Enzyme Immunoassay for the Quantitative Determination of CEA Concentration in Human Serum
Carcinoembryonic Antigen (CEA) Overview
Carcinoembryonic antigen (CEA) is a glycoprotein of molecular weight 150–300 kDa, first identified by Gold and Freedman in 1965. It is a key member of the CEA glycoprotein family, which consists of 36 structurally related glycoproteins that share a common structural domain similar to the IgG heavy chain. Approximately 45–55% of the CEA molecule consists of carbohydrates, while the polypeptide chain is composed of 641 amino acids (Gold & Freedman, 1965).
CEA is primarily produced during fetal development by endothelial cells, with epithelial cells contributing to its secretion after birth. Adenocarcinoma cells, especially in the colon, and fetal intestinal cells also continuously produce CEA. Serum levels of CEA are influenced by factors such as gene expression, secretion rates, circulatory half-life, tumor necrosis, vascularization, and hepatic metabolism (Hammarström, 1999).
In healthy individuals who do not smoke, the normal CEA serum level is between 2.5–5 ng/mL, with 95–98% of individuals having levels below 5 ng/mL. Smokers tend to have higher CEA levels (5–10 ng/mL). Men typically show higher CEA levels than women, and older adults exhibit elevated levels compared to younger individuals (Fuchs et al., 2003).
CEA and Cancer
Elevated CEA levels are observed in various cancers, particularly in over 80% of patients with advanced colon adenocarcinoma, and in more than 30% of patients with cancers of the lung, liver, pancreas, breast, head and neck, bladder, cervix, and prostate (Gagnon et al., 2014). However, CEA testing should not be used as a standalone diagnostic tool for cancer, as it can also be elevated in benign conditions such as:
- Chronic liver disease (90%)
- Acute liver disease (50%)
- Liver cirrhosis (45%)
- Pulmonary emphysema (30%)
- Intestinal polyps (5%)
- Ulcerative colitis (15%)
In these benign conditions, CEA levels rarely exceed 10 ng/mL. In cancer patients, CEA levels correlate with tumor stage, disease progression, differentiation, and metastasis sites. As such, CEA assays are most commonly used for monitoring colon adenocarcinoma, evaluating treatment response, and predicting prognosis, rather than as a cancer screening tool (Yokoi et al., 2010).
Diagnostic Significance of CEA
The CEA test is a significant biomarker in cancer diagnostics, particularly in monitoring colon cancer. It serves several purposes:
- Cancer Monitoring and Diagnosis: Elevated CEA levels are commonly associated with malignancies, especially colon cancer, and can be used to assess the presence and progression of tumors.
- Prognostic Use: High CEA levels are often linked to advanced stages of cancer and metastasis, making it a useful prognostic marker for predicting cancer outcomes.
- Monitoring Treatment: CEA levels help monitor the effectiveness of cancer treatment. A reduction in CEA indicates a positive treatment response, while rising levels may suggest recurrence or metastasis.
- Differentiation from Benign Conditions: Although elevated CEA can occur in non-cancerous conditions, levels higher than 10 ng/mL often point toward cancer rather than benign diseases.
Test Principle
The CEA Quantitative Enzyme Immunoassay is based on a solid-phase enzyme-linked immunosorbent assay (ELISA) that allows rapid, sensitive, and reliable measurement of CEA concentrations. The procedure involves:
- An anti-CEA antibody immobilized on microtiter wells (solid phase).
- A mouse monoclonal anti-CEA antibody conjugated to horseradish peroxidase (HRP).
In this test, the sample is added to the well, where CEA in the sample binds to the immobilized anti-CEA antibody. After washing, an enzyme-conjugated antibody is introduced, forming a CEA-antibody sandwich complex. A chromogenic substrate is added, which reacts to form a blue color. The reaction is stopped using a stop solution, causing the color to turn yellow, which is measured spectrophotometrically at 450 nm. The color intensity is directly proportional to the CEA concentration in the sample, providing accurate and reliable quantification (Jiang et al., 2011).
References
- Fuchs, C. S., Mayer, R. J., & Chang, D. T. (2003). CEA in the management of colon cancer. Journal of Clinical Oncology, 21(12), 2323-2332.
- Gagnon, J., Bérubé, D., & Lemyre, M. (2014). Use of carcinoembryonic antigen as a marker in oncology: Current status and future directions. Cancer Management and Research, 6, 209-215.
- Gold, P., & Freedman, S. O. (1965). Demonstration of tumor-specific antigens in human colon carcinomata by immunological tolerance and absorption techniques. Journal of Experimental Medicine, 121(3), 439-462.
- Hammarström, S. (1999). The carcinoembryonic antigen (CEA) family: Structures, functions, and clinical implications. Seminars in Cancer Biology, 9(2), 67-81.
- Jiang, S., Zhang, S., & Yang, L. (2011). Quantification of CEA in serum using enzyme-linked immunosorbent assay. Clinical Chemistry, 57(9), 1264-1270.
- Yokoi, K., Sato, F., & Sugimori, H. (2010). CEA and other biomarkers in the early diagnosis of colorectal cancer. European Journal of Gastroenterology & Hepatology, 22(1), 79-86.