FIB-4 formula: how the fibrosis-4 index is calculated

FIB-4 (fibrosis-4) is an index built from age, two lab values (AST and ALT) and the platelet count: age times AST, divided by platelets times the square root of ALT. Age 50, AST 100 U/L, ALT 40 U/L and platelets 200 × 10^9/L give a FIB-4 of 3.95.

Not a medical device. Not a diagnosis or dosing recommendation.

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Liver calculators
Sterling RK et al. · 2006
The FIB-4 formula
Worked examples
Frequently asked questions

What is FIB-4?

FIB-4 comes from Richard Sterling and colleagues, who described it in Hepatology in 2006. Their abstract reports a retrospective look at liver histology in patients with HIV/HCV coinfection, with the aim of predicting liver fibrosis from routine tests. Platelet count, age, AST and ALT made up the model that became the index.

Magent's calculator returns the index to 2 decimal places and a label, sterling_band, from the two cut-points in the 2006 paper: low below 1.45, high above 3.25, and indeterminate from 1.45 to 3.25.

This page explains the arithmetic and the labels. It doesn't say what a value means for a particular person.

The FIB-4 formula

FIB-4 = (age in years × AST in U/L) ÷ (platelets in 10^9/L × √ALT in U/L)
rounded to 2 decimals
label: low below 1.45 · indeterminate 1.45 to 3.25 · high above 3.25

Exact expression the calculator uses:

FIB-4 = (age_years * AST_U_L) / (platelets_10e9_L * sqrt(ALT_U_L))

In words: multiply age by AST, then divide by the platelet count times the square root of ALT.

Inputs

Input	Meaning	Accepted range
`age`	Age in years, a whole number	18 to 120
`ast_u_l`	Aspartate aminotransferase (AST), U/L	1 to 10000
`alt_u_l`	Alanine aminotransferase (ALT), U/L	1 to 10000
`platelets_10e9_l`	Platelet count, 10^9/L	1 to 2000

How the calculation works

Top: age × AST.
Bottom: platelet count × the square root of ALT.
Divide the top by the bottom.
Round to 2 decimal places.
Label the rounded value: below 1.45 is low, above 3.25 is high, and 1.45 to 3.25, both ends included, is indeterminate.

Age and AST sit on top, so a higher age or AST raises the index. Platelets and ALT sit underneath, so a higher platelet count or ALT lowers it. ALT enters as its square root, which damps its effect.

Units

Age is in whole years. AST and ALT are in U/L. The platelet count is in 10^9/L. A missing value, an age that isn't a whole number, or a value outside its range is rejected with an error. The calculator returns the four inputs, the rounded index and the label.

Worked examples

These use Magent's own expected values for the calculator. ALT is 40 U/L in each.

Example 1: age 50, AST 100, platelets 200

Top: 50 × 100.
Bottom: 200 × √40.
Divide and round to 2 decimals: FIB-4 3.95.
3.95 is above 3.25 → label high.

Example 2: age 50, AST 60, platelets 200

Top: 50 × 60.
Bottom: 200 × √40.
Divide and round to 2 decimals: FIB-4 2.37.
2.37 is from 1.45 to 3.25 → label indeterminate.

Example 3: age 40, AST 30, platelets 250

Top: 40 × 30.
Bottom: 250 × √40.
Divide and round to 2 decimals: FIB-4 0.76.
0.76 is below 1.45 → label low.

Examples 1 and 2 differ in AST alone, and the result moves from high to indeterminate.

History of FIB-4

The index comes from Sterling RK, Lissen E, Clumeck N and colleagues, Development of a simple noninvasive index to predict significant fibrosis in patients with HIV/HCV coinfection, Hepatology, 2006 (volume 43, issue 6, pages 1317–1325). The abstract describes grading fibrosis on liver histology with the Ishak score, splitting patients into one set to build the index and a second set to test it, and reporting results in that second set at a cutoff below 1.45 and a cutoff above 3.25.

When it's used

The 2006 study set out to predict liver fibrosis from routine tests instead of histology alone, in patients with HIV/HCV coinfection.

Where it's used

The index needs age and three routine lab results: AST, ALT and the platelet count.

Who uses it

The person using the calculator enters age and the three lab values. The calculator multiplies, divides, rounds and applies the 2006 labels.

Limits of the calculation

The labels are the 2006 cut-points. They aren't a biopsy and don't stage fibrosis.
The calculator doesn't apply other cut-points, such as ones proposed for NAFLD, unless they happen to match the 2006 values.
The 2006 study was in patients with HIV/HCV coinfection.
The label is set from the rounded value.
This page doesn't say what to do with a result.

Clinical decisions belong to a qualified clinician who knows the patient.

APRI uses AST, the lab's AST upper limit of normal and the platelet count. It doesn't use age or ALT.
NAFLD fibrosis score adds BMI, IFG/diabetes and albumin to age, AST, ALT and platelets.
MELD-Na uses creatinine, bilirubin, INR, sodium and dialysis, not liver enzymes or platelets.
Maddrey discriminant function uses patient PT, control PT and bilirubin.

Glossary

AST: aspartate aminotransferase, entered in U/L.
ALT: alanine aminotransferase, entered in U/L.
Platelet count: entered in 10^9/L.
Ishak score: the histology scale the 2006 study used to grade fibrosis.
HIV/HCV coinfection: infection with both HIV and hepatitis C virus.

Common mistakes

Swapping AST and ALT. AST goes on top; ALT goes under the square root on the bottom.
Forgetting the square root. ALT enters as √ALT, not ALT.
Entering platelets in the wrong unit. The calculator expects 10^9/L.
Reading 1.45 or 3.25 as outside the middle label. Both are indeterminate.
Treating a label as a stage. The labels are cut-points from one study, not a fibrosis stage.

Source

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. DOI: 10.1002/hep.21178. PMID: 16729309

Frequently asked questions

How do you calculate FIB-4?

Multiply age by AST, then divide by the platelet count times the square root of ALT. Age 50, AST 100, ALT 40 and platelets 200 give 3.95.

What do the FIB-4 labels low, indeterminate and high mean?

They're the cut-points from the 2006 paper: below 1.45 is low, above 3.25 is high, and anything from 1.45 to 3.25 is indeterminate. The calculator applies them to the rounded value. They aren't a fibrosis stage.

Is a FIB-4 of exactly 3.25 high?

No. 3.25 is indeterminate. The high label starts above 3.25.

What units does FIB-4 use?

Age in years, AST and ALT in U/L, and platelets in 10^9/L.

How many decimals does the calculator report?

The label is set from that rounded value.