NASH (nonalcoholic steatohepatitis) is diagnosed through a combination of blood tests, imaging, and sometimes a liver biopsy. There is no single test that confirms it. Instead, doctors work through a step-by-step process: screening for liver fat and fibrosis with simple blood calculations, confirming findings with specialized imaging, and in some cases taking a small tissue sample to examine under a microscope. The condition has recently been renamed MASH (metabolic dysfunction-associated steatohepatitis), though both terms refer to the same disease.
Why Diagnosis Takes Multiple Steps
NASH is a specific, more severe form of fatty liver disease. It means your liver has excess fat along with active inflammation and damage to liver cells. Simple fatty liver (steatosis alone) is far more common and much less dangerous, so the diagnostic process is really about figuring out where you fall on that spectrum and whether your liver is scarring.
Complicating things further, liver enzyme levels in standard blood work are unreliable indicators. ALT, the enzyme most associated with liver damage, misses over 50% of fatty liver cases. In one large national study, more than 61% of men with confirmed fatty liver disease had ALT levels that fell within the “normal” range. The numbers were similar for women. So normal blood work does not mean your liver is fine, and elevated enzymes alone don’t confirm NASH.
First Step: Risk Scoring With Blood Tests
Most diagnostic pathways start with a simple calculation called the FIB-4 index. Your doctor doesn’t need to order any special labs for it. The formula uses four things already included in routine blood work: your age, AST level, ALT level, and platelet count.
The result places you into one of three categories:
- Below 1.3: Low risk of significant liver scarring. If imaging also shows no concerns, you can typically be monitored over time without further workup.
- Between 1.3 and 2.67: Indeterminate. You need additional testing, usually with imaging or a specialized blood panel.
- Above 2.67: High risk of advanced fibrosis. Referral to a liver specialist is recommended.
Another calculation called the NAFLD Fibrosis Score uses six inputs: age, BMI, whether you have diabetes or impaired fasting glucose, AST, ALT, platelet count, and albumin. It serves a similar purpose, with its own cutoffs (below -1.455 for low risk, above 0.675 for high risk). Some clinics use one or both scores as a first filter.
Imaging: FibroScan and MRI
If your FIB-4 score lands in the indeterminate or high-risk zone, the next step is usually a FibroScan (transient elastography). This is a painless, noninvasive test that takes about 10 minutes. It measures two things simultaneously: 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 suggests minimal or no significant fibrosis. Between 8 and 12 kPa is considered indeterminate and needs further evaluation. Above 12 kPa indicates advanced fibrosis or cirrhosis. Fat content is measured by something called the CAP score, reported in dB/m. A CAP score above 290 dB/m reliably identifies severe fat accumulation, correctly flagging about 90% of patients with the highest grade of steatosis on biopsy.
When combined, FIB-4 below 1.3 and FibroScan below 8 kPa together are considered strong enough to rule out significant fibrosis. In one study, about 70% of patients who met both low-risk cutoffs had minimal scarring (stage 0 or 1) on biopsy.
MRI offers a more precise measurement of liver fat. A technique called MRI-PDFF (proton density fat fraction) quantifies the exact percentage of fat in your liver. A fat fraction of 6.4% or higher indicates at least mild steatosis. Above 17.4% suggests moderate fat accumulation, and above 22.1% indicates severe steatosis. MRI-PDFF is more accurate than ultrasound-based methods but is also more expensive and less widely available, so it’s typically reserved for cases where FibroScan results are unclear or for tracking response to treatment.
The Enhanced Liver Fibrosis (ELF) Test
The ELF test is a specialized blood panel that measures three proteins involved in the scarring process. It’s particularly useful when FibroScan results fall in the gray zone or aren’t available. A score below 9.8 has a negative predictive value above 95% for serious liver outcomes, meaning it’s very good at identifying people who are unlikely to develop complications. A score of 9.8 or above warrants referral to a specialist.
At the higher end, ELF scores carry serious prognostic weight. A score of 11.3 or above raises concern for cirrhosis. Scores of 13 or higher signal urgent risk: in one study, 26% of patients at that level needed hospitalization for liver decompensation within 90 days, and 38% died within one year from any cause. The ELF test also outperformed FIB-4 at predicting short-term liver-related death, with an accuracy of 94% compared to 83% at six months.
Liver Biopsy: The Definitive Test
A liver biopsy remains the only way to definitively confirm NASH. A needle is inserted through the skin to remove a tiny sample of liver tissue, which a pathologist examines under a microscope. The procedure is done with local anesthesia and typically takes less than an hour, though you’ll be monitored for several hours afterward.
Three specific features must all be present for a NASH diagnosis: at least 5% of liver cells filled with fat droplets, swollen (ballooned) hepatocytes, and inflammation in the liver lobules. Pathologists score these features using the NASH Clinical Research Network (CRN) system, which assigns points for steatosis (0 to 3), inflammation (0 to 3), and ballooning (0 to 2). The total, called the NAS score, ranges from 0 to 8. A score of 5 or higher strongly correlates with definite NASH, while 3 or below correlates with “not NASH.” Fibrosis is staged separately on a 0 to 4 scale.
Despite being the gold standard, biopsy is not performed on every patient. Nationally, fewer than a third of people with a clinical MASH diagnosis actually undergo one. It carries small risks of pain, bleeding, and infection, and a single sample can miss areas of the liver with different levels of damage. For many patients, noninvasive tests provide enough information to guide treatment decisions.
What Else Needs to Be Ruled Out
Before settling on a NASH diagnosis, your doctor needs to exclude other causes of liver fat and inflammation. The most important distinction is from alcohol-related liver disease, since the two look identical under a microscope. NASH requires that alcohol intake falls below specific thresholds: fewer than three standard drinks per day for men and fewer than two for women (roughly 30 grams and 20 grams of alcohol, respectively).
Other conditions that can mimic fatty liver include hepatitis C, celiac disease, Wilson’s disease, certain medications (corticosteroids, methotrexate, tamoxifen, amiodarone), lipid metabolism disorders, and severe weight loss after bariatric surgery. Blood tests for viral hepatitis and autoimmune markers, along with a medication review, are standard parts of the workup.
How Diagnosis Affects Treatment Options
The level of fibrosis found during diagnosis directly shapes what happens next. Patients with early-stage disease (F0 or F1 fibrosis) are typically managed with lifestyle changes: weight loss of 7 to 10% of body weight, exercise, and control of metabolic risk factors like diabetes and high cholesterol.
For patients with moderate to advanced fibrosis (stages F2 and F3), the first targeted medication for MASH, resmetirom, became available in 2024. The FDA label does not require a liver biopsy for prescription. Fibrosis can be assessed with noninvasive tools: FibroScan readings between 8 and 13.6 kPa, ELF scores between 7.7 and 11.3, or MRI-based elastography readings between 2.56 and 4.71 kPa all qualify. Some healthcare systems, like the Veterans Administration, do still require a biopsy showing stage 2 or 3 fibrosis and a NAS score of 4 or higher within three years, but this is not a universal requirement.