Fatty liver disease is diagnosed through a combination of blood tests, imaging, and sometimes a scoring system that estimates how much damage has occurred. There’s no single test that gives the full picture. Instead, doctors typically start with routine blood work and an ultrasound, then use more specialized tools if those initial results raise concerns.
The condition was recently renamed from NAFLD (non-alcoholic fatty liver disease) to MASLD (metabolic dysfunction-associated steatotic liver disease). The new name reflects what doctors now understand: fatty liver is closely tied to metabolic health. To meet the current diagnostic criteria, you need evidence of fat in the liver plus at least one cardiometabolic risk factor, such as a BMI over 25, fasting blood sugar above 100 mg/dL, blood pressure above 130/85, triglycerides above 150, or low HDL cholesterol.
Blood Tests: The Usual Starting Point
Most cases of fatty liver are first suspected because of abnormal results on a routine blood panel. The key markers are two liver enzymes called ALT and AST. When liver cells are inflamed or damaged, these enzymes leak into the bloodstream at higher-than-normal levels. A reading above 40 for either enzyme is generally considered elevated, though doctors weigh the degree of elevation alongside your overall health profile.
Mildly elevated enzymes, defined as less than twice the upper limit of normal, don’t automatically mean serious liver disease. If your levels fall in that range, your doctor may recommend dietary changes and exercise first, then recheck your blood work in about three months. Persistently elevated results trigger a deeper workup to rule out other causes of liver inflammation, such as hepatitis, autoimmune conditions, or medication side effects.
It’s worth knowing that liver enzymes can be completely normal even when significant fat is present. Blood tests alone cannot confirm or rule out fatty liver disease, which is why imaging plays a central role.
Ultrasound: The First Imaging Step
Abdominal ultrasound is the most common first-line imaging tool for detecting liver fat. It’s painless, widely available, and doesn’t involve radiation. The technician applies gel to your abdomen and moves a handheld probe over the area. A fatty liver appears brighter than normal on the screen because fat reflects sound waves differently than healthy tissue.
Ultrasound has an overall sensitivity of about 74.5% and specificity of 86.6% for detecting liver fat. That means it catches roughly three out of four cases while correctly identifying most people who don’t have it. Its accuracy improves dramatically as the amount of fat increases: sensitivity jumps from 65% for mild fat accumulation to 95% for moderate and 96% for severe. The main limitation is with early or mild steatosis, where ultrasound can miss about one in three cases.
FibroScan: Measuring Fat and Stiffness
A FibroScan (technically called vibration-controlled transient elastography) is a more specialized test that measures two things at once: how much fat is in your liver and how stiff the tissue has become. Stiffness matters because it signals fibrosis, the scarring that develops when fatty liver progresses to more serious disease.
The test feels similar to an ultrasound. A probe placed on your right side sends a gentle vibration through the liver and measures how quickly the wave travels. Stiffer tissue transmits waves faster. The whole process takes about 10 to 15 minutes.
For fat content, the FibroScan produces a CAP (controlled attenuation parameter) score measured in decibels per meter:
- 238 to 260 dB/m (Grade S1): Less than one-third of the liver is affected
- 260 to 290 dB/m (Grade S2): Between one-third and two-thirds affected
- 290 to 400 dB/m (Grade S3): More than two-thirds affected
For fibrosis, readings below 8 kPa generally suggest no significant scarring. Values above 12 kPa raise concern for advanced fibrosis. Results between 8 and 12 fall into a gray zone that may require additional testing.
The FIB-4 Score: A Simple Fibrosis Estimate
Not every clinic has a FibroScan, so doctors often use a calculation called the FIB-4 index as a first screening step for fibrosis. It uses four numbers you probably already have from routine blood work: your age, platelet count, and ALT and AST levels. No extra blood draw is needed.
A FIB-4 score below 1.30 effectively rules out advanced fibrosis in most people, with a negative predictive value of about 97%. A score above 3.25 strongly suggests advanced scarring, with 97% specificity. Scores between those two numbers are inconclusive, and your doctor will typically follow up with a FibroScan or another imaging study to get a clearer answer.
This two-step approach (FIB-4 first, then elastography for borderline results) is now the recommended screening strategy for people at high risk, particularly those with type 2 diabetes. In a large multicenter study of diabetes patients, this algorithm ruled out advanced fibrosis in over half of participants at the first step, sparing them from additional testing.
MRI-Based Tests for Greater Precision
When results from ultrasound or FibroScan are unclear, or when doctors need a highly precise measurement, MRI-based techniques offer the most accurate non-invasive assessment available. MRI-proton density fat fraction can quantify liver fat down to individual percentage points, making it far more precise than ultrasound for mild steatosis.
Magnetic resonance elastography (MRE) measures liver stiffness with exceptional accuracy. For detecting significant fibrosis (stage 2 or higher), MRE achieved an accuracy score of 0.981 out of 1.0 in a meta-analysis, compared to 0.794 for FibroScan. That’s a meaningful gap, especially for catching fibrosis at earlier stages. The tradeoff is cost and availability. MRI-based liver tests are significantly more expensive, take longer to perform, and aren’t offered at every facility.
Liver Biopsy: When It’s Still Needed
Liver biopsy remains the definitive way to assess exactly what’s happening inside liver tissue. A thin needle is inserted through the skin to collect a small sample, which a pathologist examines under a microscope. This is the only test that can definitively distinguish simple fat accumulation from steatohepatitis, the inflammatory form where liver cells are actively being damaged.
Biopsies are not routine. They’re typically reserved for situations where non-invasive tests give conflicting results, when another liver condition needs to be ruled out, or when the degree of inflammation needs to be assessed before starting treatment. The procedure carries a small risk of pain, bleeding, and other complications, so doctors use it selectively rather than as a screening tool.
Who Should Be Screened
Fatty liver disease affects an estimated one in three adults, and most people have no symptoms until the disease has progressed significantly. Screening is particularly important if you have type 2 diabetes, obesity, high triglycerides, or metabolic syndrome. The American Diabetes Association and major liver disease guidelines now recommend routine fibrosis screening for people with diabetes, using the FIB-4 score as a starting point.
If you fall into a high-risk group, the diagnostic path typically looks like this: your doctor calculates your FIB-4 score from existing blood work. A low score means monitoring over time. A borderline or high score leads to a FibroScan or similar elastography test. If that test also raises concerns, you may be referred to a hepatologist for further evaluation, potentially including MRI-based imaging or biopsy. The goal at each step is to determine not just whether fat is present, but whether scarring has started, because that’s what determines the urgency of treatment.