NASH (now officially called MASH) is diagnosed through a stepwise process that starts with simple blood tests and may progress to specialized imaging or a liver biopsy. Most people are screened and risk-stratified without ever needing an invasive procedure. The majority of patients, somewhere between 69% and 98% in clinical pathways studied, turn out to be low risk and stay under the care of their primary doctor.
A quick note on naming: in 2023, a group of international liver disease societies officially renamed NASH (nonalcoholic steatohepatitis) to MASH (metabolic dysfunction-associated steatohepatitis). The old term “nonalcoholic” didn’t describe what the disease actually was, and “fatty” was considered stigmatizing. You’ll still see both names used interchangeably, and the diagnostic process is identical regardless of which term your doctor uses.
Step One: The FIB-4 Blood Test
The first screening tool is usually the FIB-4 score, a calculation that uses four numbers your doctor already has or can easily get: your age, platelet count, and two liver enzymes (AST and ALT). No special blood draw is needed beyond routine labs. Current international guidelines from European liver, diabetes, and obesity societies recommend FIB-4 as the starting point for anyone suspected of having fatty liver disease.
The score places you into one of three categories. A FIB-4 below 1.30 means low risk for significant liver scarring, and most people stop here with routine follow-up from their primary care doctor. A score above 2.67 signals high risk and typically triggers a referral to a liver specialist. Scores between those two values fall into an intermediate zone that needs further testing to clarify.
Step Two: FibroScan and Other Imaging
If your FIB-4 score isn’t clearly low risk, the next step in most diagnostic pathways is a FibroScan (formally called vibration-controlled transient elastography). This is a painless, non-invasive test that takes about 10 to 15 minutes. A probe is placed on your skin over the liver area and sends gentle vibrations through the tissue. The device measures two things: how stiff your liver is (which reflects scarring) and how much fat it contains.
Liver stiffness is reported in kilopascals (kPa). A reading below 8 kPa, combined with a low FIB-4 score, effectively rules out advanced scarring. Values between 10 and 15 kPa suggest possible advanced chronic liver disease. Readings above 15 kPa are highly suggestive of it, and a measurement at or above 20 kPa can confirm cirrhosis without needing a biopsy.
Fat content is measured separately using what’s called the controlled attenuation parameter, reported in decibels per meter. Normal values fall below 247 dB/m. Readings of 248 or higher indicate at least mild fat accumulation, above 268 suggests moderate levels, and above 280 points to severe steatosis. This helps your doctor gauge how much fat is in the liver but doesn’t, on its own, confirm whether active inflammation (the “H” in NASH/MASH) is present.
Ruling Out Other Causes
Part of diagnosing MASH involves making sure something else isn’t causing the liver damage. The most important distinction is between metabolic liver disease and alcohol-related liver disease, since they look similar on imaging and blood work. Your doctor will ask about your drinking habits. Under the older naming system, NAFLD required that women drink fewer than 14 standard drinks per week and men fewer than 21. One standard drink equals about 14 grams of pure alcohol, roughly a 12-ounce beer or 5-ounce glass of wine.
The updated classification also recognizes an overlap category called MetALD, for people whose liver disease involves both metabolic factors and moderate alcohol use (20 to 50 grams per day for women, 30 to 60 grams per day for men). Your doctor may also check for hepatitis B and C, autoimmune liver conditions, and medication-related liver injury before settling on a MASH diagnosis.
When a Liver Biopsy Is Needed
Liver biopsy remains the gold standard for confirming MASH because it’s the only test that can directly show inflammation and a specific type of cell damage called hepatocyte ballooning. These two features, along with fat accumulation, are what separate MASH from simple fatty liver (which carries much less risk). Blood tests and imaging can estimate scarring and fat content, but they can’t reliably detect the active inflammation that defines MASH.
A pathologist scores the biopsy tissue using the NAFLD Activity Score, which grades three components on a point scale. Fat accumulation is scored 0 to 3 based on how much of the liver tissue is affected (under 5% scores zero, over 66% scores three). Inflammation is scored 0 to 3 based on the number of inflammatory clusters visible under the microscope. Cell ballooning is scored 0 to 2 based on how many damaged, swollen liver cells are present. The combined score helps determine how active the disease is.
That said, not everyone needs a biopsy. It’s typically reserved for cases where the diagnosis is uncertain, when your doctor needs to confirm the stage of scarring before starting treatment, or when competing causes of liver disease need to be sorted out. The procedure is done as an outpatient visit. Most people undergo a percutaneous biopsy, where a needle is inserted through the skin into the liver using ultrasound guidance. You’ll need to lie still for 2 to 4 hours afterward while your vital signs are monitored. A less common approach goes through a vein in the neck (transjugular biopsy), which requires at least 4 hours of monitoring. Either way, expect to avoid intense physical activity and heavy lifting for up to a week.
Why Diagnosis Matters More Now
For years, a MASH diagnosis didn’t change treatment much beyond lifestyle recommendations. That shifted in March 2024 when the FDA approved the first medication specifically for MASH with moderate to advanced liver scarring in adults who don’t yet have cirrhosis. This approval means an accurate diagnosis, including the specific stage of fibrosis, now directly affects whether you’re eligible for pharmaceutical treatment.
The two-tiered pathway (FIB-4 first, then FibroScan if needed) is designed to catch the people who are progressing toward serious liver damage while avoiding unnecessary procedures for the majority who aren’t. If your doctor has flagged elevated liver enzymes or you have risk factors like type 2 diabetes, obesity, or metabolic syndrome, asking about your FIB-4 score is a reasonable place to start the conversation.