FIB-4 Liver Fibrosis Calculator

Understanding the FIB-4 liver fibrosis calculator — concept and principle

The FIB-4 index estimates the likelihood of advanced liver scarring (fibrosis) from age and three routine blood tests. It is the recommended first step for people with metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD) and is also used in viral hepatitis. Its main job is to identify who can safely stay in primary care and who needs a second test such as elastography.

Age and AST sit on top, platelets and √ALT below; the default result 0.95 is under 1.3, the low-risk band.

How it is calculated

FIB-4 = (age × AST) ÷ (platelets × √ALT)

Age in years.
AST and ALT in U/L.
Platelets in × 10⁹/L (the same number as thousands per µL).

The calculator uses the MASLD cut-offs: below 1.3 (2.0 from age 65) low risk of advanced fibrosis, above 2.67 high risk, and values in between indeterminate.

The principle

Each ingredient changes in a predictable way as fibrosis advances. AST rises relative to ALT because damaged liver releases more AST and clears it less well. Platelets fall because portal hypertension enlarges the spleen, which traps them, and because the liver makes less thrombopoietin. Age is included because fibrosis accumulates over time. Multiplying the "up" factors and dividing by the "down" factors makes the index grow as scarring increases. The square root on ALT simply gave the best fit in the original data.

Worked example

Numerator: 35 × 40 = 1,400.
√45 = 6.708.
Denominator: 220 × 6.708 = 1,475.8.
FIB-4 = 1,400 ÷ 1,475.8 = 0.9486, rounded to 0.95.
At age 35 the lower cut-off used is 1.3; 0.95 is below it, so the result is low risk of advanced fibrosis.

Each point is the calculator re-run with only “Platelets (× 10⁹/L)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Platelets (× 10⁹/L)	FIB-4
55	3.79
110	1.9
165	1.26
220	0.95
275	0.76
330	0.63
440	0.47

How to read the result

Where does your result fall?

A value below 1.3 has a high negative predictive value: advanced fibrosis is unlikely and repeating the test in a few years is a common approach. Above 2.67, advanced fibrosis is likely and specialist referral is usual. The middle zone is where a second test, typically vibration-controlled transient elastography or the ELF blood test, is recommended. The original 2006 study in HIV/hepatitis C co-infection used cut-offs of 1.45 and 3.25.

Common mistakes

Forgetting the age rule: from age 65 the calculator switches the lower cut-off to 2.0 to avoid false positives, and shows which cut-off it used.
Using it under about 35 years of age, where it is less reliable.
Calculating it during acute hepatitis, when very high transaminases distort the ratio.

Keep in mind: FIB-4 is not a diagnosis. Low platelets from other causes, intense exercise or muscle injury (raising AST) and age can mislead it. It does not measure fat or inflammation. Interpretation belongs to a clinician.

Frequently asked questions

Is a FIB-4 of 1.5 bad?

It is indeterminate, not proof of fibrosis; it means a second test is advised.

Can FIB-4 go down?

Yes. Weight loss, treating hepatitis or stopping alcohol can improve liver tests and lower the index.

Is FIB-4 better than an ultrasound?

They answer different questions. Routine ultrasound shows fat and gross changes but is poor at staging fibrosis.

Sources: Sterling RK et al. Development of a simple noninvasive index to predict significant fibrosis in patients with HIV/HCV coinfection. Hepatology 2006. · Shah AG et al. Comparison of noninvasive markers of fibrosis in patients with nonalcoholic fatty liver disease. Clin Gastroenterol Hepatol 2009.

Checked 8 Nov 2026. Sterling et al., Hepatology 2006.

Your numbers