Diagnosing NASH (now officially called MASH, or metabolic dysfunction-associated steatohepatitis) requires a combination of blood tests, imaging, and sometimes a liver biopsy. No single test can confirm it on its own. The process typically moves through stages: screening for liver fat and risk factors, measuring liver scarring with noninvasive tools, and in some cases, examining a tissue sample under a microscope to confirm inflammation and cell damage.
NASH Is Now Called MASH
Before diving into diagnosis, a quick note on terminology. In 2023, the American Association for the Study of Liver Diseases officially renamed NASH to MASH, and NAFLD to MASLD (metabolic dysfunction-associated steatotic liver disease). The new name reflects the fact that this condition is driven by metabolic problems, not just the absence of alcohol. If your doctor uses either term, they’re talking about the same disease.
Under the updated framework, a MASLD diagnosis requires two things: fat buildup in the liver and at least one of five cardiometabolic risk factors. Those include a BMI of 25 or higher (23 in Asian populations), fasting blood sugar of 100 mg/dL or above (or an existing diabetes diagnosis), blood pressure at or above 130/85, triglycerides of 150 mg/dL or higher, or low HDL cholesterol (below 40 mg/dL for men, below 50 for women). Being treated for any of these conditions also counts. MASH is the more severe form of MASLD, where fat accumulation has progressed to active inflammation and liver cell damage.
Step One: Blood Tests and Risk Scoring
The diagnostic process usually starts with a simple blood-based calculation called the FIB-4 index. It uses four numbers your doctor already has or can easily get: your age, platelet count, and two liver enzymes (AST and ALT). The formula produces a score that estimates your likelihood of significant liver scarring.
For adults between 35 and 65, a FIB-4 score below 1.3 means advanced fibrosis is unlikely, and you can typically be monitored by your primary care doctor without further workup. A score of 1.3 or higher triggers the next round of testing. Above 2.67, advanced fibrosis is likely, though doctors will still confirm with additional tools. For people over 65, the “low risk” cutoff is higher at 2.0, because age alone can push the score up. The threshold for probable advanced fibrosis stays the same at 2.67.
FIB-4 is a screening tool, not a diagnosis. Its main job is to sort people into categories so that those at genuine risk get more thorough evaluation while those at low risk avoid unnecessary procedures.
Imaging: Measuring Fat and Stiffness
If blood tests raise concern, imaging helps quantify two things: how much fat is in your liver and how stiff the tissue has become from scarring.
Ultrasound and FibroScan
A standard abdominal ultrasound can detect moderate to severe fat accumulation but often misses milder cases. It’s cheap and widely available, which makes it a common first step, but it can’t measure inflammation or distinguish MASH from simple fatty liver.
FibroScan (vibration-controlled transient elastography) is more informative. It sends a painless pulse through your abdomen and measures how quickly the vibration travels through liver tissue. Stiffer tissue, meaning more scarring, transmits the pulse faster. Results are reported in kilopascals (kPa), with a normal value around 5 kPa. Values in the 11 to 14 kPa range generally suggest cirrhosis, though exact cutoffs vary depending on the underlying condition. FibroScan also provides a controlled attenuation parameter (CAP) score that estimates fat content.
MRI-Based Fat Measurement
MRI-PDFF (proton density fat fraction) is the most accurate noninvasive way to measure liver fat. It calculates the percentage of fat in liver tissue, and anything above 5% confirms hepatic steatosis. This technique is precise enough to track small changes over time, which makes it especially useful in clinical trials and for monitoring treatment response. The downside is cost and availability. Not every facility offers it, and insurance coverage varies.
Liver Biopsy: The Definitive Test
A liver biopsy remains the only way to definitively confirm MASH. It’s the one test that can directly show inflammation and a specific type of cell damage called hepatocyte ballooning, the two features that separate MASH from simple fatty liver. No imaging tool or blood test can reliably detect these yet.
During the procedure, a doctor inserts a thin needle through the skin and removes a small sample of liver tissue. A pathologist then examines the sample and assigns a score called the NAFLD Activity Score (NAS), which ranges from 0 to 8. It grades three features:
- Steatosis (0 to 3 points): how much of the tissue surface is covered by fat, from less than 5% (score of 0) to more than 66% (score of 3).
- Lobular inflammation (0 to 3 points): the number of inflammatory clusters visible under the microscope, from none to more than four per field of view.
- Hepatocyte ballooning (0 to 2 points): the presence and prominence of swollen, damaged liver cells. A score of 1 means a few definite ballooned cells; a score of 2 means many are visible.
A NAS of 5 or higher, with points from all three categories, is generally considered diagnostic for MASH. The pathologist also stages fibrosis separately on a 0 to 4 scale, since fibrosis determines long-term prognosis more than inflammation alone.
Biopsy isn’t routine for everyone with suspected fatty liver disease. It carries small but real risks, including pain, bleeding, and rarely, infection. Doctors typically reserve it for cases where the diagnosis is uncertain, when other causes of liver disease need to be ruled out, or when knowing the exact degree of inflammation and scarring would change the treatment plan.
Ruling Out Other Causes
Before confirming MASH, doctors need to exclude other explanations for liver fat and inflammation. Alcohol use is the most obvious one. Certain medications, including corticosteroids, tamoxifen, and some anti-seizure drugs, can cause fat to accumulate in the liver. Endocrine disorders like hypothyroidism and polycystic ovary syndrome can mimic or worsen the picture. Rarer possibilities include celiac disease, Wilson disease, and certain inherited metabolic conditions. This step usually involves a thorough medical history, targeted blood work, and sometimes additional testing depending on what your doctor suspects.
What the Diagnostic Pathway Looks Like in Practice
For most people, the journey starts with a routine blood test that reveals mildly elevated liver enzymes, or with an ultrasound done for another reason that incidentally shows fatty liver. From there, the typical sequence looks like this: your doctor calculates a FIB-4 score using existing lab work. If it’s low, you’ll likely be told to focus on weight management and come back for monitoring in a year or two. If it’s intermediate or high, you’ll be referred for a FibroScan or similar test to assess stiffness. If that confirms significant fibrosis, or if the picture is ambiguous, a liver biopsy may be recommended.
The whole process can take weeks to months, depending on referral wait times and insurance approvals. Many people with early-stage MASLD never need a biopsy at all. The noninvasive tools have become reliable enough to guide treatment decisions for the majority of patients, reserving biopsy for the cases where it genuinely changes what happens next.