FIB-4 calculator

What the FIB-4 number tells you

Fatty liver is common, and most people who have it never develop serious scarring. The hard part is finding the few who do without giving everyone a biopsy or a scan. FIB-4 is the cheap first filter that guidelines recommend: it uses a routine blood count and liver enzymes. The AASLD 2023 guidance calls it the most validated of the blood-based scores and says it is best at identifying patients with a low probability of advanced fibrosis. A low result is the useful one. An intermediate or high result means another test is needed, not that a diagnosis has been made.

What is the FIB-4 index?

FIB-4 is a blood-test score that estimates how likely advanced liver scarring (fibrosis stage F3 or F4) is. It uses four numbers that almost every patient with liver disease already has: age, AST, ALT and platelet count. It was published by Sterling and colleagues in Hepatology in 2006, built on biopsies from 832 people with HIV and hepatitis C. It was later tested in fatty liver disease, which is where it is most used today.

What is the FIB-4 formula?

FIB-4 = (age in years × AST in U/L) ÷ (platelet count in 10⁹/L × the square root of ALT in U/L). Platelets are the same number as the “K/µL” most US labs print, so 200 K/µL is 200. The square root is the part people forget: dividing by ALT itself, not its square root, gives a number that is far too small.

What do the cut-offs 1.3 and 2.67 mean?

They come from a 2009 study of 541 adults with fatty liver disease (Shah and colleagues). A score of 1.30 or lower had a negative predictive value of 90 percent for advanced fibrosis, and 2.67 or higher had a positive predictive value of 80 percent. About 30 percent of the patients fell between the two numbers, where the score alone cannot answer the question. The AASLD 2023 practice guidance uses the same three bands: below 1.3 low, 1.3 to 2.67 intermediate, above 2.67 high.

Why are some sources quoting 1.45 and 3.25?

Those were the cut-offs in the original 2006 paper, which studied people with HIV and hepatitis C, not fatty liver. In that group a value under 1.45 ruled out advanced fibrosis with a negative predictive value of 90 percent, and a value over 3.25 had a positive predictive value of 65 percent. Different diseases have different fibrosis rates, so the cut-offs differ. This page uses the 1.3 and 2.67 values that the AASLD guidance uses for fatty liver disease.

Why is there a different cut-off after age 65?

Age is in the numerator, so older people score higher even with the same liver. The AASLD guidance says that over age 65 a cut-off of 2.0 should be used instead of 1.3. The page shows both the standard band and the age-adjusted band when the age is above 65. In the second example below, a 70-year-old scores 1.87: intermediate with the standard cut-offs, but low with the age-adjusted lower limit.

What does an intermediate result mean?

The guidance treats it as a result that needs a second step. The AASLD document describes vibration-controlled transient elastography (VCTE, often called FibroScan) as a common next test, where a stiffness under 8 kPa makes advanced fibrosis unlikely. That choice, and what to do with the answer, belongs to the clinician who knows the patient.

Who should not be assessed with FIB-4?

The AASLD guidance says FIB-4 should not be used in acutely ill patients, and gives no cut-off for that setting. It also says the score is less accurate under age 35. The original studies were done in people with chronic liver disease, so a value in someone with a short-lived illness is hard to interpret.

Worked example

A 55-year-old has an AST of 40 U/L, an ALT of 36 U/L and platelets of 200. The numerator is 55 × 40 = 2,200. The denominator is 200 × √36 = 200 × 6 = 1,200. FIB-4 = 1.83, which sits between 1.3 and 2.67: the intermediate band. A 70-year-old with AST 28, ALT 25 and platelets 210 scores 1.87: intermediate by the standard cut-offs but under the age-adjusted lower limit of 2.0. A 30-year-old with AST 22, ALT 20 and platelets 250 scores 0.59, well below 1.3, though the guidance warns that accuracy is lower under 35.

Limits of this method

FIB-4 was built to rule out advanced fibrosis, not to stage it. Shah and colleagues found that when the specificity was fixed at 90 percent (a FIB-4 of 1.93) the sensitivity for advanced fibrosis was only 50 percent, so a middling score misses many patients. The score depends on AST and ALT, which fluctuate; one reading can be unrepresentative. Platelet counts can fall for reasons unrelated to the liver, and that pushes FIB-4 up (our reasoning from the formula, not a quoted finding). The AASLD guidance also notes that the cut-offs may change as more data accumulate for people with type 2 diabetes, in whom the tests perform less robustly.

Common mistakes

Entering the platelet count as 200,000 instead of 200 makes the result a thousand times too small. Using the same-day ALT and AST from an acute illness, or from a blood draw after a hard workout, is another. A third is reading 1.3 as a diagnosis threshold: it is the point below which advanced fibrosis is unlikely, and a value of 1.4 does not mean a patient has scarring.

Sources: 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;43:1317–1325; Shah AG, Lydecker A, Murray K, et al., “Comparison of noninvasive markers of fibrosis in patients with nonalcoholic fatty liver disease,” Clin Gastroenterol Hepatol 2009;7:1104–1112; Rinella ME, Neuschwander-Tetri BA, Siddiqui MS, et al., “AASLD Practice Guidance on the clinical assessment and management of nonalcoholic fatty liver disease,” Hepatology 2023;77:1797–1835. Related: MELD score, Child-Pugh score, BMI calculator.

For general information only — not medical advice, and not a tool for patient care decisions. Talk with a doctor about any liver test result. How we check our calculators.