Calculate the FIB-4 index from age, AST, ALT, and platelet count.
Calculator inputs
Enter the available information
FIB-4
Calculated result
Result summary
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Complete every required field, then select the calculate button.
FIB-4 score(age × AST) ÷ (platelets × √ALT)
FIB-4 score
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Reported interpretation
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Educational estimate only—not medical advice, a diagnosis, or a treatment instruction. Cutoffs vary by age, condition, and guideline. FIB-4 is a screening index, not a diagnosis of liver fibrosis.
What is Fibrosis-4 (FIB-4)?
The Fibrosis-4 index, commonly called FIB-4, combines age, aspartate aminotransferase, alanine aminotransferase and platelet count into a noninvasive liver-fibrosis risk estimate. It was first developed in adults with HIV and hepatitis C coinfection and is now used in several liver-disease pathways as an initial risk-stratification aid.
This result is not a diagnosis and does not measure scarring directly. Its meaning depends on the condition being assessed, the patient’s age, current illness and the threshold adopted by a particular guideline. The page displays the classic research thresholds of 1.45 and 3.25 so users can see where the numerical score falls, while avoiding diagnostic language.
How to Use This Calculator
Enter age in completed years. This page is limited to adults.
Enter AST and ALT from the same laboratory draw, both in U/L.
Enter platelet count in 109/L, numerically equivalent to thousands per microlitre.
Select Calculate to obtain the rounded index and its position relative to the displayed reference thresholds.
The equation multiplies age by AST, then divides by platelet count multiplied by the square root of ALT. Because ALT is under a square root, zero and negative laboratory values cannot be used. A very low platelet count or elevated AST can raise the score, but those findings can have causes other than chronic fibrosis.
Use results from the same clinical episode. Acute hepatitis, alcohol-related injury, infection, medication effects and other temporary changes may distort aminotransferases or platelets. A clinician may repeat testing or use elastography, imaging or another assessment rather than interpreting one isolated number.
Formula Used
FIB-4 = (age × AST) ÷ (platelet count × √ALT)
Age is in years, AST and ALT are in U/L, and platelet count is entered in 109/L. The index is reported without a physical unit. The page rounds to two decimal places after completing the calculation.
The displayed bands are below 1.45, from 1.45 through 3.25, and above 3.25. Other pathways, especially those for metabolic dysfunction-associated steatotic liver disease, may use 1.3 and 2.67 and may raise the lower threshold for adults older than 65. Follow the thresholds specified for the actual clinical setting.
Variables Explained
Laboratory values used in the FIB-4 equation
Input
Required unit
Role in the score
Age
Completed years
Appears in the numerator; the index tends to rise with age even if laboratory values stay fixed.
AST
U/L
Aspartate aminotransferase appears directly in the numerator.
ALT
U/L
Alanine aminotransferase appears as its square root in the denominator.
Platelet count
109/L
Appears in the denominator; confirm the scale on the laboratory report.
FIB-4
No unit
A calculated index requiring clinical context and a suitable interpretation pathway.
Worked Example
For a 50-year-old adult with AST 40 U/L, ALT 40 U/L and platelets 200 × 109/L:
FIB-4 = (50 × 40) ÷ (200 × √40). The numerator is 2,000. The square root of 40 is about 6.3249, making the denominator about 1,264.98. Dividing gives 1.581, which rounds to 1.58.
On this page, 1.58 is between the classic reference thresholds of 1.45 and 3.25, so the result is labeled “between the reference thresholds.” That wording does not determine whether advanced fibrosis is present. It indicates that this single index does not sit in either outer band under that threshold set.
Why Use This Calculator?
A score below the lower threshold may help identify a lower-risk group in an appropriate population, while a score above an upper threshold may prompt further assessment. Neither band is definitive. Predictive value changes with disease prevalence, age and the reason testing was ordered.
Age deserves particular attention. Scores can be falsely reassuring in younger adults and rise in older adults partly because age is built into the numerator. Several contemporary pathways therefore use age-adjusted cutoffs or recommend different handling for people younger than 35 or older than 65.
Discuss an unexpected result with a qualified healthcare professional who can review symptoms, medicines, alcohol exposure, metabolic risks, prior results and the complete blood and liver panels. Seek urgent medical care for severe symptoms such as confusion, vomiting blood, black stools, marked abdominal swelling or rapidly developing jaundice.
Common Mistakes
Using the wrong platelet scale: enter 200 for 200 × 109/L, not 200,000.
Mixing dates: AST, ALT and platelets should normally come from the same blood draw.
Applying one threshold set everywhere: clinical pathways use different cutoffs for different diseases and ages.
Reading the band as a diagnosis: FIB-4 estimates risk and cannot confirm or exclude fibrosis on its own.
Ignoring acute illness: temporary enzyme or platelet changes can make the index misleading.
Rounding inputs: use the reported laboratory values and round only the final score.
Frequently Asked Questions
Can FIB-4 diagnose cirrhosis?
No. It is a risk-stratification index. Diagnosis may require clinical evaluation and additional tests such as elastography, imaging or, in selected cases, biopsy.
Which platelet unit should I enter?
Use 10 to the ninth per litre. This has the same numerical value as thousands per microlitre, so a reported count of 200 thousand per microlitre is entered as 200.
Why does age change the score?
Age is multiplied directly by AST in the numerator. This improves performance in some studied groups but also means age-specific interpretation is often needed.
Can I use results from different blood draws?
That is best avoided. Enzymes and platelet counts can change, and combining dates creates an index that did not describe one point in time.
What if my score is between the thresholds?
An intermediate band commonly means the index alone does not settle risk. The next step depends on the clinical pathway and may include repeat tests or another noninvasive assessment.
Why do some websites use 1.3 and 2.67?
Those cutoffs are common in newer pathways for certain liver conditions, while this page displays the classic validation thresholds. Neither set should be transferred blindly between populations.
Can medicines or acute illness affect FIB-4?
Yes. Anything that changes AST, ALT or platelet count can change the score, even without a corresponding change in chronic liver scarring.