Cytokeratin-18, often shortened to CK-18 or K18, is a protein found inside many epithelial cells, including liver cells. When liver cells are injured or die, fragments of CK-18 can enter the bloodstream. That is why CK-18 has been studied as a blood marker of liver cell death, especially in fatty liver disease and steatohepatitis. In older wording, steatohepatitis related to fatty liver is often called NASH. In newer medical wording, it is usually called MASH, which stands for metabolic dysfunction-associated steatohepatitis. The CK-18 blood test is most useful to understand as a research and specialist test, not as a stand-alone diagnosis. A high result can suggest more active liver cell injury, but it cannot prove NASH or MASH by itself. Results need context from liver enzymes, fibrosis scores, imaging, metabolic risk factors, medications, alcohol intake, and sometimes liver biopsy.
- CK-18 measures liver cell injury indirectly by detecting cytokeratin-18 fragments released into blood after cell death.
- High CK-18 can be seen with steatohepatitis, but it is not specific to fatty liver and does not diagnose NASH or MASH alone.
- CK-18 is different from the CK or creatine kinase blood test used for muscle injury.
- There is no single universal CK-18 normal range; results depend on the assay, lab method, and clinical setting.
- Fasting is usually not required for CK-18 alone, but fasting may be needed if it is ordered with glucose, insulin, triglycerides, or other metabolic tests.
- Abnormal CK-18 results are usually interpreted with ALT, AST, GGT, platelet count, FIB-4, ELF, liver imaging, and metabolic risk markers.
Table of Contents
- What the CK-18 Blood Test Measures
- Why CK-18 Is Studied in Fatty Liver and NASH
- When CK-18 Testing May Be Used
- How to Understand CK-18 Results
- CK-18 vs Liver Enzymes and Fibrosis Tests
- Accuracy, Limits, and Common Mistakes
- What to Do After an Abnormal CK-18 Result
- Preparation, Safety, and Follow-Up Questions
What the CK-18 Blood Test Measures
Cytokeratin-18 is a structural protein that helps support the internal framework of epithelial cells. Hepatocytes, the main working cells of the liver, contain CK-18. When these cells are injured, stressed, or die, CK-18 fragments can leak or be released into the bloodstream.
The test is mainly discussed in liver disease because cell death is part of the process that separates simple fat buildup from more active inflammatory disease. A liver can contain excess fat without much cell injury. A different picture appears when fat accumulation is accompanied by inflammation, ballooning injury, and cell death. That more active pattern is why CK-18 has attracted interest.
There are two important CK-18 testing concepts:
- Caspase-cleaved CK-18, often measured by M30-type assays, reflects apoptosis. Apoptosis is a controlled form of cell death in which enzymes called caspases cut proteins inside the cell.
- Total soluble CK-18, often measured by M65-type assays, reflects a broader amount of cell death, including apoptotic and necrotic injury.
That distinction matters because the result may describe a biological process, not just a liver diagnosis. A high M30-type result suggests more caspase-related epithelial cell death. A high M65-type result suggests a larger total burden of epithelial cell death. Neither result tells the full cause on its own.
CK-18 can also be confusing because “CK” is commonly used for creatine kinase, a muscle enzyme. A creatine kinase test is often used when doctors are checking muscle injury, rhabdomyolysis, or exercise-related muscle breakdown. Cytokeratin-18 is a different marker. If a lab report says “CK,” “CK total,” or “creatine kinase,” it is not the same as cytokeratin-18.
CK-18 is a cell-death marker, not a routine liver function test
Routine liver blood panels usually include enzymes and proteins such as ALT, AST, ALP, GGT, bilirubin, albumin, and sometimes INR. These tests show patterns of liver irritation, bile flow problems, or liver synthetic function. CK-18 is narrower: it is meant to reflect epithelial cell death, especially hepatocyte death when the clinical setting points to liver disease.
That narrower focus can be helpful, but it also limits the test. A high CK-18 result does not automatically mean a person has severe liver scarring. A normal or low result does not rule out fatty liver, fibrosis, or early cirrhosis. It gives one piece of information about active cell injury at the time the blood sample was taken.
Why CK-18 Is Studied in Fatty Liver and NASH
Fatty liver disease exists on a spectrum. At one end, fat builds up in liver cells with little inflammation or damage. At the more active end, fat buildup is accompanied by liver cell ballooning, inflammation, and cell death. This active form was traditionally called nonalcoholic steatohepatitis, or NASH. Under newer nomenclature, metabolic dysfunction-associated steatohepatitis, or MASH, is the preferred term when the disease is linked to metabolic risk factors.
CK-18 is studied because liver cell death is one of the biological features of steatohepatitis. When hepatocytes are injured by lipotoxicity, oxidative stress, mitochondrial strain, insulin resistance, inflammatory signals, and related metabolic stress, CK-18 fragments may increase in blood.
Fat alone is not the whole story. Many people have fatty liver on ultrasound, CT, or MRI without having aggressive disease. The larger clinical concern is whether the liver is developing active inflammation and fibrosis. Fibrosis means scar tissue. Over years, fibrosis can progress to advanced fibrosis or cirrhosis in some people.
CK-18 gained attention because it may better reflect the “injury” part of fatty liver disease than ALT or AST alone. Someone can have normal liver enzymes and still have meaningful fatty liver disease. Another person can have mildly high ALT for reasons that do not match the amount of fibrosis. CK-18 offers a different window: cell death rather than enzyme leakage alone.
Metabolic risk is central. Fatty liver and steatohepatitis are more common in people with insulin resistance, type 2 diabetes, higher waist size, high triglycerides, low HDL cholesterol, high blood pressure, and sleep apnea. When CK-18 is interpreted in this setting, doctors usually also look at glucose and insulin patterns, lipid markers, blood pressure, body weight trends, and family history. Tests such as HOMA-IR may be used in some settings to estimate insulin resistance, although they do not diagnose liver inflammation.
The terminology can feel messy because older studies often use NAFLD and NASH, while newer sources use MASLD and MASH. In everyday lab interpretation, the clinical idea is similar: CK-18 is being considered as a marker of active liver cell injury in people whose fatty liver appears tied to metabolic dysfunction.
When CK-18 Testing May Be Used
CK-18 is not usually part of a standard yearly physical, a routine liver panel, or a basic workup for mildly abnormal liver enzymes. It is more often used in research, clinical trials, specialty liver clinics, or selected cases where a clinician wants extra information about liver cell death.
A clinician may consider CK-18 testing when a person has known or suspected fatty liver disease and the question is whether there may be active steatohepatitis. It may also be ordered when routine tests and imaging do not clearly explain the level of concern. For example, a person may have metabolic syndrome, fatty liver on imaging, and only mildly abnormal ALT. CK-18 might be used as one additional clue, especially if the clinician is trying to avoid unnecessary invasive testing.
CK-18 may also appear in studies that track whether a treatment reduces liver cell injury. If a medication, weight-loss intervention, or metabolic treatment lowers CK-18 over time, that can suggest reduced cell death. Even then, improvement in CK-18 does not automatically prove that fibrosis has improved.
Common situations where CK-18 may be discussed include:
- Suspected MASH or NASH in a person with metabolic risk factors
- Fatty liver with persistent ALT or AST elevation
- Clinical trial screening or treatment monitoring
- Research comparing blood markers with liver biopsy findings
- Specialist evaluation when other noninvasive results are mixed
- Monitoring trends when the same lab and same assay are used over time
CK-18 is less helpful when the cause of liver injury is unclear and no broader evaluation has been done. Viral hepatitis, alcohol-related liver disease, autoimmune liver disease, drug-induced liver injury, biliary obstruction, iron overload, and other conditions can all affect liver health. CK-18 should not be used to skip that evaluation.
A more typical first step is a liver panel and risk assessment. A hepatic function panel can show ALT, AST, bilirubin, albumin, and related markers. Depending on the pattern, clinicians may add viral hepatitis tests, iron studies, autoimmune markers, ultrasound, elastography, or metabolic testing.
How to Understand CK-18 Results
CK-18 results should be read using the reference interval and method shown on the specific lab report. There is no single normal range that applies across all CK-18 tests. Results can vary by assay type, antibody target, calibration, specimen handling, and whether the lab reports caspase-cleaved CK-18 or total CK-18.
Some reports use units such as U/L, while others may use assay-specific units. A result that looks high by one method may not match a cutoff used in a different study. For this reason, CK-18 is often more meaningful as a trend when repeated using the same assay than as a one-time number compared with a general internet cutoff.
| CK-18 pattern | What it may suggest | What it does not prove |
|---|---|---|
| Within the lab’s reference range | No strong CK-18 signal of increased epithelial cell death at that time | Does not rule out fatty liver, fibrosis, or early cirrhosis |
| Mildly elevated | Possible low-grade liver cell injury or another epithelial cell injury source | Does not diagnose NASH, MASH, or fibrosis stage |
| Clearly elevated | More active cell death may be present, especially if liver disease is already suspected | Does not identify the exact cause without other tests |
| Falling over time | Cell injury may be improving if the same assay is used | Does not guarantee fibrosis reversal |
| Rising over time | Injury may be worsening or a new trigger may be present | Does not show whether scarring has advanced |
A high CK-18 result is most concerning when it fits the rest of the picture: fatty liver on imaging, metabolic risk factors, persistently high ALT or AST, high GGT, rising fibrosis scores, or liver stiffness on elastography. A high result is less specific if the person has another active illness that could injure epithelial tissues or liver cells.
High CK-18
High CK-18 usually means more CK-18 fragments were found in the blood than expected for that assay. In a liver-focused setting, this can suggest hepatocyte apoptosis or broader hepatocyte death. In fatty liver disease, this may raise suspicion for steatohepatitis rather than simple steatosis.
Possible reasons for high CK-18 include:
- Metabolic dysfunction-associated steatohepatitis
- Alcohol-related liver injury
- Viral hepatitis
- Drug-induced liver injury
- Autoimmune hepatitis
- Cholestatic or bile duct disease
- Advanced inflammatory liver disease
- Other epithelial tissue injury, depending on the clinical context
A high CK-18 result should not be treated as a diagnosis by itself. It should trigger a more careful look at the full liver pattern. For example, a person with high ALT, high GGT, high triglycerides, central weight gain, and fatty liver on ultrasound has a different risk pattern than a person with sudden jaundice, abdominal pain, and very high bilirubin.
Low or normal CK-18
A low CK-18 result usually has little clinical meaning unless the lab provides a specific interpretive comment. It does not mean the liver is “extra healthy.” It usually means the test did not detect a high level of the CK-18 fragment being measured.
Normal CK-18 can occur in people with simple steatosis. It can also occur in some people with fibrosis, especially if active cell death is not high at the time of testing. Fibrosis is scar tissue from prior or ongoing injury; CK-18 is more about active cell death. Those are related but not identical.
CK-18 vs Liver Enzymes and Fibrosis Tests
CK-18 should not be compared with liver enzymes as though one is simply “better” than the other. They measure different things.
ALT and AST are enzymes that can rise when liver cells are irritated or damaged. ALT is more liver-focused than AST, while AST can also rise from muscle injury and other tissues. Patterns involving ALT and AST can help identify hepatocellular injury, but they do not reliably separate simple fatty liver from steatohepatitis.
GGT can rise with fatty liver, alcohol use, bile duct irritation, certain medications, and metabolic risk. A GGT and ALT pattern can be helpful when interpreting liver stress, but it still does not directly measure liver cell death in the way CK-18 attempts to do.
Fibrosis tests answer a different question. FIB-4, APRI, ELF, transient elastography, and magnetic resonance elastography are used to estimate the chance of liver scarring. A person may have active cell injury with little fibrosis, or advanced fibrosis with only modest current inflammation. That is why CK-18 and fibrosis tests can disagree.
A common liver risk assessment may include:
- ALT, AST, ALP, GGT, and bilirubin
- Albumin, platelet count, and INR when liver function or advanced disease is a concern
- Fasting glucose, A1c, insulin, triglycerides, HDL cholesterol, and blood pressure
- FIB-4 or APRI using routine lab values
- Ultrasound, FibroScan, MRI-PDFF, or magnetic resonance elastography
- ELF or other fibrosis-focused blood tests in selected settings
- Liver biopsy when the diagnosis or fibrosis stage remains uncertain and the result would change care
Scores such as FIB-4 and APRI are not cell-death tests. They use age, AST, ALT, and platelet count to estimate fibrosis risk. The Enhanced Liver Fibrosis test uses markers related to extracellular matrix turnover. CK-18 adds another biological angle: epithelial cell death.
The most useful interpretation comes from combining these categories. A high CK-18 with low fibrosis risk may suggest active injury that has not yet produced advanced scarring. A normal CK-18 with high liver stiffness may suggest that fibrosis risk needs attention even if active apoptosis is not high. A high CK-18 with high fibrosis markers deserves more urgent specialist review.
Accuracy, Limits, and Common Mistakes
CK-18 has shown promise as a biomarker for steatohepatitis, but its performance is not strong enough for it to replace clinical evaluation, imaging, fibrosis assessment, or biopsy when biopsy is truly needed. Studies have found that CK-18 can help distinguish some people with steatohepatitis from those with simple steatosis, but accuracy varies.
Several factors limit the test:
- Different assays measure different CK-18 forms.
- Research cutoffs are not identical across populations.
- Results can overlap between simple steatosis and steatohepatitis.
- CK-18 does not stage fibrosis well by itself.
- Other liver diseases can increase liver cell death.
- CK-18 may rise from non-liver epithelial cell injury in some settings.
- A single result may miss changing disease activity over time.
One common mistake is treating CK-18 as a yes-or-no NASH test. It is better understood as a probability marker. A high result can raise suspicion, especially when the rest of the pattern supports steatohepatitis. A normal result can lower suspicion in some contexts, but it cannot fully exclude disease.
Another mistake is comparing a result with a cutoff from an unrelated study. Research thresholds may depend on biopsy definitions, patient age, diabetes status, body mass index, assay brand, and disease severity. A cutoff from a trial may not apply to an individual outpatient lab report.
A third mistake is using CK-18 to ignore fibrosis risk. Fibrosis stage is one of the strongest predictors of long-term liver outcomes in fatty liver disease. CK-18 may reflect active injury, but fibrosis-focused tests still matter. Someone with diabetes, low platelets, high AST, or increased liver stiffness needs fibrosis assessment even if CK-18 is not very high.
A fourth mistake is overlooking other causes of abnormal liver tests. Fatty liver is common, so it can be blamed too quickly. A careful workup may need to consider hepatitis B, hepatitis C, alcohol intake, autoimmune liver disease, hemochromatosis, Wilson disease in younger patients, alpha-1 antitrypsin deficiency, celiac disease, thyroid disease, medications, supplements, and biliary disease.
Ferritin is a good example of why context matters. Ferritin can rise from inflammation, metabolic liver disease, alcohol use, or iron overload. A person with fatty liver, high ALT, and high ferritin may need a more complete iron and inflammation review rather than assuming one marker tells the whole story. A related pattern is discussed in high ferritin with liver enzymes.
What to Do After an Abnormal CK-18 Result
An abnormal CK-18 result should lead to a structured review, not panic. The next step depends on how high the result is, why the test was ordered, and whether other liver or metabolic markers are abnormal.
A practical follow-up often starts with confirming the basics:
- Check which CK-18 assay was used. M30-type and M65-type tests do not mean exactly the same thing.
- Review the lab’s reference interval. Use the range on the report rather than a general online cutoff.
- Compare with prior CK-18 results if available. Trends are more useful when the same lab method was used.
- Review routine liver tests. ALT, AST, ALP, GGT, bilirubin, albumin, platelet count, and INR can change the level of concern.
- Look at metabolic risk. A1c, fasting glucose, triglycerides, HDL cholesterol, waist size, blood pressure, and medications matter.
- Assess fibrosis risk. FIB-4, elastography, ELF, or other tools may be needed.
- Consider competing causes. Alcohol, viral hepatitis, medications, supplements, autoimmune disease, and bile duct problems may need review.
Lifestyle and metabolic treatment can matter even when CK-18 is only mildly high. Weight loss, improved insulin sensitivity, better triglyceride control, physical activity, and reduced alcohol exposure can lower liver stress in many people with fatty liver disease. For people with obesity or type 2 diabetes, medical treatment choices may also affect liver fat and inflammation, but those decisions should be individualized.
A clinician may refer someone to a liver specialist when CK-18 is high and other risk markers are concerning. Referral is especially reasonable if there is increased liver stiffness, high FIB-4, low platelets, abnormal bilirubin, low albumin, elevated INR, imaging signs of cirrhosis, or a strong family or personal history of liver disease.
Urgent care is different from routine follow-up. Seek prompt medical attention if abnormal liver tests occur with yellow skin or eyes, confusion, severe abdominal pain, vomiting blood, black stools, fainting, new abdominal swelling, fever with right-upper-abdominal pain, severe weakness, or easy bleeding. CK-18 is not the deciding test in those situations; symptoms and standard liver function markers are more important.
Preparation, Safety, and Follow-Up Questions
CK-18 testing is a standard blood draw. A healthcare professional collects blood from a vein, usually in the arm. The risks are small and can include brief pain, bruising, lightheadedness, or mild bleeding at the needle site.
Fasting is usually not needed for CK-18 by itself. However, the test may be ordered with fasting glucose, fasting insulin, triglycerides, or other metabolic markers that do require fasting. Follow the instructions on the lab order, not just the CK-18 test name.
Before testing, it is helpful to tell the clinician about:
- Alcohol intake, including recent heavier use
- New prescription medications
- Over-the-counter pain relievers, especially acetaminophen
- Supplements, bodybuilding products, or herbal products
- Recent infection, hospitalization, or inflammatory illness
- Known viral hepatitis or autoimmune disease
- Pregnancy or major recent weight change
- Prior liver imaging or biopsy results
Because CK-18 is not a routine marker, turnaround time may be longer than standard liver enzymes. Some samples may be sent to a specialty lab. If the result is being used for trend monitoring, try to repeat it through the same lab or same assay when possible.
Useful follow-up questions include:
- Was my test measuring caspase-cleaved CK-18, total CK-18, or both?
- Is the result mildly or clearly above this lab’s range?
- Do my ALT, AST, GGT, bilirubin, platelets, albumin, or INR change the interpretation?
- Do I need fibrosis assessment with FIB-4, ELF, FibroScan, or MRI-based testing?
- Could alcohol, medication, viral hepatitis, iron overload, or autoimmune disease explain the pattern?
- Should the result be repeated, and if so, when?
- Would the result change my treatment plan?
CK-18 can be a useful clue when it is ordered for the right reason. Its strongest role is not as a simple screening test, but as one part of a broader liver assessment. In fatty liver disease, the most important clinical task is to identify who has active steatohepatitis, who has fibrosis risk, and who needs more intensive monitoring or treatment. CK-18 may contribute to that assessment, but it should not carry the full weight of the diagnosis.
References
- AASLD Practice Guidance on the clinical assessment and management of nonalcoholic fatty liver disease 2023 (Guideline)
- EASL-EASD-EASO Clinical Practice Guidelines on the management of metabolic dysfunction-associated steatotic liver disease (MASLD) 2024 (Guideline)
- A multisociety Delphi consensus statement on new fatty liver disease nomenclature 2023 (Position Statement)
- EASL Clinical Practice Guidelines on non-invasive tests for evaluation of liver disease severity and prognosis – 2021 update 2021 (Guideline)
- Cytokeratin-18 fragment levels as noninvasive biomarkers for nonalcoholic steatohepatitis: a multicenter validation study 2009 (Clinical Study)
- Differentiation between cell death modes using measurements of different soluble forms of extracellular cytokeratin 18 2004 (Clinical Study)
Disclaimer
CK-18 results should be interpreted by a qualified healthcare professional who can review your full liver panel, imaging, medical history, medications, alcohol intake, and metabolic risk factors. This information is for education only and cannot diagnose fatty liver disease, NASH, MASH, fibrosis, or cirrhosis. Seek urgent medical care for jaundice, confusion, vomiting blood, black stools, severe abdominal pain, fainting, or new abdominal swelling.
