What does a FIB-4 score mean, and what number rules out liver scarring?

FIB-4 was published in 2006 by Sterling and colleagues, who built it from routine blood results in 832 patients with HIV and hepatitis C coinfection who had liver biopsies. The formula they printed is:

FIB-4 = (age in years x AST in U/L) / (platelet count in 10⁹/L x square root of ALT in U/L)

If your report prints platelets as thousands per microliter (for example 220 x10³/uL), that is the same number as 220 x10⁹/L, so the figure goes into the formula unchanged (our arithmetic: both units equal 220 billion per liter). Age and AST sit on top, so the score rises with age and with AST. Platelets and ALT sit underneath, so a falling platelet count pushes it up. The AGA's 2021 pathway notes that a low platelet count (thrombocytopenia) is highly suggestive of cirrhosis, which on our reading is the logic of having platelets in the formula. The age term is what causes trouble at both ends of adult life, covered below.

The first three rows carry identical blood results. Only the age changes, and the score more than doubles between 30 and 70.

The cutoffs used in fatty liver disease today, 1.3 and 2.67, were set in a 2009 study by Shah and colleagues in 541 adults with biopsy-proven fatty liver from the NIH NASH Clinical Research Network. They chose them as the points that best separated the absence (1.3) and presence (2.67) of advanced fibrosis. A score at or under 1.30 had a 90 percent negative predictive value, and a score at or above 2.67 had an 80 percent positive predictive value.

The AASLD's 2023 practice guidance on fatty liver disease and the AGA's 2021 clinical care pathway both adopted those lines. Both were written when the condition was called NAFLD; it has since been renamed MASLD. An AASLD commentary in 2024 states that its guidance can be used for the diagnosis, risk stratification and management of MASLD. In March 2026 the AGA replaced its 2021 pathway with one written for MASLD; in the AGA's published summary, which we read (the full 2026 text was not open to us), it keeps the 1.3 line and applies 2.0 to those 65 and older.

The Shah data show what those percentages mean in people. Of the 327 participants who scored under 1.3, 294 truly had no advanced fibrosis and 33 did; the score missed them. Of 125 participants who did have advanced fibrosis, the 2.67 line caught 41, while wrongly flagging 10 of the 416 without it. And 163 participants, 30 percent, landed in the indeterminate middle. On our reading, that pattern (few false alarms above 2.67, a wide uncertain middle) is why both bodies treat FIB-4 as a first filter rather than an answer.

The phrase people search for is whether FIB-4 rules out liver scarring. Strictly, it rules out advanced scarring. In the NASH CRN system used by the Shah study that set the 1.3 cutoff, fibrosis is staged from 0 to 4, and advanced fibrosis meant stage 3 or 4: bridging fibrosis or cirrhosis. The AGA's 2021 pathway said it directly: all noninvasive fibrosis scores are more accurate in distinguishing patients with from those without advanced fibrosis (F3 or higher), so the cutoffs might miss some patients with F2 at an initial assessment, who should be detected at later assessments as they progress. Its 2026 replacement, in the AGA's summary, goes further: clinically significant risk begins at fibrosis stage 2, and the pathway sets out to identify F2 or higher.

That matters because the AASLD treats stage F2 as the start of what it calls at-risk disease: it writes that patients with steatohepatitis and F2 to F4 fibrosis are at higher risk for liver-related events and mortality. A low FIB-4 is therefore a reason to repeat the score on schedule, not a permanent all-clear.

A second limit appears when FIB-4 is used on the general population rather than on people already suspected of fatty liver. In a 2022 study of 5,129 people from five cohorts in Spain, Hong Kong, Denmark, England and France, 3,979 from the general population and 1,150 from groups at risk through alcohol, diabetes or obesity (we read the abstract), 552 had a liver stiffness of 8 kPa or more, and 239 of those, 43 percent, had a normal FIB-4. FIB-4 was falsely negative in 11 percent of people with diabetes, and 28 to 29 percent of raised FIB-4 or NAFLD fibrosis score results were false positives. The AASLD does not advise general population screening for fatty liver; it recommends FIB-4 for people with fatty liver on imaging, or suspected fatty liver with obesity and metabolic risk factors.

FIB-4, the NAFLD fibrosis score and the AST/ALT ratio were developed and validated principally in patients aged 35 to 65, McPherson and colleagues write. McPherson and colleagues tested what happens outside that window in 634 patients with biopsy-proven fatty liver from four European specialist clinics, published in the American Journal of Gastroenterology in 2017.

Both guidelines took this up. The AASLD guidance states: in patients older than age 65, a FIB-4 cutoff of greater than 2.0 should be used, and FIB-4 has low accuracy in those under age 35, so secondary assessment should be considered in those under 35 with increased metabolic risk or elevated liver chemistries. The AGA's 2021 pathway wrote the low-risk line as under 2.0 in those older than 65; the AGA's summary of its 2026 pathway says below 2.0 for those 65 and older, which matches the study's own group. The AASLD still says older than 65.

The worked example shows the under-35 problem: at 30, the age term is so small that the score sits low, so a reassuring number at that age reassures less.

FIB-4 began in hepatitis C, and the cutoffs printed with it there are different numbers. In the HIV and hepatitis C derivation, a score under 1.45 in the validation set had a negative predictive value of 90 percent for excluding advanced fibrosis (Ishak stage 4 to 6), with a sensitivity of 70 percent, and a score over 3.25 had a positive predictive value of 65 percent and a specificity of 97 percent. The University of Washington's Hepatitis C Online FIB-4 calculator, read on 28 October 2026, interprets the score with that pair.

The formula is identical; the cutoffs are not. If your result is being read for fatty liver, 1.3 and 2.67 are the pair in the AASLD's 2023 guidance. At the upper line the difference is small in fatty liver data: the AASLD's 2025 guideline on blood-based tests found a pooled specificity of 0.94 for both 2.67 and 3.25 across 32 studies.

Acute illness. The AASLD says FIB-4 should not be used in acutely ill patients.

Under 35. Low accuracy, per the AASLD and the McPherson data above.

Diabetes. The AASLD notes that the performance of noninvasive tests may be less robust in patients with diabetes, that the FIB-4 cutoffs for type 2 diabetes may change as data accumulate. Separately, its table of tests notes beside FIB-4 a "lower rule-out threshold among high-risk individuals who have high pretest probability" without printing a number.

Anything that moves an input for a non-liver reason. On our reading of the formula, a raised AST from hard exercise or muscle injury, or a platelet count lowered by a blood or bone marrow condition, will raise the score without any change in the liver. No guideline we read names these as exclusions; it is arithmetic, so ask whether an unusual input reflects the liver.

The next step in both pathways is usually a liver stiffness measurement, most often vibration-controlled transient elastography (FibroScan). In the AGA's summary of its 2026 pathway, a stiffness under 8 kPa or an ELF score under 9.2 generally means low risk, with repeat testing every one to two years; higher values should prompt referral to hepatology; and 10 kPa or more, or an ELF of 9.8 or more, suggests clinically significant fibrosis. The AGA's 2021 pathway had used 8.0 and 12.0 kPa, and treated a FIB-4 above 2.67 together with a stiffness of 12.0 kPa or more as highly suggestive of advanced fibrosis. The AASLD also accepts the ELF blood test or magnetic resonance elastography as the second step.

For cirrhosis specifically, the AASLD's table gives separate FIB-4 points: 3.48 or above to rule cirrhosis in and under 1.67 to rule it out, chosen for 90 percent specificity and 90 percent sensitivity respectively.

Find the four numbers: your age and the AST, ALT and platelet count from one blood draw. The AGA's 2026 pathway summary notes that routine lab results provide the data needed to calculate it, and the formula above does it. Our ALT reference page carries the ACG's healthy range with its source and date, the above the healthy limit page sets out the guideline workup for a raised result, and the lab result lookup will place an ALT value against those bands. If the score is 1.3 or more (2.0 or more if you are older than 65), ask the clinician who ordered the test whether a liver stiffness scan is the next step, as both pathways recommend.

Our guide to a high ALT on a blood test covers the causes to rule out before anyone reaches for a score, and the metabolic numbers worth knowing after 40 covers the blood sugar, lipid and blood pressure results that decide how often the AASLD wants FIB-4 repeated. To see which screenings apply at your age, build your personal screening plan.