Tumor Markers: How Clinicians Use CA19-9, CA125 and CEA in 2026

Key takeaways

• Tumor markers remain essential but context-dependent tools, helping to assess cancer presence and behavior

• These markers are most useful for tracking treatment response, disease progression and recurrence

• Each marker has a key clinical niche:

  • CA19-9: Mainly used in pancreatic cancer management

  • CA125: Central in ovarian cancer care

  • CEA: Widely used in colorectal cancer follow-up

• Trends over time matter more than single results, as falling or rising marker levels can signal treatment outcomes, progression or relapse

• Limitations are significant and varied, making standalone use of classic tumor markers unreliable

• Advances such as multi-marker panels, liquid biopsies and AI-driven analysis are enhancing accuracy, with traditional markers increasingly used alongside newer technologies rather than being replaced

• Medix Biochemica is a trusted provider of high-quality tumor markers for use in cancer testing and monitoring 

Tumor markers are molecules that can be detected in blood, tissue or other body fluids and provide information about the presence or behavior of cancer. They’re produced either by cancer cells themselves or by the body in response to cancer.¹ 

Classic examples of tumor markers include CA19-9, CA125 and CEA, which have long been used to help oncologists monitor disease progression and guide treatment.²

In 2026, these traditional tumor markers are still clinically significant, even with the use of new biomarkers and personalized medicine approaches on the rise. It’s important for clinicians to understand the contemporary roles of these key markers in patient care.

There are certain limitations that mean tumor marker measurements must be interpreted carefully and in context:^3,4

• Many healthy people have a little CA19-9 or CEA circulating normally, although cancer can elevate these levels significantly
• Not all cancers have a known tumor marker, and not every patient’s cancer will produce these markers
• Some benign diseases can also sometimes cause elevated marker levels

Clinically, tumor markers are valued for offering information beyond what scans or symptoms alone can provide. Doctors measure these substances to assess how aggressive a cancer might be, predict prognosis, monitor response to treatment and check for recurrence after treatment. For example, a falling marker level during chemotherapy can signal that a tumor is shrinking, but a rising level after surgery might raise concern that cancer has returned.¹ 

Some tumor markers are even used to tailor therapy. For example, certain markers in tumor tissue can determine eligibility for targeted drugs.¹

Overall, tumor marker testing has become a routine tool in oncology care, guiding decisions alongside imaging and biopsies.¹ But it’s crucial to recognize what tumor markers cannot do perfectly, especially when it comes to early cancer detection. 

Let’s look at three quintessential tumor markers in detail.

CA19-9 (cancer antigen 19-9, carbohydrate antigen 19-9)

CA19-9 is a tumor marker commonly linked to pancreatic cancer. In patients with pancreatic ductal adenocarcinoma (PDAC), CA19-9 is often elevated, which makes it the go-to marker for this disease’s diagnosis and management.⁴

CA19-9 is also produced by some other gastrointestinal cancers, like bile duct cholangiocarcinoma, colorectal and gastric cancers, and even by certain non-digestive malignancies in the gynecologic, pulmonary or thyroid systems. However its primary application is in pancreatic cancer.⁴

Uses of CA19-9

In modern clinical settings, CA19-9 is best used as a tool for monitoring disease progression rather than for early detection. Oncologists track a pancreatic cancer patient’s CA19-9 levels over time to gauge how the cancer is responding to treatment.^4,5

Typically, CA19-9 levels drop when treatment (such as chemotherapy) is effectively shrinking the tumor, and rise if the tumor is growing or progressing. This dynamic makes CA19-9 an important adjunct to scans:^4,5

• A sharp increase in CA19-9 can be an early warning of recurrence or treatment failure, prompting further imaging or a change in therapy
• After curative surgery for pancreatic cancer, a rising CA19-9 can be one of the first signs of recurrence, often detected before symptoms appear on a scan

These early signals allow doctors to intervene sooner, which in some cases might improve outcomes or eligibility for additional surgery.^4,5

Limitations of CA19-9

CA19-9 has some notable limitations and quirks:

• About 5%–10% of people have a genetic trait (Lewis-negative blood type) that prevents them from producing CA19-9 at all. So a CA19-9 test will stay falsely low even if they have an advanced pancreatic cancer⁴
• CA19-9 levels can be elevated by benign conditions like pancreatitis (inflammation of the pancreas), bile duct obstruction (like gallstones or cholangitis) and liver diseases (such as cirrhosis)^4,5
• Because high CA19-9 levels can mean different things, a CA19-9 test is not used by itself to screen for or diagnose cancer or other diseases^4,5 

The role of CA19-9 today

In 2026 and beyond, CA19-9 remains a workhorse in pancreatic cancer management. But on its own, CA19-9 is ineffective for catching early pancreatic tumors in asymptomatic people. Even in high-risk groups, CA19-9 often fails to flag cancer early enough. Researchers are therefore striving to improve early detection of this deadly cancer.⁴

Emerging strategies are trying to harness CA19-9 in combination with other factors. For example, new studies suggest that in patients over a certain age with new-onset diabetes (a group at elevated risk for pancreatic cancer), checking CA19-9 might have some predictive value – especially if CA19-9 remains abnormally high after blood sugar is brought under control. The idea is that unexplained persistent CA19-9 elevation in a diabetic patient could warrant closer imaging surveillance for a hidden pancreatic tumor.⁴

Scientists are also exploring other biomarkers (such as DNA mutations from tumor cells) to pair with CA19-9, aiming for a more reliable panel for early pancreatic cancer detection.⁶

For now, CA19-9’s chief value is in known cancer patients, helping doctors monitor treatment and look out for relapse.^4,5

CA125 (cancer antigen 125)

CA125 is best known as the blood tumor marker for ovarian cancer, particularly epithelial ovarian carcinoma.⁶ 

Uses of CA125

High levels of CA125 are often seen in ovarian cancer patients, especially those with advanced-stage disease. CA125 has been used in multiple aspects of care, from helping evaluate an ovarian mass, to monitoring response to chemotherapy to checking for cancer recurrence after treatment.⁷

CA125 plays a supporting role in cancer diagnosis. When a patient presents with a suspicious ovarian tumor or persistent bloating and pelvic pain, a CA125 test might be ordered. If the level is high, it increases concern that the ovarian mass is cancerous.⁷

Limitations of CA125

CA125 is not perfect as a diagnostic marker. Some ovarian cancers (like certain subtypes or early-stage tumors) may not elevate CA125 much. On the other hand, numerous benign conditions can raise CA125 (common non-cancer causes of high CA125 include uterine fibroids and menstruation).⁷ Therefore, a single CA125 result cannot confirm ovarian cancer. Rather, it’s used together with imaging tests and a surgical biopsy for definitive diagnosis.

Treatment response and recurrence

The role of CA125 becomes central after an ovarian cancer diagnosis is established. During treatment, especially chemotherapy, doctors track CA125 levels to gauge how the cancer is responding. Oncologists often establish a baseline CA125 level at diagnosis and then look for significant changes over time as a trigger for further action.¹⁰

In monitoring for recurrence, CA125 has long been used as an early indicator that ovarian cancer might be coming back. Often, CA125 levels start to rise months before clinical symptoms of recurrence appear.¹⁰

The role of CA125 today

In the landscape of ovarian cancer markers, the CA125 biomarker is no longer alone. HE4 (human epididymis protein 4) is another blood marker that has been approved and, when combined with CA125 in the ROMA (Risk of Ovarian Malignancy Algorithm), may improve accuracy in evaluating pelvic masses.¹¹

Tests like OVA1 also reflect a modern trend, using a panel of biomarkers rather than a single marker to improve diagnostic performance.⁹

CA125 remains a valuable tumor marker, but it’s increasingly being used as part of a broader personalized medicine approach, complemented by other markers and tests to guide decision-making.¹⁰ 

CEA (carcinoembryonic antigen)

CEA tumor markers are associated with several cancers and most commonly used in the management of colorectal cancer.¹²

Many gastrointestinal (GI) tumors can cause elevated CEA levels, including cancers of the colon/rectum, stomach and pancreas, and also non-GI malignancies like lung, breast, thyroid and ovarian cancer.¹²

Uses of CEA

The principal use of CEA today is in colorectal cancer patients, especially to monitor for disease recurrence after initial treatment.¹²

High levels of CEA are produced during fetal development, but it’s usually present only at very low levels in healthy adults. In colon cancer, however, tumor cells often resume production of CEA, secreting it into the bloodstream.¹³

Oncologists will typically check a baseline CEA level when a patient is diagnosed with colon cancer. If it’s elevated pre-surgery, that can correlate with a higher tumor burden or metastatic disease.^12,13

Normal CEA doesn’t rule out cancer, as not all colon cancers produce CEA.^12,13

Recurrence

After surgical removal of a colon tumor, the patient’s CEA level often falls to normal (assuming it was high to begin with), providing a tumor-marker baseline for that patient. From then on, regular CEA tests are done during follow-up appointments.¹³

Serial CEA monitoring can detect recurrent colorectal cancer with about 80% sensitivity and 70% specificity, and it often flags recurrence a few months earlier than imaging might. This early detection can be life-saving.¹³

CEA is also used during treatment monitoring for metastatic colorectal cancer. For example, a patient on chemotherapy may have their CEA checked periodically and, if it was elevated at baseline, a downward trend is a good sign that the tumors are responding. Rising CEA during therapy could signal progression.¹³

CEA levels are used in prognosis too. For example, very high preoperative CEA often means a higher likelihood of metastasis. CEA also has roles in other cancers’ follow-up, including some lung cancers and medullary thyroid carcinoma. However, while CEA is the flagship tumor marker for colon cancer, for most of these cancers, other markers or imaging are primary and CEA is secondary.¹³ 

Limitations of CEA

Just like CA19-9 and CA125, CEA is not suitable as a general screening test for cancer. Healthy adults should have virtually no CEA in their blood, but many non-cancerous conditions can cause mild CEA elevations, especially conditions that affect the liver’s ability to metabolize CEA or that cause inflammation. Therefore, an isolated CEA result is not diagnostic.¹³

The role of CEA today

CEA continues to be a critical part of colorectal cancer follow-up, but we’re seeing advancements in how recurrence is detected. 

One development is the rise of ctDNA (circulating tumor DNA) tests for minimal residual disease detection. These high-tech blood tests look for tiny fragments of tumor-specific DNA in the blood after surgery. Early studies have found that ctDNA can sometimes identify a recurrence before CEA rises or imaging shows anything, potentially redefining surveillance in the coming decade.⁶

While ctDNA assays are not yet standard of care, they represent the direction of personalized cancer monitoring. In the near future, CEA testing could potentially be complemented – or even partially supplanted – by these genetic marker tests, for a more sensitive detection of recurrence. 

For now, CEA remains a widely used blood test that offers great value in managing colorectal cancer, improving the chances of returning cancer being caught early enough to act on.^12,13 

CEA exemplifies how a classic tumor marker continues to serve as a backbone in oncology, even as new innovations emerge.

Emerging biomarkers and the future of tumor marker testing

While CA19-9, CA125 and CEA are well-established in clinical practice, the field of cancer diagnostics is continually evolving. A wave of emerging biomarkers and technologies is enriching what we can do beyond these traditional markers. 

Here are a few of the current trends oncology is shifting towards:

Multi-analyte testing 

Instead of looking at a single protein, newer tests examine multiple signals simultaneously to improve accuracy.⁶

Liquid biopsies

These are blood tests that detect cancer-derived material released by tumors into the blood. Liquid biopsy tests, particularly those looking at ctDNA, are already being used in some settings to guide treatment and to monitor for recurrence with high sensitivity.⁶

Personalized medicine

Tumor markers are expanding beyond diagnosis and monitoring into guiding therapy selection. We now often use the term “biomarker” to include genetic alterations in tumors that predict response to treatments.¹

Artificial intelligence (AI)

AI technology is being applied to biomarker discovery and interpretation. Machine learning (ML) algorithms can sift through complex patterns (e.g., thousands of proteins or gene expression data points) to find combinations that best predict early cancer or treatment response.⁶

Frequently asked questions

What is a tumor marker?

A tumor marker is a biological “signpost” of cancer in the body. It’s any substance present in or produced by cancer cells that provides information about a tumor. Most commonly, tumor markers are proteins that are made in higher amounts by cancerous tissue and detected through blood tests. Tumor markers can be found in blood, urine, stool, tumor tissue or other bodily fluids. Some are organ-specific (made mostly by one type of cancer), while others can be elevated in a variety of diseases.¹

How are tumor markers used in early cancer detection?

Traditional tumor markers are not standalone diagnostic tools. Doctors may use them to screen high-risk patients or to check for cancer in a patient with certain symptoms. They can also be used to monitor a patient’s response to treatment and to detect signs of recurrence early on.^10,13

Partner with Medix Biochemica, a reliable supplier of excellent quality tumor markers

For an IVD (in vitro diagnostics) raw materials provider like Medix Biochemica, these developments mean that the demand is growing not only for high-quality antibodies and antigens for the classic markers (like CA19-9, CA125 and CEA), but also for new reagents to detect emerging biomarkers. Test-kit manufacturers are looking for robust components to build multiplex assays and liquid biopsy platforms. 

The continued relevance of CA19-9, CA125 and CEA in patient management means that reliable sources of these antigens and corresponding antibodies remain crucial for the diagnostic industry. At the same time, companies are investing in materials for newer markers, positioning themselves to support the next generation of cancer detection tools. 

The future of tumor marker testing is undoubtedly moving toward a more comprehensive and personalized approach, but it builds on the solid foundation that markers like CA19-9, CA125 and CEA have provided over years of clinical use.

Tumor marker testing provides critical information that can guide care at every step, and ongoing improvements in biomarker science are making these signals even more powerful. At Medix Biochemica, ensuring a reliable supply of marker test raw components (such as high-quality antibodies, antigens and control materials) is vital for the laboratories we partner with to deliver accurate results.¹⁴ 

Download Medix Biochemica’s Tumor Markers Catalog to see our full selection of antibodies and biospecimens for use in cancer-detection immunoassays 

Speak to our sales team for more information on our tumor markers and other raw materials

References

1. Tumor markers. National Cancer Institute. Accessed April 27, 2026. https://www.cancer.gov/about-cancer/diagnosis-staging/diagnosis/tumor-markers-fact-sheet.
2. Tumor marker tests in common use. National Cancer Institute. Accessed April 27, 2026. https://www.cancer.gov/about-cancer/diagnosis-staging/diagnosis/tumor-markers-list.
3. Tumor marker tests. Medlineplus. Accessed April 27, 2026. https://medlineplus.gov/lab-tests/tumor-marker-tests/.
4. Lee T, Teng TZJ, Shelat VG. Carbohydrate antigen 19-9 - tumor marker: Past, present, and future. World J Gastrointest Surg. 2020 Dec 27;12(12):468-490. doi: 10.4240/wjgs.v12.i12.468.
5. CA 19-9 blood test (Pancreatic cancer). Medlineplus. Accessed April 27, 2026. https://medlineplus.gov/lab-tests/ca-19-9-blood-test-pancreatic-cancer/.
6. Qoronfleh MW, Al-Dewik N. Cancer biomarkers: Reflection on recent progress, emerging innovations, and the clinical horizon. Cancers (Basel). 2025 Sep 12;17(18):2981. doi: 10.3390/cancers17182981.
7. CA 125 test. Mayo Clinic. Accessed April 27, 2026. https://www.mayoclinic.org/tests-procedures/ca-125-test/about/pac-20393295.
8. Gandhi T, Zubair M, Bhatt H. Cancer antigen 125. In: StatPearls. StatPearls Publishing; 2026. Accessed April 27, 2026. http://www.ncbi.nlm.nih.gov/books/NBK562245/.
9. OVA1. National Cancer Institute Early Detection Research Network. Accessed April 27, 2026. https://edrn.cancer.gov/data-and-resources/biomarkers/ova1/.
10. Momenimovahed Z, Mazidimoradi A, Allahqoli L, et al. The role of CA‐125 in the management of ovarian cancer: A systematic review. Cancer Rep (Hoboken). 2025;8(3):e70142. doi:10.1002/cnr2.70142.
11. Van Gorp T, Cadron I, Despierre E, et al. HE4 and CA125 as a diagnostic test in ovarian cancer: Prospective validation of the Risk of Ovarian Malignancy Algorithm. Br J Cancer. 2011;104(5):863-870. doi:10.1038/sj.bjc.6606092.
12. CEA test. Medlineplus. Accessed April 27, 2026. https://medlineplus.gov/lab-tests/cea-test/.
13. Kankanala VL, Zubair M, Mukkamalla SKR. Carcinoembryonic antigen. In: StatPearls. StatPearls Publishing; 2026. Accessed April 27, 2026. http://www.ncbi.nlm.nih.gov/books/NBK578172/.
14. Tumor markers. Medix Biochemica. Accessed April 27, 2026. https://info.medixbiochemica.com/tumor-markers.