LSM stands for liver stiffness measurement, a noninvasive test that checks for scarring (fibrosis) in your liver. The test works by sending a painless vibration through your skin and measuring how quickly the wave travels through liver tissue. Stiffer tissue means more scarring, and the result is reported as a number in kilopascals (kPa). A healthy liver typically measures around 5 kPa, while readings above 14 kPa suggest cirrhosis.
How the Test Works
LSM is most commonly performed using a device called FibroScan, which uses a technique called transient elastography. A technician places a probe against your right side, between your ribs, and the device sends a low-frequency vibration into your liver. An ultrasound sensor on the same probe tracks how fast that vibration moves through the tissue. The faster it travels, the stiffer your liver is, and stiffer generally means more scar tissue has built up.
The whole process takes about 10 to 15 minutes, requires no needles or sedation, and you can go about your day immediately afterward. The device takes at least 10 valid readings during a session, and the result is reported as the median of those readings. For the result to be considered reliable, the readings need to be fairly consistent with one another, with the spread between them staying within 30% of the median value.
What the Numbers Mean
In a large study of 502 healthy adults, the average liver stiffness was 5.10 kPa. Men tend to run slightly higher (around 5.45 kPa) than women (around 4.89 kPa). Your result will fall somewhere on a scale that corresponds to fibrosis stages labeled F0 through F4, but the exact cutoffs depend on your underlying condition.
For hepatitis C:
- 2 to 7 kPa (F0–F1): Normal liver, no significant scarring
- 8 to 9 kPa (F2): Moderate scarring
- 9 to 14 kPa (F3): Severe scarring
- 14 kPa or higher (F4): Cirrhosis
For non-alcoholic fatty liver disease (NAFLD/NASH):
- 2 to 7 kPa (F0–F1): Normal liver, no significant scarring
- 7.5 to 10 kPa (F2): Moderate scarring
- 10 to 14 kPa (F3): Severe scarring
- 14 kPa or higher (F4): Cirrhosis
The threshold for cirrhosis is the same across both conditions, but the middle stages differ. Fatty liver disease uses slightly lower cutoffs for moderate and severe scarring, which means a reading of 8 kPa could be interpreted differently depending on your diagnosis. Your doctor will interpret your number in the context of your specific liver condition.
LSM vs. CAP: Scarring vs. Fat
During the same FibroScan session, you may also get a second number called a controlled attenuation parameter, or CAP score. These two measurements capture different things. LSM measures stiffness, which reflects scarring. CAP measures how much the ultrasound signal fades as it passes through tissue, which reflects fat content. Together, the two numbers give a more complete picture of liver health, especially in fatty liver disease where both fat buildup and scarring can progress independently.
One useful distinction: LSM becomes more accurate as fibrosis gets worse, meaning it’s especially good at catching advanced scarring and cirrhosis. CAP, on the other hand, becomes less precise at distinguishing between higher grades of fat accumulation.
What Can Throw Off Your Results
Several things besides actual scarring can make your liver temporarily stiffer, leading to a falsely elevated reading. Active inflammation is a common one. If your liver is currently inflamed from a hepatitis flare or another cause, your LSM score may read higher than it would once the inflammation settles. This is one reason doctors sometimes repeat the test after treatment or after inflammation has resolved.
Other factors that can push your number up include increased pressure in the veins that drain the liver (which can happen with heart failure), bile duct blockages, and even eating recently. Food intake raises liver blood flow and temporarily increases stiffness, so you’ll typically be asked to fast overnight or for at least a few hours before the test. Body weight, interestingly, affects the CAP score but does not directly inflate the LSM number itself.
Why LSM Replaced Liver Biopsy for Many Patients
Before LSM became widely available, the standard way to assess liver scarring was a biopsy, where a needle is inserted into the liver to extract a small tissue sample. Biopsies are still used in certain situations, but they come with real drawbacks: pain, a small risk of bleeding, the need for recovery time, and the possibility that the tiny sample doesn’t represent the whole organ. LSM offered a rapid, painless, and repeatable alternative. Because it can be done in a clinic visit and repeated over time without risk, it’s now the first-line tool for tracking fibrosis progression or improvement in many liver diseases.
The trade-off is precision. A biopsy can reveal the exact pattern and cause of scarring under a microscope, while LSM gives a single stiffness number that can be influenced by the confounding factors described above. For straightforward monitoring of known liver conditions, LSM is often sufficient. For ambiguous or complex cases, a biopsy may still be necessary to get a definitive answer.