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Fib 4 (Fibrosis 4) Index Calculator

Calculate the FIB-4 index using age, AST, ALT, and platelet count to estimate liver fibrosis risk noninvasively.

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FIB-4 Score—

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What Is the FIB-4 Index?

The Fibrosis-4 (FIB-4) index is a noninvasive scoring tool used to estimate the degree of liver fibrosis in patients with chronic liver disease. Originally developed by Sterling RK et al. (Hepatology, 2006) for patients with HIV/HCV coinfection, the FIB-4 score has since been validated across multiple liver disease etiologies including nonalcoholic fatty liver disease (NAFLD), hepatitis B, and alcoholic liver disease.

The FIB-4 Formula

The FIB-4 score is derived from four readily available laboratory and clinical values:

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

  • Age: Patient age in years (best validated for adults aged 35–65)
  • AST: Aspartate aminotransferase in units per liter (U/L)
  • ALT: Alanine aminotransferase in units per liter (U/L)
  • Platelet Count: In 10⁹/L — e.g., a count of 200,000/µL is entered as 200

Worked Example

Consider a 52-year-old patient with AST of 60 U/L, ALT of 45 U/L, and a platelet count of 180 × 10⁹/L:

FIB-4 = (52 × 60) ÷ (180 × √45) = 3,120 ÷ (180 × 6.71) = 3,120 ÷ 1,207.4 ≈ 2.58

This result falls in the indeterminate zone (1.30–2.67), indicating that further evaluation — such as transient elastography (FibroScan) — is warranted before a clinical decision is made.

Interpreting FIB-4 Scores

Established cutoff values stratify fibrosis risk into three clinically actionable categories:

  • FIB-4 < 1.30: Low risk of advanced fibrosis (stages F0–F1). Negative predictive value (NPV) approximately 90%. Liver biopsy may be safely deferred in most patients.
  • FIB-4 1.30–2.67: Indeterminate zone. Additional testing such as FibroScan, FibroTest, or liver biopsy is recommended to clarify fibrosis stage.
  • FIB-4 > 2.67: High risk of advanced fibrosis (stages F3–F4). Positive predictive value (PPV) approximately 65–80%. Hepatologist referral is strongly advised.

The original HIV/HCV derivation cohort used cutoffs of <1.45 (NPV 90%) and >3.25 (PPV 65%). The NAFLD-specific cutoffs of 1.30 and 2.67 were subsequently validated by Shah AG et al. (Clinical Gastroenterology and Hepatology, 2009), establishing FIB-4 as a first-line triage tool across disease populations.

Clinical Applications

The FIB-4 index supports multiple evidence-based clinical decisions:

  • NAFLD/NASH risk stratification: The AASLD Practice Guidance on NAFLD/NASH endorses FIB-4 as a first-line noninvasive fibrosis test, helping clinicians triage among the estimated 90 million Americans with NAFLD and avoid unnecessary invasive procedures.
  • Hepatitis C monitoring: Tracks fibrosis regression or progression before, during, and after antiviral therapy.
  • Hepatitis B management: Assists in determining optimal timing for treatment initiation based on fibrosis burden.
  • Alcoholic liver disease: Offers rapid fibrosis stratification alongside clinical history and imaging findings.

Variable Details and Known Limitations

Age

Because age appears as a direct multiplier in the numerator, patients over 65 may score above 2.67 purely due to age, leading to potential overestimation of fibrosis risk. Conversely, patients under 35 may have fibrosis risk underestimated due to their lower age value. Clinical judgment and supplementary testing remain essential in both groups.

AST and ALT

AST and ALT reflect hepatocellular injury. Acute transaminase spikes from ischemic hepatitis, acute viral hepatitis, or rhabdomyolysis can inflate the FIB-4 score independently of true fibrosis stage. Stable, chronic values from repeat testing provide more reliable estimates.

Platelet Count

Platelet count inversely correlates with portal hypertension and splenic sequestration — hallmarks of advanced cirrhosis. Enter the value in 10⁹/L. A laboratory result showing 175,000/µL or 175 K/µL should be entered as 175.

Why FIB-4 Reduces the Need for Liver Biopsy

Liver biopsy carries procedural risks including pain, bleeding, and a mortality rate of approximately 1 in 10,000 procedures. The FIB-4 index uses standard values from a routine complete metabolic panel and CBC. According to MDCalc, a FIB-4 below 1.30 correctly excludes advanced fibrosis in roughly 90% of cases, making it a powerful, cost-effective front-line screening tool. FIB-4 is a screening index only. All results require interpretation by a qualified healthcare professional and do not replace clinical evaluation, imaging, or biopsy when clinically indicated.

Reference

Frequently asked questions

What is a normal FIB-4 score?
A FIB-4 score below 1.30 is considered low risk for advanced liver fibrosis (stages F0–F1) and carries a negative predictive value of approximately 90%, meaning roughly 9 out of 10 patients in this range do not have significant fibrosis. Scores between 1.30 and 2.67 fall into an indeterminate zone requiring further testing, while scores above 2.67 indicate a high probability of advanced fibrosis or cirrhosis (stages F3–F4).
What does a FIB-4 score above 2.67 mean?
A FIB-4 score above 2.67 signals a high probability of advanced liver fibrosis (stages F3–F4), corresponding to a positive predictive value of approximately 65–80%. Patients in this range should be referred to a hepatologist for confirmatory testing such as transient elastography (FibroScan), advanced imaging, or liver biopsy to verify fibrosis stage and guide treatment planning. This threshold was validated by Shah AG et al. in a NAFLD population.
How accurate is the FIB-4 index for detecting liver fibrosis?
The FIB-4 index achieves an area under the ROC curve (AUROC) of approximately 0.77 to 0.85 for detecting advanced fibrosis (stages F3–F4), depending on the study population. In the Sterling et al. (2006) derivation cohort, the cutoff of 3.25 yielded a positive predictive value of 65% and the cutoff of 1.45 yielded a negative predictive value of 90%. Accuracy is highest in patients aged 35–65 years with stable, chronic liver disease.
Can the FIB-4 calculator be used for NAFLD and fatty liver disease?
Yes. Shah AG et al. (2009) validated FIB-4 specifically in a NAFLD population, confirming cutoffs of 1.30 and 2.67 for low and high fibrosis risk respectively. The American Association for the Study of Liver Diseases (AASLD) endorses FIB-4 as a first-line noninvasive fibrosis test in its NAFLD/NASH Practice Guidance. It effectively identifies which patients need further evaluation with FibroScan or liver biopsy, avoiding unnecessary invasive procedures in low-risk individuals.
What platelet count units does the FIB-4 formula require?
The FIB-4 formula requires platelet count entered in units of 10⁹/L, which is equivalent to 10³/µL or K/µL on most laboratory reports. For example, a platelet count reported as 200,000/µL or 200 K/µL should be entered as 200 in the calculator. Entering the full count value of 200,000 instead of 200 would produce a dramatically incorrect and unusably low FIB-4 score, so unit verification is critical before calculation.
Is the FIB-4 score reliable in patients over 65 or under 35 years old?
FIB-4 accuracy is reduced at both age extremes. In patients over 65, age functions as a direct multiplier in the numerator, which can push the score above the 2.67 threshold even in the absence of significant fibrosis, leading to overestimation of risk. In patients under 35, the relatively low age value may underestimate true fibrosis burden. For both groups, clinicians should interpret FIB-4 cautiously and supplement the result with transient elastography or liver biopsy when the clinical picture is unclear.