FIB-4 Index Calculator

Q: What is the formula for the FIB-4 index?

FIB-4 = (Age × AST) ÷ (Platelet count × √ALT), where age is in years, AST and ALT are in U/L, and platelet count is in ×10⁹/L (the same as the standard CBC unit ×10³/µL).

Q: What is the low-risk cutoff for FIB-4?

FIB-4 below 1.30 suggests a low probability of advanced fibrosis in patients under 65. For patients 65 and older, guidelines use an age-adjusted low-risk cutoff of below 2.00, since FIB-4 rises with age independent of true fibrosis.

Q: What FIB-4 value is considered high risk?

FIB-4 above 2.67 is generally considered high risk for advanced fibrosis regardless of age, and typically warrants referral for elastography or biopsy to confirm the fibrosis stage.

Q: What units does the platelet count use?

Enter platelet count in ×10⁹/L, which is numerically identical to the ×10³/µL unit reported on most standard complete blood count (CBC) panels — no conversion is needed.

Q: What is FIB-4 used for?

FIB-4 is a non-invasive screening tool used primarily in chronic hepatitis C, HIV/HCV coinfection, and nonalcoholic fatty liver disease (NAFLD) to estimate the probability of advanced fibrosis, helping avoid unnecessary liver biopsies in low-risk patients.

Q: Do I need further testing after a FIB-4 result?

Low-risk results often mean no further fibrosis-staging tests are needed. Indeterminate or high-risk results typically call for confirmatory testing such as transient elastography (FibroScan) or liver biopsy, interpreted alongside the full clinical picture.

Risk band	FIB-4 (age < 65)	What it means
Low risk	< 1.30	Advanced fibrosis is unlikely; further invasive testing is often unnecessary.
Indeterminate	1.30 – 2.67	Does not rule fibrosis in or out; secondary testing (elastography or biopsy) is often needed.
High risk	> 2.67	High probability of advanced fibrosis (F3–F4); referral for confirmatory testing is typically warranted.

Risk band	Age < 65	Age ≥ 65
Low risk	FIB-4 < 1.30	FIB-4 < 2.00
Indeterminate	1.30 – 2.67	2.00 – 2.67
High risk	FIB-4 > 2.67	FIB-4 > 2.67

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Walk-through

How to Use This Calculator

3 steps

1

Enter age, AST, and ALT

Enter the patient's age in years, plus AST and ALT in U/L from a recent liver panel or comprehensive metabolic panel.

2

Enter platelet count

Enter the platelet count in ×10⁹/L — numerically the same as the standard CBC unit ×10³/µL, so no conversion is needed.

3

Read the FIB-4 index and interpretation

The calculator instantly computes FIB-4 and classifies it as low, indeterminate, or high risk, applying the age-adjusted cutoff automatically for patients 65 and older. Check the Interpretation tab for what each band means, and the Age-Adjusted Cutoffs tab for the full cutoff comparison.

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Reference

Formula & Methodology

1 formula

FIB-4 index

FIB-4 = (Age × AST) ÷ (Platelets × √ALT)

Age is entered in years; AST and ALT are entered in U/L; platelet count is entered in ×10⁹/L. Sterling RK et al. first described this index in 2006 as a low-cost, non-invasive alternative to liver biopsy for estimating fibrosis stage in HIV/HCV coinfection, later validated more broadly in HCV and NAFLD.

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High YMYL

Trust, Methodology & Sources

Reviewed by Calculover Editorial Review · Updated 2026-07-22 · 3 sources

Editorial accountability

Author: Calculover Editorial Team - Editor

Owner: Calculover Editorial Team - Editorial owner

Reviewer: Calculover Editorial Review - Medical-source review

Last reviewed: 2026-07-22

Last verified: 2026-07-22

Data effective: 2026-07-22

Methodology

Implements the FIB-4 index described by Sterling RK et al., Hepatology 2006;43(6):1317-1325: FIB-4 = (Age × AST) ÷ (Platelets × √ALT). General-population cutoffs (low risk < 1.30, high risk > 2.67) follow Vallet-Pichard A et al., Hepatology 2007;46(1):32-36. The age-adjusted low-risk cutoff (< 2.00 for age 65+) follows McPherson S et al., Am J Gastroenterol 2017;112(5):740-751.

Assumption: Age, AST, ALT, and platelet count are assumed to be from the same blood draw, entered in the units specified (years; U/L; U/L; ×10⁹/L).

Assumption: The age-adjusted low-risk cutoff (< 2.00) is applied automatically for patients 65 and older; the standard cutoff (< 1.30) is used below that age.

Assumption: A single FIB-4 result is treated as a screening data point, not a diagnosis — the calculator does not track serial values over time.

Limitations & guidance

FIB-4 was validated primarily in hepatitis C, HIV/HCV coinfection, and NAFLD populations, and may not perform as well outside those contexts (e.g. acute liver injury).

An indeterminate or high-risk result does not itself diagnose fibrosis stage — confirmatory testing (elastography or biopsy) is typically required.

Results are for clinical decision support and education only and do not replace a treating clinician's full assessment.

Professional guidance: This calculator is for clinical decision support and educational purposes only and is not a substitute for professional medical judgment, diagnosis, or treatment. Always consult a qualified clinician for patient care decisions.

Primary sources

Sterling RK, Lissen E, Clumeck N, et al. Development of a simple noninvasive index to predict significant fibrosis in patients with HIV/HCV coinfection. Hepatology. 2006;43(6):1317-1325.

MDCalc. Fibrosis-4 (FIB-4) Index for Liver Fibrosis.

McPherson S, Hardy T, Dufour JF, et al. Age as a Confounding Factor for the Accurate Non-Invasive Diagnosis of Advanced NAFLD Fibrosis. Am J Gastroenterol. 2017;112(5):740-751.

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Glossary

Key Terms Explained

7 terms

FIB-4 index A non-invasive score calculated from age, AST, ALT, and platelet count that estimates the probability of advanced liver fibrosis without requiring a liver biopsy.

Liver fibrosis Scarring of liver tissue that develops from chronic liver injury or inflammation. Advanced fibrosis (bridging fibrosis or cirrhosis) carries a meaningfully higher risk of liver-related complications.

Non-invasive screening Estimating disease severity from blood tests and calculations rather than an invasive procedure such as a liver biopsy, reducing risk and cost for lower-risk patients.

AST Aspartate aminotransferase, a liver enzyme released into the blood when liver cells are damaged. Elevated AST is one of the two enzymes used directly in the FIB-4 formula.

ALT Alanine aminotransferase, a liver enzyme more specific to liver injury than AST. FIB-4 uses the square root of ALT in the denominator, so a lower ALT raises the calculated FIB-4.

Platelet count The number of platelets circulating in the blood, measured in ×10⁹/L. Platelet counts tend to fall as fibrosis and portal hypertension progress, so a lower count raises FIB-4.

Advanced fibrosis Significant liver scarring (METAVIR stage F3 or F4) that FIB-4 is designed to flag, since it carries substantially higher risk of cirrhosis-related complications than earlier fibrosis stages.

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Scenarios

Real-World Examples

3 worked examples

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Low risk

Age 40, AST 25 U/L, ALT 30 U/L, platelets 250 ×10⁹/L

Age 40 AST 25 U/L ALT 30 U/L Platelets 250 ×10⁹/L

(40 × 25) ÷ (250 × √30) ≈ 0.73. A FIB-4 of 0.73 falls below the 1.30 low-risk cutoff for a patient under 65, suggesting advanced fibrosis is unlikely and further invasive testing is often unnecessary.

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Indeterminate

Age 55, AST 45 U/L, ALT 35 U/L, platelets 180 ×10⁹/L

Age 55 AST 45 U/L ALT 35 U/L Platelets 180 ×10⁹/L

(55 × 45) ÷ (180 × √35) ≈ 2.32. This falls in the indeterminate band (1.30–2.67), which does not rule advanced fibrosis in or out — clinicians typically order elastography or biopsy to confirm the stage.

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High risk

Age 60, AST 90 U/L, ALT 40 U/L, platelets 120 ×10⁹/L

Age 60 AST 90 U/L ALT 40 U/L Platelets 120 ×10⁹/L

(60 × 90) ÷ (120 × √40) ≈ 7.12. A FIB-4 this far above the 2.67 high-risk cutoff suggests a high probability of advanced fibrosis, and prompt referral for elastography or biopsy is typically warranted.

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Deep Dive

Understanding the FIB-4 Index

3 chapters · 3 min read

The FIB-4 index combines age, two liver enzymes, and platelet count into a single non-invasive score that estimates the probability of advanced liver fibrosis. It is widely used in hepatitis C and NAFLD care to decide who can skip a biopsy and who needs further testing.

How FIB-4 works

FIB-4 is calculated as (Age × AST) ÷ (Platelets × √ALT). Sterling RK and colleagues first described it in 2006 as a simple, low-cost alternative to liver biopsy for staging fibrosis in patients with HIV/HCV coinfection, and it was later validated more broadly in chronic hepatitis C and nonalcoholic fatty liver disease (NAFLD).

The formula rewards a combination that reflects advancing fibrosis: rising liver enzymes, a falling platelet count (a marker of portal hypertension), and older age. Because it only requires values already on a routine blood panel, FIB-4 can be calculated at essentially no additional cost.

Inputs and units

Age is entered in years. AST and ALT are entered in U/L, as reported on a standard liver panel or comprehensive metabolic panel. Platelet count is entered in ×10⁹/L — the same numeric value as the ×10³/µL unit used on most complete blood count (CBC) reports, so no conversion is required.

All four values should come from the same blood draw whenever possible, since mixing labs from different dates can produce a misleading score.

Why age matters — and the limits of FIB-4

Because age sits directly in the numerator, FIB-4 rises with age even when true fibrosis has not progressed, which inflates false-positive rates in older adults. To correct for this, guidance derived from McPherson et al. (2017) uses an age-adjusted low-risk cutoff of below 2.00 for patients 65 and older, instead of the standard 1.30 cutoff used under 65. The high-risk cutoff of above 2.67 is unchanged across age groups.

FIB-4 was validated primarily in HCV, HIV/HCV coinfection, and NAFLD populations — it is not intended for acute liver injury, and an indeterminate or high score should prompt confirmatory testing (transient elastography or biopsy) rather than a standalone diagnosis.

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Questions

Frequently Asked Questions

6 questions

What is the formula for the FIB-4 index?+

FIB-4 = (Age × AST) ÷ (Platelet count × √ALT), where age is in years, AST and ALT are in U/L, and platelet count is in ×10⁹/L (the same as the standard CBC unit ×10³/µL).

What is the low-risk cutoff for FIB-4?+

FIB-4 below 1.30 suggests a low probability of advanced fibrosis in patients under 65. For patients 65 and older, guidelines use an age-adjusted low-risk cutoff of below 2.00, since FIB-4 rises with age independent of true fibrosis.

What FIB-4 value is considered high risk?+

FIB-4 above 2.67 is generally considered high risk for advanced fibrosis regardless of age, and typically warrants referral for elastography or biopsy to confirm the fibrosis stage.

What units does the platelet count use?+

Enter platelet count in ×10⁹/L, which is numerically identical to the ×10³/µL unit reported on most standard complete blood count (CBC) panels — no conversion is needed.

What is FIB-4 used for?+

FIB-4 is a non-invasive screening tool used primarily in chronic hepatitis C, HIV/HCV coinfection, and nonalcoholic fatty liver disease (NAFLD) to estimate the probability of advanced fibrosis, helping avoid unnecessary liver biopsies in low-risk patients.

Do I need further testing after a FIB-4 result?+

Low-risk results often mean no further fibrosis-staging tests are needed. Indeterminate or high-risk results typically call for confirmatory testing such as transient elastography (FibroScan) or liver biopsy, interpreted alongside the full clinical picture.

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Labs & Age

FIB-4 & Interpretation

FIB-4 Risk Bands

Age-Adjusted FIB-4 Cutoffs

How to Use This Calculator

Enter age, AST, and ALT

Enter platelet count

Read the FIB-4 index and interpretation

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Low risk

Indeterminate

High risk

Understanding the FIB-4 Index

How FIB-4 works

Inputs and units

Why age matters — and the limits of FIB-4

Frequently Asked Questions

Get a branded PDF of your results

FIB-4 Index Calculator

Labs & Age

FIB-4 & Interpretation

FIB-4 Risk Bands

Age-Adjusted FIB-4 Cutoffs

How to Use This Calculator

Enter age, AST, and ALT

Enter platelet count

Read the FIB-4 index and interpretation

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Understanding the FIB-4 Index

How FIB-4 works

Inputs and units

Why age matters — and the limits of FIB-4

Frequently Asked Questions

Get a branded PDF of your results

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