminate | 1.30 to 2.67 | 2.00 to 2.67 |
| High risk | Above 2.67 | Above 2.67 |
Limits
- FIB-4 estimates fibrosis risk; it does not measure liver stiffness or stage fibrosis directly.
- The score is less accurate in people younger than 35 and in acutely ill patients.
- Inflammatory, hematologic, and renal conditions can change AST, ALT, or platelets independently of fibrosis.
- Cutoffs vary by cause of liver disease and by clinical setting.
Common mistakes
- Using the 1.30 cutoff for a patient older than 65 when the 2.00 cutoff applies.
- Entering platelets as 180,000 instead of 180 x 10^9/L.
- Comparing results from different laboratories without checking the AST and ALT upper limits.
- Treating an indeterminate score as either normal or diagnostic of cirrhosis.
- Using FIB-4 as the only test in a patient with an acute illness or suspected decompensation.
Try it yourself
Quick checks. Answers sit tucked under each question.
What is FIB-4 for a 50-year-old with AST 30 U/L, ALT 25 U/L, and platelets 250 x 10^9/L?
1.20. The calculation is 50 x 30 / (250 x sqrt(25)) = 1,500 / 1,250. This is below 1.30, so it is low risk.
What is FIB-4 for a 70-year-old with AST 45 U/L, ALT 30 U/L, and platelets 200 x 10^9/L?
2.88. The age-specific lower cutoff is 2.00 and the upper cutoff is 2.67, so the result is high risk.
A 70-year-old has FIB-4 1.70. Is that low or indeterminate?
Low risk. For age above 65, the lower cutoff is 2.00 rather than 1.30. The same score in a 50-year-old would be indeterminate.
Questions
What does FIB-4 measure?
It estimates the probability of advanced liver fibrosis from age, AST, ALT, and platelet count. It is a risk marker, not a direct measurement of scar tissue.
Why is the cutoff different after age 65?
Age is part of the formula, so scores rise with age even without more fibrosis. Guidance uses a lower cutoff of 2.00 for adults older than 65 to reduce false positives.
Can FIB-4 diagnose cirrhosis?
No. A high score raises concern and usually leads to elastography, a serum fibrosis panel, or specialist evaluation. Cirrhosis is diagnosed from the full clinical picture, imaging, elastography, or biopsy.
Does FIB-4 require fasting?
No. It uses routine blood counts and liver enzymes. The same laboratory units and reference ranges should be used when comparing results over time.
What should happen after an indeterminate result?
Common next steps are vibration-controlled transient elastography, an enhanced liver fibrosis panel, or referral, depending on the cause of liver disease and local resources.
Is FIB-4 useful in younger adults?
It is less accurate under age 35. A low score is reassuring, but symptoms, liver enzymes, imaging, and the underlying disease still guide assessment.
Sources
- EASL, EASD, and EASO guidelines on MASLD, 2024
- AASLD practice guidance on NAFLD, 2023
- AASLD blood-based noninvasive liver disease assessment guideline