FIB-4 = (age × AST) ÷ (platelets × √ALT). In fatty liver disease, a score below 1.3 means advanced fibrosis is unlikely, 1.3 to 2.67 is indeterminate and needs a second test such as elastography, and above 2.67 means advanced fibrosis is likely. From age 65, the low cutoff rises to 2.0.
This calculator gives the FIB-4 index with the AASLD 2023 risk band, applies the age 65 rule automatically, shows the original 2006 cutoffs for comparison and accepts platelets in 10⁹/L or per µL. It is written for clinicians and students.
How to use this calculator
- Enter the patient's age in years.
- Enter AST and ALT in U/L (IU/L is the same thing).
- Enter the platelet count. Most labs report it as 10⁹/L, which equals 10³/µL; switch the unit if yours reads like 250,000.
- Read the FIB-4 index and the band, then plan the next step.
FIB-4 formula
Sterling and colleagues (Hepatology, 2006) derived the index with logistic regression on liver biopsies from 832 people with HIV and hepatitis C:
FIB-4 = (age [years] × AST [U/L]) ÷ (platelets [10⁹/L] × √ALT [U/L])
Each term pushes the score in the direction scarring would. Older age and higher AST raise it. Low platelets, a sign of portal hypertension and spleen enlargement, raise it too. ALT sits under a square root in the denominator, so a high ALT with a normal AST lowers the score. That reflects the AST-to-ALT ratio rising as fibrosis advances. The index had an area under the curve of 0.765 for telling Ishak stage 0–3 from stage 4–6, and using its two cutoffs would have avoided biopsy in 71% of the validation group.
FIB-4 cutoffs and what they mean
| FIB-4 (age 35–64) | FIB-4 (age 65+) | Band | Next step (AASLD 2023) |
|---|---|---|---|
| <1.3 | <2 | Low risk: advanced fibrosis unlikely | Manage in primary care; repeat every 1–3 years depending on metabolic risk |
| 1.3–2.67 | 2–2.67 | Indeterminate | Secondary test: vibration-controlled elastography or ELF, or refer |
| >2.67 | >2.67 | High risk | Second test and referral to hepatology |
The AASLD guidance lists FIB-4 below 1.3 as the rule-out cutoff and 2.67 or more as the rule-in cutoff for advanced fibrosis. It also notes FIB-4 is not accurate under 35, that a lower rule-out threshold applies to high-risk people with a high pre-test probability, and that its cutoffs in type 2 diabetes may change as more data appear. For cirrhosis specifically, it gives a rule-out cutoff below 1.67 and a rule-in cutoff of 3.48 or more.
Original and newer cutoffs compared
| Cutoff set | Population | Low (rule out) | High (rule in) |
|---|---|---|---|
| Sterling 2006 | HIV/hepatitis C coinfection | <1.45 | >3.25 |
| AASLD 2023 | Fatty liver disease (NAFLD/MASLD) | <1.3 | ≥2.67 |
| McPherson 2017, adopted by AASLD | Fatty liver disease, age 65+ | <2 | — |
| AASLD 2023, cirrhosis | Fatty liver disease | <1.67 | ≥3.48 |
Worked examples
Five sample patients scored with the calculator's own function (platelets in 10⁹/L).
| Patient | Age | AST | ALT | Platelets | FIB-4 | Band |
|---|---|---|---|---|---|---|
| Normal liver tests, middle-aged | 45 | 22 | 25 | 260 | 0.76 | low |
| Fatty liver, raised ALT | 52 | 38 | 64 | 240 | 1.03 | low |
| Same patient 20 years later | 72 | 38 | 64 | 240 | 1.43 | low |
| AST above ALT, lower platelets | 58 | 55 | 40 | 150 | 3.36 | high |
| Advanced disease pattern | 63 | 80 | 50 | 95 | 7.50 | high |
The second and third rows are the same blood results 20 years apart. At 52 the score is 1.03; at 72 it is 1.43, purely because of age. With the standard 1.3 cutoff the older patient would look indeterminate; with the age 65 cutoff of 2.0 the result is low. That is the false-positive problem McPherson set out to fix.
Why FIB-4 is the first test in fatty liver disease
The AASLD calls FIB-4 statistically inferior to the ELF panel, FIBROSpect II and elastography for spotting advanced fibrosis, but still recommends it first for general practitioners and endocrinologists because it is simple and costs little or nothing extra. Its strength is a high negative predictive value: in a population where advanced fibrosis is uncommon, a low score reliably means it is absent. The guidance also notes that high FIB-4 values are linked to all-cause and liver-related outcomes in population studies, and that movement between bands over time can track progression.
Metabolic risk matters for timing. People with prediabetes, type 2 diabetes or two or more metabolic risk factors progress faster, so the AASLD suggests repeating FIB-4 every 1–2 years, against every 2–3 years for lower-risk patients.
Common mistakes
- Wrong platelet units. Entering 250,000 as if it were 10⁹/L makes FIB-4 a thousand times too small. Use 250.
- Using 1.3 in older adults. From 65, use 2.0 as the low cutoff or many healthy livers will look indeterminate.
- Trusting a low score under 35. FIB-4 is unreliable at this age.
- Scoring during acute illness. Acute hepatitis, heavy drinking or another cause of low platelets distorts the result.
- Mixing up the cutoff sets. 1.45 and 3.25 are for HIV/hepatitis C; 1.3 and 2.67 are for fatty liver disease.
More lab tools: albumin/globulin ratio, BUN/creatinine ratio, anion gap, A1C to average glucose and CHA₂DS₂-VASc stroke risk.
For healthcare professionals and students only. FIB-4 estimates the likelihood of advanced liver fibrosis to guide further testing; it does not diagnose liver disease, and this tool offers no medical advice. Yellowing of the skin, vomiting blood, black stools or confusion need urgent care. Patients should discuss their results with their own clinician.
Frequently asked questions
What is the FIB-4 score?
FIB-4 (fibrosis-4 index) estimates the chance of advanced liver scarring from four routine values: age, AST, ALT and platelet count. Sterling and colleagues developed it in 2006 in 832 people with HIV and hepatitis C. It is now the first-line fibrosis test for fatty liver disease in primary care.
How is FIB-4 calculated?
FIB-4 = (age in years × AST in U/L) ÷ (platelets in 10⁹/L × √ALT in U/L). For example, a 50-year-old with AST 40, ALT 25 and platelets 200 has FIB-4 = (50 × 40) ÷ (200 × 5) = 2.00.
What is a normal FIB-4 score?
There is no single normal value, but in fatty liver disease a FIB-4 below 1.3 is low risk: advanced fibrosis is unlikely and the AASLD says patients can be followed in primary care. From age 65 the AASLD uses 2 instead.
What does a FIB-4 above 2.67 mean?
It means advanced fibrosis (stage F3–F4) is likely. The AASLD 2023 guidance lists 2.67 as the rule-in cutoff for advanced fibrosis, and anyone at 1.3 or above should have a second test, such as vibration-controlled elastography (FibroScan) or the ELF blood test, or be referred.
What does an indeterminate FIB-4 mean?
A score from 1.3 to 2.67 is neither reassuring nor diagnostic. The AASLD recommends a secondary assessment, preferably elastography or ELF, rather than a liver biopsy as the next step.
Why is FIB-4 different for people over 65?
Age is in the formula, so FIB-4 rises with age even when the liver is unchanged. McPherson and colleagues found specificity for advanced fibrosis fell to 35% in people 65 and over with the 1.3 cutoff. A cutoff of 2.0 raised specificity to 70% with 77% sensitivity, and the AASLD adopted 2.0 for older adults.
Is FIB-4 accurate under 35?
No. McPherson found FIB-4 performed poorly at 35 and under (area under the curve 0.60). The AASLD says a second test should be considered in people under 35 with raised liver tests or several metabolic risk factors, whatever their FIB-4.
What are the original FIB-4 cutoffs of 1.45 and 3.25?
They come from Sterling's 2006 HIV/hepatitis C study. Below 1.45, FIB-4 had a 90% negative predictive value for advanced fibrosis (Ishak stage 4–6) with 70% sensitivity; above 3.25, it had a 65% positive predictive value and 97% specificity. Fatty liver guidance uses 1.3 and 2.67 instead.
Can FIB-4 detect cirrhosis?
The AASLD lists FIB-4 ≥3.48 as a rule-in cutoff for cirrhosis (90% specificity) and below 1.67 as a rule-out cutoff (90% sensitivity). Elastography or specialist assessment is still needed to confirm it.
How often should FIB-4 be repeated?
The AASLD suggests every 2–3 years for low-risk patients without prediabetes or diabetes and with one or two metabolic risk factors, and every 1–2 years for those with prediabetes, type 2 diabetes or two or more metabolic risk factors.
When should FIB-4 not be used?
The AASLD says FIB-4 should not be used in acutely ill patients. A sudden rise in AST or ALT, for example from hepatitis or alcohol, or a low platelet count from another cause will push the score up without reflecting scarring.
Is NAFLD the same as MASLD?
Yes, broadly. In 2023 a multisociety Delphi consensus renamed nonalcoholic fatty liver disease (NAFLD) as metabolic dysfunction-associated steatotic liver disease (MASLD). The FIB-4 cutoffs in the AASLD guidance were published under the older name and are applied to MASLD.
Sources & method
- Sterling et al. (2006), Hepatology — Development of a simple noninvasive index to predict significant fibrosis in patients with HIV/HCV coinfection (FIB-4)
- Rinella et al. (2023), Hepatology — AASLD Practice Guidance on the clinical assessment and management of nonalcoholic fatty liver disease
- McPherson et al. (2017), Am J Gastroenterol — Age as a confounding factor for the accurate non-invasive diagnosis of advanced NAFLD fibrosis
- Rinella et al. (2023), Hepatology — A multisociety Delphi consensus statement on new fatty liver disease nomenclature (MASLD)
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