FIB-4 Score Calculator

Estimate Liver Fibrosis Risk from Age, AST, ALT and Platelets

FIB-4 Score Formula

Formula

\[ \text{FIB-4} = \frac{\text{Age} \times \text{AST}}{\text{PLT} \times \sqrt{\text{ALT}}} \]

Where:

\(\text{Age}\) — age in years
\(\text{AST}\) — aspartate aminotransferase in U/L
\(\text{ALT}\) — alanine aminotransferase in U/L
\(\text{PLT}\) — platelet count in ×10⁹/L

The formula combines age and liver enzymes in the numerator with platelets (which fall as fibrosis progresses) in the denominator, giving a single number that rises with the likelihood of advanced fibrosis.

FIB-4 Score – Calculation Example

Example: a 55-year-old with AST of 45 U/L, ALT of 50 U/L and a platelet count of 180 ×10⁹/L.

\[ \text{FIB-4} = \frac{55 \times 45}{180 \times \sqrt{50}} = \frac{2475}{180 \times 7.07} \approx \frac{2475}{1272.8} \approx 1.94 \]

Result: FIB-4 = 1.94 — intermediate risk. The score falls in the indeterminate range (1.45–3.25), so additional testing such as liver elastography is usually recommended to clarify the risk.

How to Use the FIB-4 Calculator

Enter your Age in years (the score is validated for adults aged 18–100).
Enter your AST value from your blood test and select the unit — U/L or µkat/L. If your lab reports µkat/L, the calculator converts it automatically (1 µkat/L = 60 U/L).
Enter your ALT value the same way, choosing the matching unit.
Enter your Platelet count in ×10⁹/L. This is the same number your lab reports in 10³/µL — for example, a lab value of "180 ×10⁹/L" or "180 K/µL" is simply entered as 180.
Read your FIB-4 score, the risk category, and an explanation of what the result means.

What Is the FIB-4 Score?

The FIB-4 index is a non-invasive fibrosis score built from four routine measurements: age, AST, ALT and platelet count. It was originally developed by Sterling and colleagues (Hepatology, 2006) in patients with HIV/HCV coinfection and has since been widely validated in chronic hepatitis C, fatty liver disease (NAFLD/MASLD) and other chronic liver conditions. Because it uses only standard blood work, it is often the first step in deciding whether a patient with abnormal liver enzymes needs further evaluation.

The logic behind the formula is straightforward: liver enzymes (AST and ALT) tend to rise as the liver is damaged, while platelet count falls as fibrosis progresses — advanced scarring causes portal hypertension and splenic sequestration of platelets. Age adds weight because fibrosis accumulates over time. The result is a dimensionless number that increases with the likelihood of advanced fibrosis (stage F3–F4 in the METAVIR classification).

The standard interpretation uses two cut-offs: a score below 1.45 indicates low risk, a score above 3.25 indicates high risk of advanced fibrosis, and values in between are indeterminate. In the original validation cohorts, more than 90% of patients below 1.45 had no advanced fibrosis, while a score above 3.25 carried a high probability of significant scarring. An indeterminate result does not mean "unknown" — it means a second-line test (elastography, a proprietary fibrosis panel, or direct fibrosis markers) is warranted.

Interpreting Your Result

A low-risk result (FIB-4 < 1.45) generally rules out advanced fibrosis, and no immediate specialist referral is needed. An intermediate result (1.45–3.25) calls for additional non-invasive testing rather than invasive procedures. A high-risk result (> 3.25) usually prompts referral to a hepatologist, who may order liver elastography (FibroScan) or, in selected cases, a liver biopsy.

Note that slightly different thresholds are used in fatty liver disease: many guidelines for NAFLD/MASLD use 1.3 as the low-risk cut-off and 2.67 as the high-risk cut-off, because these values perform better in that population. If your result falls between the hepatitis C and NAFLD thresholds (for example 1.35 or 2.9), the calculator flags this in the interpretation.

Limitations and Practical Tips

Age extremes: the score is less reliable in adults under 35 and over 65. In younger patients it tends to underestimate risk; in older patients it may overestimate it.
Not a diagnosis: FIB-4 is a screening tool, not a substitute for elastography, imaging or biopsy. It estimates probability, not certainty.
Other causes of abnormal values: thrombocytopenia from non-liver causes, acute hepatitis flares, or AST/ALT elevations from muscle injury or intense exercise can distort the score.
Transient results: if your liver enzymes were elevated during an acute illness, it is worth recalculating the score once values have stabilized.
Not validated in: transplant recipients, patients on anticoagulants affecting platelet counts, or children.

For a broader picture of liver-related risk, the MELD-Na Score estimates short-term mortality in advanced liver disease, while the eGFR calculator (CKD-EPI 2021) and the LDL cholesterol calculator help assess metabolic and cardiovascular factors that often accompany fatty liver disease. Always discuss your FIB-4 result with a healthcare professional who knows your full medical history.

Frequently Asked Questions

What is a normal FIB-4 score?

There is no "normal" reference range like for lab values — FIB-4 is a risk index. A score below 1.45 is considered low risk of advanced fibrosis in most guidelines (below 1.3 in fatty liver disease). A healthy middle-aged adult with normal liver tests often scores well under 1.0. Values above 3.25 (2.67 in NAFLD) indicate high risk and warrant specialist evaluation.

Why is my FIB-4 high even though my AST and ALT are normal?

The score depends on more than enzymes: age sits in the numerator, so adults over 65 naturally score higher, and the result is easily pushed up by a mildly low platelet count in the denominator. If your enzymes are normal and the score is only moderately elevated, the result may be a false positive — commonly seen in older patients. Discuss it with your doctor rather than assuming advanced fibrosis.

Can I calculate FIB-4 by hand with a regular calculator?

Yes. Multiply your age by your AST (in U/L), then divide by your platelet count (in ×10⁹/L) multiplied by the square root of your ALT (in U/L). For example: \(55 \times 45 = 2475\), \(\sqrt{50} \approx 7.07\), \(180 \times 7.07 \approx 1272.8\), and \(2475 / 1272.8 \approx 1.94\). Just make sure all inputs use the correct units before you start.

My lab reports platelets as 10³/µL or K/µL — how do I enter that?

These units are numerically identical to ×10⁹/L, so just enter the number as printed. A result of "180 K/µL", "180 ×10³/µL" and "180 ×10⁹/L" all mean 180 in the calculator. If your lab reports platelets per liter as "180,000/µL" is not typical, but "180 ×10⁹/L" is — a value like 180,000 would be a unit mix-up and produce a meaningless score.

How is FIB-4 different from APRI or FibroTest?

All three are non-invasive fibrosis markers, but they use different inputs. FIB-4 needs only age, AST, ALT and platelets — the cheapest and most widely available option. APRI uses only AST, the AST upper limit of normal and platelets. FibroTest (FibroSure) is a proprietary panel including GGT, haptoglobin and bilirubin that must be ordered commercially. FIB-4 is usually the first-line screen; the others are typically second-line.

Can FIB-4 detect cirrhosis or liver cancer?

FIB-4 estimates the probability of advanced fibrosis (METAVIR F3–F4), which includes cirrhosis, but it cannot diagnose either condition or detect liver cancer at all. A high score means further testing is needed, not that you have cirrhosis. Cancer surveillance (e.g. ultrasound and AFP every 6 months) is a separate process for patients already known to have cirrhosis.

How often should I recalculate my FIB-4 score?

For monitoring in fatty liver disease, guidelines generally suggest repeating FIB-4 every 1–3 years alongside routine blood work, since fibrosis develops slowly. Also recalculate after any acute illness, medication change or exercise-related enzyme spike has settled, because transient AST/ALT elevations distort the result. A single score is a snapshot — trends over time are more informative than one value.