Fatty liver is most often diagnosed through a combination of blood tests and imaging, starting with routine lab work that flags elevated liver enzymes and confirmed by an ultrasound or more advanced scan showing fat buildup in the liver. Many people first learn they have fatty liver incidentally, when blood work or an abdominal scan done for another reason reveals something unexpected. Here’s what each step of the diagnostic process involves and what the results actually mean.
Blood Tests: The Usual Starting Point
A standard liver panel measures enzymes that signal inflammation or damage to liver cells. The two most important are ALT and AST. For fatty liver screening, the American Association for the Study of Liver Diseases considers ALT elevated at anything above 33 U/L in men and above 25 U/L in women. These thresholds are lower than what many labs print as the “normal” range on your results, which means some cases get overlooked if your doctor relies solely on the lab’s reference values.
Elevated liver enzymes alone don’t confirm fatty liver. They tell your doctor that something is irritating the liver, which could be alcohol use, a medication side effect, a viral infection, or fat accumulation. That’s why abnormal blood work typically triggers the next step: imaging.
It’s also worth knowing that liver enzymes can be completely normal even when significant fat is present. Up to a quarter of people with fatty liver have ALT and AST values that fall within standard ranges, so normal blood work doesn’t rule it out if other risk factors are present.
Ultrasound: The First-Line Imaging Test
A standard abdominal ultrasound is the most common way fatty liver is initially spotted. It’s painless, widely available, and doesn’t involve radiation. Radiologists look for a characteristic “bright liver,” where the liver tissue appears noticeably brighter than the adjacent kidney on the screen. This brightness comes from sound waves bouncing off fat deposits differently than they bounce off normal tissue.
Ultrasound findings are graded on a simple scale:
- Grade 0: Normal echogenicity, no signs of fat
- Grade 1 (mild): Slight increase in brightness, but blood vessels and the diaphragm are still clearly visible
- Grade 2 (moderate): More pronounced brightness with some blurring of blood vessels and the diaphragm
- Grade 3 (severe): Marked brightness with poor or no visibility of vessel borders, the diaphragm, or the back portion of the liver
Ultrasound works well for moderate to severe fat accumulation but can miss mild cases. It also can’t precisely measure how much fat is in the liver or tell you whether scarring (fibrosis) has started. For those answers, you need more specialized tools.
FibroScan: Measuring Fat and Stiffness Together
A FibroScan is a specialized form of ultrasound that takes about 10 to 15 minutes and gives two separate measurements in one session. You lie on your back with your right arm raised above your head while a technician presses a small probe against your skin between your ribs.
The first measurement is the CAP score (controlled attenuation parameter), which quantifies how much fat is in your liver. Results are reported in decibels per meter:
- 238 to 260 dB/m (S1): Mild fat accumulation
- 260 to 290 dB/m (S2): Moderate fat accumulation
- 290 to 400 dB/m (S3): Severe fat accumulation
The second measurement is liver stiffness, reported in kilopascals. Stiffer tissue suggests scarring. This matters because fatty liver itself is reversible, but once significant fibrosis develops, the stakes change considerably. You’ll need to fast for at least two hours before a FibroScan, as recent food intake can temporarily increase liver stiffness and skew results.
MRI for Precise Fat Measurement
MRI-based fat measurement, known as proton density fat fraction (MRI-PDFF), is the most accurate non-invasive way to quantify liver fat. It’s primarily used in clinical trials and specialized centers rather than routine screening because of cost and availability. A liver fat fraction above 5% is the standard threshold for diagnosing clinical fatty liver. In research settings, two cutoffs are commonly used: 7% is optimized for catching more cases (higher sensitivity), and 12% is optimized for being more certain the fat is truly abnormal (higher specificity).
Your doctor might order this test if ultrasound results are ambiguous, if they need precise measurements to track whether your liver fat is improving over time, or if you’re being evaluated for a clinical trial.
Checking for Liver Scarring
Once fatty liver is confirmed, the most important clinical question becomes whether fibrosis has developed. Your doctor can estimate this using a simple calculation called the FIB-4 index, which combines your age, platelet count, and AST and ALT levels into a single score. No extra blood draw is needed since these values come from standard lab work.
A FIB-4 score below 1.45 reliably rules out advanced fibrosis about 90% of the time. A score above 3.25 strongly suggests advanced scarring. Scores between those two numbers fall into a gray zone where further testing, typically a FibroScan or MRI elastography, is needed to clarify the picture.
Liver biopsy, where a small tissue sample is taken with a needle, remains the gold standard for grading both fat and fibrosis. But it’s invasive and carries a small risk of bleeding or pain, so it’s generally reserved for cases where non-invasive tests give conflicting results or when your doctor suspects more advanced disease that would change treatment decisions.
What Makes It a Formal Diagnosis
The current diagnostic name is metabolic dysfunction-associated steatotic liver disease, or MASLD. To meet the formal criteria, you need evidence of fat in the liver (from imaging or biopsy) plus at least one metabolic risk factor: overweight or obesity, elevated waist circumference, high blood pressure, prediabetes or type 2 diabetes, high triglycerides, or low HDL cholesterol. Alcohol intake also needs to be below certain thresholds (under 20 grams per day for women, under 30 grams per day for men) to distinguish MASLD from alcohol-related liver disease.
This combination of criteria means the diagnosis isn’t just about fat on a scan. It ties the liver finding to the broader metabolic picture, which helps guide treatment priorities like blood sugar management, weight loss, or cardiovascular risk reduction.
Fatty Liver in People Who Aren’t Overweight
An important diagnostic nuance: you don’t need to be obese, or even overweight, to have fatty liver. Lean MASLD affects people at a normal BMI who still carry metabolic risk factors like insulin resistance, elevated triglycerides, or prediabetes. These cases are easier to miss because doctors may not think to screen for liver fat in someone who appears metabolically healthy.
Younger adults in particular can develop isolated fat in the liver with insulin resistance as their only metabolic abnormality. If you have persistently elevated liver enzymes and no obvious explanation, fatty liver is worth investigating regardless of your body size. Early identification opens the door to lifestyle changes that can reverse fat accumulation before scarring begins.