Fatty Liver Index (FLI)
The FLI (Fatty Liver Index) is a simple calculated index that uses routine lab tests and two tape-measure readings to estimate the likelihood of steatosis — the buildup of fat in the liver in non-alcoholic (metabolic dysfunction-associated) fatty liver disease. It needs just four inputs: triglycerides, GGT, body mass index and waist circumference.
What FLI is and why it's needed
Fat in the liver stays silent for a long time: a person feels fine, and liver tests may remain almost normal. To avoid missing the buildup of fat at an early — still reversible — stage, simple calculated indices were devised. FLI (Fatty Liver Index) is one of the best known and best validated of them. It requires neither an ultrasound nor expensive work-ups: two blood values and two tape-measure readings are enough.
It's important to understand that FLI is a screening tool, not a diagnosis. It doesn't “see” the liver directly and doesn't measure how much fat is actually in it. Its job is to sort: who is unlikely to have steatosis and can simply be monitored, and who should get an ultrasound and see a physician. That's exactly how it's used in large studies and in population-wide screening.
Steatosis, NAFLD and MASLD — what they are
Hepatic steatosis is the accumulation of fat in the liver cells (hepatocytes). Steatosis is diagnosed when fat (neutral fats — triglycerides) is present in more than 5% of hepatocytes. At this stage the changes are still reversible: with weight loss and dietary changes, fat leaves the liver.
Non-alcoholic fatty liver disease (NAFLD) is a chronic liver disease associated with metabolic dysfunction, in which macrovesicular steatosis is present in more than 5% of hepatocytes and there is no significant alcohol intake as a cause. Today the same condition is increasingly called metabolic dysfunction-associated steatotic liver disease (MASLD) — the new name more accurately reflects its essence: the disease is closely linked to excess weight, insulin resistance and metabolic syndrome. FLI was created specifically to assess the degree of steatosis in patients with NAFLD/MASLD.
What you need for the calculation
FLI is calculated from four inputs. Two come from a routine blood chemistry panel, and two are your anthropometric measurements:
- Triglycerides (mmol/L). The main type of fat in the blood. A rise in them is a common companion of fat buildup in the liver and of metabolic syndrome.
- Gamma-glutamyl transferase (GGT), U/L. A sensitive liver enzyme; in fatty liver disease it's often one of the first to rise.
- Body mass index (BMI). Reflects total body weight relative to height. In the calculator, BMI is computed automatically from the height and weight you enter — there's no need to enter it separately.
- Waist circumference, cm. An indirect marker of visceral fat — the fat that surrounds the internal organs and is most strongly linked to fatty liver disease.
The calculation itself is a logistic model: each of the four inputs makes its own contribution, and the output is a single number from 0 to 100. The calculator below performs the correct computation and interpretation — this avoids manual-calculation errors (including converting triglyceride units) and ensures the result matches the original model.
How to measure correctly
The accuracy of FLI depends directly on how carefully the measurements are taken. Take them under the same conditions, preferably in the morning.
Waist circumference
Per the World Health Organization data-collection protocol, waist circumference is measured at the midpoint between the lowest palpable rib and the top of the iliac crest. The tape is held parallel to the floor, without compressing the skin, and the reading is taken at the end of a normal exhalation.
Height
Measure without shoes, standing straight, heels together, with your gaze directed horizontally.
Weight
Weigh yourself on the same scale, if possible in the morning, on an empty stomach, in light clothing or without it. That way the result will be comparable across repeat measurements.
FLI Calculator
Enter the four inputs — the calculator will compute the fatty liver index, determine the risk zone and explain the result. The computation runs on the server; the model's coefficients are not sent to the browser.
FLI interpretation thresholds (Bedogni et al., 2006): <30 — steatosis ruled out, 30–59 — indeterminate, ≥60 — predictor of steatosis (probability >78%).
How to read the result
The FLI scale — from 0 to 100 — is split by two thresholds into three zones. This is deliberate: between “definitely not” and “highly likely” there's a middle ground where it's more honest to call the result indeterminate than to force it into a “yes” or “no.”
| Zone | FLI | What it means |
|---|---|---|
| Steatosis ruled out | < 30 | Fatty liver is unlikely. The test has a high negative predictive value. |
| Indeterminate | 30–59 | Steatosis can be neither confirmed nor ruled out. Further evaluation is needed. |
| Predictor of steatosis | ≥ 60 | Fatty liver is highly likely — probability of steatosis over 78%. |
Fatty liver is unlikely. The test has a high negative predictive value.
Steatosis can be neither confirmed nor ruled out. Further evaluation is needed.
Fatty liver is highly likely — probability of steatosis over 78%.
What each zone means
FLI < 30 — steatosis ruled out
A value below 30 reliably rules out fatty liver: this is precisely where FLI is strong — it does a good job of “filtering out” those who don't have steatosis. It's not a reason to relax forever, but a reason not to worry right now. It's enough to maintain your weight, diet and physical activity and to repeat the assessment at routine check-ups.
FLI 30–59 — an indeterminate result
This is the “gray zone”: steatosis is possible, but the index can't say for sure. Here the right next step is not to guess but to undergo a direct examination. Usually that's a liver ultrasound (and elastography if needed) and an assessment of metabolic factors: blood sugar, the lipid profile, blood pressure and waist circumference. The physician decides what the work-up should include.
FLI ≥ 60 — a predictor of steatosis
A value of 60 or above indicates a high probability of fatty liver — over 78%. This is neither a verdict nor a measure of severity: the index doesn't distinguish simple steatosis from inflammation (steatohepatitis) and doesn't measure fibrosis. But it's a strong reason to show the result to a physician, get an ultrasound and work on the modifiable factors — first and foremost reducing weight and waist circumference, along with diet and physical activity. At an early stage, steatosis is reversible.
Why these particular inputs
FLI is not made up of these four quantities by chance — each reflects its own facet of the process of fat accumulating in the liver:
- Triglycerides — a direct marker of fat metabolism. Their excess in the blood accompanies an excess of fat in the tissues too, including the liver.
- GGT — a liver enzyme that responds sensitively to fatty liver disease, oxidative stress and metabolic overload; it often rises earlier than other liver markers.
- BMI — an overall measure of excess body weight, which is closely linked to the risk of steatosis.
- Waist circumference — an indicator of visceral fat around the internal organs. It's visceral fat, not subcutaneous fat, that is most strongly linked to fatty liver disease and insulin resistance.
Together they give a rounded picture: you can see fat metabolism (triglycerides), the liver's response (GGT), and both general (BMI) and abdominal (waist) excess of fat tissue. That's why FLI works better than any of these markers on its own.
Limitations and important caveats
- It's not a diagnosis. FLI estimates only the probability of steatosis and doesn't replace ultrasound, elastography or biopsy.
- It doesn't measure severity. The index doesn't distinguish simple steatosis from steatohepatitis (with inflammation) and doesn't assess liver fibrosis.
- The “gray zone” is normal. A result of 30–59 isn't an error but an honest signal that direct liver imaging is needed.
- It can be distorted. Acute liver disease, marked loss of muscle mass, certain medications and comorbidities can shift individual inputs.
- Context matters more than a single number. FLI is assessed together with your history, lifestyle and other tests — and that's done by a physician.
What to do with the result
FLI is a starting point, not a finish line. What to do next depends on the zone:
- Green zone (<30): keep up your current lifestyle, keep an eye on your weight and waist, and repeat the assessment at check-ups.
- Yellow zone (30–59): discuss a liver ultrasound and a check of metabolic factors with your physician — don't put it off.
- Red zone (≥60): show the result to a physician, get an ultrasound and actively work on the modifiable factors: weight loss, diet, physical activity and cutting out excess alcohol.
The good news is that early steatosis is reversible: a moderate reduction in body weight (even by 7–10%) reduces the amount of fat in the liver for many people. But the specific plan for work-up and action should be drawn up by a physician — don't prescribe tests, let alone treatment, for yourself.
Frequently asked questions
What units should I enter triglycerides in?
In mmol/L (SI units). The calculator converts them to the units the model uses automatically — there's no need to recalculate anything by hand.
Do I need to enter BMI separately?
No. Enter your height and weight and the body mass index will be computed for you. That's more reliable than plugging in BMI by hand.
Does FLI ≥ 60 mean I definitely have cirrhosis?
No. A high FLI indicates only a high probability of steatosis — the buildup of fat. That's an early and often reversible stage, and it's not the same as cirrhosis. The index doesn't assess fibrosis or disease severity at all; other tests are needed for that.
How does FLI differ from the R-factor?
They serve different purposes. The R-factor, based on ALT and alkaline phosphatase, determines the type of liver injury once tests have already moved out of the normal range. FLI is a screening index that estimates the probability of fat accumulating in the liver, including when there are no obvious abnormalities yet.
Can I track changes over time with FLI?
Yes, it's a convenient way to see the trend: if the index falls as your weight and waist circumference come down, that's a good sign. But decisions about diagnosis and treatment are made based on direct examinations, not on the index trend alone.
