FIB-4 Index Calculator

Learn about the Fibrosis-4 (FIB-4) Index for assessing liver fibrosis. Explore the formula, cutoff values, interpretation, clinical significance, indications, and limitations.

FIB-4 Index Calculator

Age (years)

AST (U/L)

ALT (U/L)

Platelet Count (10⁹/L)

Disclaimer: The FIB-4 Index is a screening tool and should not replace clinical judgment or liver biopsy when indicated. Results should be interpreted in the context of the patient's overall clinical presentation, medical history, and other diagnostic findings. Always consult with a hepatology specialist for complex cases and consider additional testing when appropriate.

FIB-4 Index Formula Explained

FIB-4 Index Formula

The FIB-4 (Fibrosis-4) Index is a non-invasive biomarker panel used to estimate liver fibrosis by combining age, aspartate aminotransferase (AST), alanine aminotransferase (ALT), and platelet count. It helps stratify patients based on their risk of advanced liver fibrosis.

FIB-4 Formula:

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

Where:

Age: Patient's age in years
AST: Aspartate aminotransferase level in U/L
ALT: Alanine aminotransferase level in U/L
Platelets: Platelet count in 10⁹/L (thousands per microliter)

FIB-4 Risk Categories

FIB-4 Score	Risk Category	Clinical Significance	Interpretation
< 1.45	Low Risk	90% NPV	Advanced fibrosis unlikely
1.45 - 3.25	Indeterminate	-	Additional assessment needed
> 3.25	High Risk	97% specificity, 65% PPV	High probability of advanced fibrosis

Clinical Applications

Screening Tool

• Non-invasive assessment of liver fibrosis
• Risk stratification for advanced fibrosis
• Screening tool for chronic liver diseases
• Monitoring disease progression over time

Clinical Indications

• Chronic hepatitis B and C infection
• Non-alcoholic fatty liver disease (NAFLD)
• Alcoholic liver disease
• Autoimmune hepatitis
• Drug-induced liver injury
• Metabolic liver diseases

Advantages and Limitations

Advantages

• Non-invasive and cost-effective
• Uses routine laboratory tests
• Good negative predictive value
• Easy to calculate and interpret
• Widely validated in various populations
• Useful for screening large populations

Limitations

• Indeterminate range requires additional testing
• Less accurate in elderly patients
• May be affected by acute liver injury
• Not a replacement for liver biopsy
• Limited accuracy in certain populations
• Should be used with clinical context

Calculation Examples

Example 1: Low Risk Patient

45-year-old patient with AST 25 U/L, ALT 20 U/L, Platelets 300 × 10⁹/L

FIB-4 = (45 × 25) / (300 × √20) = 1125 / (300 × 4.47) = 1125 / 1341 = 0.84

Result: FIB-4 = 0.84 (Low Risk - Advanced fibrosis unlikely)

Example 2: High Risk Patient

65-year-old patient with AST 80 U/L, ALT 60 U/L, Platelets 120 × 10⁹/L

FIB-4 = (65 × 80) / (120 × √60) = 5200 / (120 × 7.75) = 5200 / 930 = 5.59

Result: FIB-4 = 5.59 (High Risk - High probability of advanced fibrosis)

Important Clinical Notes

• FIB-4 should be interpreted in the context of the patient's clinical presentation
• Consider additional non-invasive tests (FibroScan, ELF test) for indeterminate results
• Liver biopsy remains the gold standard for fibrosis staging
• Results may be less reliable in patients with acute liver injury
• Consider patient age and comorbidities when interpreting results
• Regular monitoring may be needed to assess disease progression
• Always correlate with imaging studies and clinical findings

The Fibrosis-4 (FIB-4) Index is a non-invasive scoring system developed to estimate the degree of liver fibrosis in patients with chronic liver disease, particularly those with hepatitis C and nonalcoholic fatty liver disease (NAFLD). It combines simple, routinely available clinical and laboratory parameters—age, aspartate aminotransferase (AST), alanine aminotransferase (ALT), and platelet count—to provide an estimate of fibrosis severity without the need for liver biopsy.

Since liver biopsy carries risks such as bleeding and sampling error, non-invasive markers like FIB-4 have become essential tools for risk stratification, treatment planning, and long-term monitoring of chronic liver disease.

Calculation Formula

The FIB-4 Index is calculated using the following formula:

FIB-4 = (Age × AST) / (Platelet count × √ALT)

Age is in years
AST and ALT are measured in U/L
Platelet count is measured in 10⁹/L

Interpretation of Scores

The FIB-4 Index provides cut-off values to rule out or suggest significant fibrosis:

< 1.3: Low probability of advanced fibrosis (safe to monitor without biopsy in most cases)
1.3 – 2.67: Indeterminate; further testing with imaging or other non-invasive scores recommended
> 2.67: High probability of advanced fibrosis or cirrhosis; further evaluation and management required

In patients over 65 years, a higher cut-off of 2.0 may be used for better specificity.

Clinical Significance

The FIB-4 Index is widely used in hepatology because of its simplicity and accuracy:

Risk stratification: Helps identify patients at risk for advanced fibrosis or cirrhosis.
Monitoring chronic liver disease: Useful in hepatitis B, hepatitis C, alcoholic liver disease, and NAFLD/NASH.
Reducing need for biopsy: Non-invasive assessment reduces reliance on invasive procedures.
Guiding therapy: Identifies patients who may benefit from antiviral therapy or closer follow-up.
Public health and research: Provides an inexpensive, scalable tool for large population screening studies.

Indications for Use

The FIB-4 Index should be applied in:

Patients with chronic viral hepatitis (HBV, HCV).
Individuals with nonalcoholic fatty liver disease (NAFLD) or nonalcoholic steatohepatitis (NASH).
Patients with alcoholic liver disease.
Routine monitoring of liver fibrosis progression or regression after treatment.

Limitations

Despite its value, FIB-4 has important limitations:

Indeterminate zone: Scores between 1.3 and 2.67 require additional testing such as elastography (FibroScan) or other biomarker panels.
Age influence: Older patients may have falsely elevated scores due to age alone.
Not specific to etiology: FIB-4 does not differentiate between causes of fibrosis.
Laboratory variability: Differences in AST, ALT, and platelet measurements across labs can affect accuracy.
Acute illness interference: Elevated transaminases due to acute hepatitis or other conditions may distort results.