FIB-4 calculator: liver fibrosis score from age, AST, ALT, and platelets
FIB-4 is a score that estimates how likely it is that the liver has advanced scarring, called fibrosis. It needs nothing beyond a routine blood draw: your age, AST, ALT, and platelet count. Liver guidelines use it as the first step for people with fatty liver disease, because a low score rules out advanced fibrosis well enough that most people need no further testing. Enter results from the same draw below. AST and ALT are on the metabolic or liver panel, and platelets are on the complete blood count.
The formula
FIB-4 was published by Sterling and colleagues in 2006:
FIB-4 = (age × AST) ÷ (platelets × √ALT)
Age is in years, AST and ALT in U/L, and platelets in 10^9/L. Platelet counts printed as K/µL, 10^3/µL, or thou/µL are the same number, so a count of 230 K/µL is entered as 230. A few labs print the raw count, such as 230,000 per µL, and the calculator divides that by 1,000. If your liver enzymes are in µkat/L, multiply by 60 to get U/L, or pick µkat/L in the calculator.
The square root on ALT and the age on top are why the score behaves the way it does. It rises with age even when nothing else changes, and it rises when AST is high relative to ALT or when platelets fall. Scarring in the liver tends to do both.
| Age | AST | ALT | Platelets | FIB-4 | Reading |
|---|---|---|---|---|---|
| 45 | 25 | 30 | 250 | 0.82 | Low risk |
| 52 | 38 | 45 | 210 | 1.40 | Indeterminate |
| 58 | 45 | 40 | 160 | 2.58 | Indeterminate |
| 68 | 30 | 30 | 200 | 1.86 | Low risk (age 65 or older) |
| 62 | 70 | 55 | 110 | 5.32 | High risk |
AST and ALT in U/L, platelets in 10^9/L. Readings use the fatty liver cutoffs.
What the score means
For fatty liver disease, now called MASLD, the cutoffs come from Shah and colleagues, who tested FIB-4 in the NASH Clinical Research Network. The AASLD’s 2023 practice guidance uses the same two numbers:
| FIB-4 | Reading | What usually follows |
|---|---|---|
| Below 1.3 | Low risk of advanced fibrosis | Follow-up in primary care, with the score repeated every 1 to 3 years |
| 1.3 to 2.67 | Indeterminate | A second test, such as elastography (FibroScan) or the ELF blood test |
| Above 2.67 | High risk | Referral to a liver specialist is considered |
In Shah’s data, a score of 1.30 or less had a negative predictive value of about 90% for advanced fibrosis, and 2.67 or more a positive predictive value of about 80%. The low cutoff is the more useful one. A low score is reliable. A high score is a reason to look further, not a diagnosis of scarring.
Age 65 and older
Because age is in the formula, older people score higher whether or not their liver is scarred. McPherson and colleagues found that with the 1.3 cutoff, specificity fell to 35% in people 65 and over, so most older people with a score above 1.3 did not have advanced fibrosis. A low cutoff of 2.0 brought specificity to 70% without losing sensitivity, and the upper cutoff stayed at 2.67. The AASLD guidance adopts the 2.0 cutoff for people over 65. The calculator switches to it at 65.
Under 35
McPherson’s study also found FIB-4 performed poorly in people 35 and under, and the AASLD guidance says it has low accuracy below 35. A low score at 30 is less reassuring than at 50, and the guidance suggests a second test for younger people with metabolic risk factors or raised liver enzymes. The calculator still shows the score, with a warning.
Hepatitis C
Sterling built FIB-4 in people with HIV and hepatitis C, and set its cutoffs at 1.45 and 3.25. Vallet-Pichard and colleagues then validated those same cutoffs in hepatitis C alone, where a score below 1.45 had a negative predictive value of 94.7% and above 3.25 a positive predictive value of 82.1%. If FIB-4 is being used for hepatitis C, choose that option in the calculator. For fatty liver disease, or when no cause is known, the 1.3 and 2.67 cutoffs apply.
When the score is less reliable
- Acute liver injury. A short spike in AST and ALT from a virus, a medication, or hard exercise moves the score without any change in scarring. The AASLD guidance says FIB-4 should not be used in acutely ill patients.
- A low platelet count for another reason. Platelets are in the denominator, so anything that lowers them, from a blood disorder to some medications, raises the score.
- AST from outside the liver. Muscle releases AST, so a draw after a hard workout, or a hemolyzed sample, can push the score up.
- Type 2 diabetes. The AASLD guidance notes FIB-4 performs less well in diabetes and suggests repeating it every 1 to 2 years, with a second test when one is available.
Tracking FIB-4 over time
The AASLD guidance treats a move from one band to the next as a sign of progression, so a series of scores says more than one. Since the score is built from three results, keep AST, ALT, and the platelet count from the same draw together, and recalculate when all three are new. How to track your ALT over time covers why ALT moves, how much is noise, and the lower limits liver specialists use. Remember that the score creeps up with age alone: a rise of a few hundredths a year with steady labs is your birthday, not your liver.
FIB-4 in HealthViewer
HealthViewer is a lab results tracker that runs on your own computer. It imports liver panels and blood counts from lab report PDFs, MyChart downloads, and direct connections to your health system, and charts each test over time in Trends. The Liver tag pulls ALT, AST, and the rest of the liver panel together, and the overlay view puts AST and ALT on one chart. Add platelets to see all three FIB-4 inputs side by side, and use the Cross-Date view to read them by draw.
Everything stays in an encrypted file on your computer, with no account and no cloud service. Getting Started walks through the first import.
This calculator is for understanding your lab results, not for diagnosis or treatment. FIB-4 is a screening score. Talk to your clinician about what yours means and whether you need a second test.