Liver Fibrosis Score (FIB-4) Calculator
Calculate the FIB-4 index for non-invasive liver fibrosis assessment using age, AST, ALT, and platelet count. Includes APRI score and NAFLD-specific cutoffs.
About this calculator
FIB-4 is a widely used non-invasive index for estimating the likelihood of advanced liver fibrosis (scarring) without a liver biopsy, calculated from age and three routine lab values: AST, ALT, and platelet count, using the published formula (age x AST) / (platelet count x the square root of ALT), exactly as derived by Sterling and colleagues in their original 2006 validation study, which was conducted in a cohort of patients co-infected with HIV and hepatitis C (the APRICOT trial), not a hepatitis-C-only population. FIB-4 rises with age and with AST, and falls as ALT or platelet count rises -- lower platelets are a marker of the portal hypertension that develops with advancing fibrosis, and a higher AST relative to ALT (an elevated AST/ALT, or De Ritis, ratio) is itself a recognized fibrosis marker, which is why AST sits in the numerator and ALT in the denominator rather than the two being averaged. The standard cutoffs (validated in chronic hepatitis C but also widely applied to other chronic liver diseases) treat a score under 1.45 as low risk of advanced fibrosis, 1.45 to 3.25 as indeterminate (typically prompting further testing such as elastography), and above 3.25 as high risk.
A separate, lower set of cutoffs (below 1.30 low risk, 1.30 to 2.67 indeterminate, above 2.67 high risk) is commonly used specifically for NAFLD/MASLD (fatty liver disease) and is shown alongside the standard cutoffs here -- note that some more recent NAFLD-specific guidance further adjusts the indeterminate cutoff upward for patients over 65 to reduce false positives from age alone, an adjustment this calculator does not apply. The APRI score (AST-to-platelet ratio index) is included as an older, independently validated companion non-invasive marker. No non-invasive score substitutes for elastography or biopsy when the clinical picture is ambiguous or when the result will change management.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Inputs
Results
FIB-4 Index
1.37
Risk Category (0–2)
0
Figures current as of 2006. Source: Sterling RK, Lissen E, Clumeck N, Sola R, Correa MC, Montaner J, Sulkowski MS, Torriani FJ, Dieterich DT, Thomas DL, Messinger D, Nelson M; APRICOT Clinical Investigators. Development of a simple noninvasive index to predict significant fibrosis in patients with HIV/HCV coinfection. Hepatology. 2006;43(6):1317-1325.
How to Use This Calculator
- Enter the patient's Age in years, AST in U/L, and ALT in U/L from current laboratory results.
- Enter Platelet Count in ×10⁹/L to complete the FIB-4 formula.
- Review the FIB-4 Index — the primary Risk Category uses the standard cutoffs: below 1.45 suggests low fibrosis risk, above 3.25 suggests advanced fibrosis, in between is indeterminate.
- If the underlying condition is NAFLD/MASLD (fatty liver disease), also check the separate NAFLD Risk output, which applies its own lower cutoffs (below 1.30 low, above 2.67 high) to the same FIB-4 score.
- Check the APRI Score and AST/ALT Ratio (De Ritis) for additional non-invasive fibrosis markers.
- Use the risk category to guide decisions about liver biopsy or specialist referral.
How the result changes with Age (years)
| Age (years) | FIB-4 Index | Risk Category (0–2) |
|---|---|---|
| 25 | 0.68 | 0 |
| 38 | 1.04 | 0 |
| 75 | 2.05 | 1 |
| 100 | 2.74 | 1 |
What each input means
- Age (years)
- Patient age in years. FIB-4 is validated for adults 35–65 years.
- AST (U/L)
- Aspartate aminotransferase (SGOT) in units per liter.
- ALT (U/L)
- Alanine aminotransferase (SGPT) in units per liter.
- Platelet Count (×10⁹/L)
- Platelet count in 10⁹/L (e.g., 200 = 200,000/μL).
What each result means
- FIB-4 Index
- FIB-4 score. <1.45 = low risk, 1.45–3.25 = indeterminate, >3.25 = high risk.
- Risk Category (0–2)
- 0 = low risk (F0–F1), 1 = indeterminate (need further testing), 2 = high risk (F3–F4).
- NAFLD Risk (0–2)
- NAFLD-specific cutoffs: 0 = low (<1.30), 1 = indeterminate (1.30–2.67), 2 = high (>2.67).
- AST/ALT Ratio (De Ritis)
- De Ritis ratio. >1.0 may suggest cirrhosis or alcoholic liver disease.
- APRI Score
- AST to Platelet Ratio Index. <0.5 = low risk, 0.5–1.5 = indeterminate, >1.5 = high risk.
- APRI Risk (0–2)
- APRI-based fibrosis risk: 0 = low, 1 = indeterminate, 2 = high.
- Next Step (0–2)
- 0 = routine monitoring, 1 = consider elastography/further testing, 2 = elastography + hepatology referral.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersAge (years) = 50, AST (U/L) = 30, ALT (U/L) = 30, Platelet Count (×10⁹/L) = 200 = 4 input(s) provided
- Calculate FIB-4 IndexFIB-4 Index = round(fib4 * 100) / 1001.37 = 1.37
- Calculate Risk Category0 = 0
- Calculate NAFLD Risk1 = 1
- Calculate AST/ALT RatioAST/ALT Ratio1 = 1
Figures and sources
- FIB-4 Index (non-invasive liver fibrosis assessment) (2006) — Sterling RK, Lissen E, Clumeck N, Sola R, Correa MC, Montaner J, Sulkowski MS, Torriani FJ, Dieterich DT, Thomas DL, Messinger D, Nelson M; APRICOT Clinical Investigators. Development of a simple noninvasive index to predict significant fibrosis in patients with HIV/HCV coinfection. Hepatology. 2006;43(6):1317-1325.
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does a higher platelet count lower the FIB-4 score?
Platelet count sits in the denominator of the FIB-4 formula, so a higher count lowers the calculated score. This reflects real pathophysiology: as fibrosis progresses toward cirrhosis, portal hypertension develops and causes the spleen to sequester more platelets, so a falling platelet count is itself a recognized indirect marker of worsening fibrosis, not just a mathematical artifact of the formula.
Why does ALT lower the score while AST raises it?
AST is in the numerator and ALT (under a square root) is in the denominator, so they pull the score in opposite directions. This mirrors the AST/ALT (De Ritis) ratio, a separately recognized fibrosis marker: a rising AST relative to ALT is associated with more advanced liver disease, so building that same relationship into FIB-4's formula improves its ability to separate low-risk from high-risk patients compared to using either transaminase alone.
Should I use the standard cutoffs or the NAFLD-specific cutoffs?
The original 1.45/3.25 cutoffs were validated in chronic hepatitis C, while the lower 1.30/2.67 cutoffs are commonly used for NAFLD/MASLD (fatty liver disease) specifically, since fibrosis in that population tends to develop differently. If the underlying liver disease isn't hepatitis C or NAFLD, discuss with a hepatologist which cutoff set (if either) is appropriate, since FIB-4 was not independently validated for every chronic liver disease.
Does FIB-4 replace the need for a liver biopsy or elastography?
No -- FIB-4 and the companion APRI score here are screening tools meant to reduce unnecessary biopsies in patients who are clearly low risk and to flag patients who need further workup. A result in the indeterminate range, or a high-risk result with an ambiguous clinical picture, is generally followed by elastography (such as FibroScan) or biopsy for a more definitive fibrosis assessment.
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