Carcinoembryonic antigen (CEA) is a glycoprotein, with a molecular weight of 180 kDa, involved in cell adhesion. It is normally produced during fetal development, but the production of CEA stops before birth. Therefore, it is not usually present in the blood of healthy adults, although levels are raised in heavy smokers. CEA was identified in human colon cancer tissue extracts . It was later found that serum from individuals with colorectal and other carcinomas had higher levels of CEA than healthy individuals and can be used to monitor the response to colon cancer treatment.
CEA and related genes make up the CEA family belonging to the immunoglobulin superfamily.
The most frequent cancer which causes an increased CEA is cancer of the colon and rectum. Others include cancers of the pancreas, stomach, breast, lung, and certain types of thyroid and ovarian cancer. Benign conditions which can elevate CEA include smoking, infections, inflammatory bowel disease, pancreatitis, cirrhosis of the liver, and some benign tumors in the same organs in which an elevated CEA indicates cancer. Chemotherapy and radiation therapy can cause a temporary rise in CEA due to the death of tumor cells and release of CEA into the blood stream. Benign tumours do not usually cause an increase above 10 ng/ml.

May be diagnosed by:

• Determination of specific antibodies based on the ELISA technique
• Determination of specific antibodies based on the RIA-technique

CEA ELISA:

Immunoenzymatic colorimetric method for quantitative determination of CEA in human serum.

Principle of the Assay:

In this method, CEA standards, patient specimens and/or controls containing the native antigen are first added to streptavidin coated wells. Biotinylated monoclonal and horseradish peroxidase (HRP) labeled antibodies are added and the reactants are mixed. The different types of antibodies used have high affinity and specificity and are directed against distinct and different epitopes of CEA. Reaction between the various CEA antibodies and native CEA occurs in the microwells without competition or sterics hindrance forming a soluble sandwich complex. The interaction is illustrated by the following equation:

 
BtnAb(m) = Byotinilated Monoclonal Antibody (Excess Quantity)
AgCEA = Native Antigen (Variable Quantity)
E-Ab = enzyme labeled Antibody (Excess Quantity)
HRP-Ab(p)-AgCEA-BtnAb(m) = Antigen-Antibodies Sandwich Complex  
AgTSH = Native Antigen (Variable Quantity)
HRP-Ab(p) = HRP labeled Antibody (Excess Quantity)
HRP-Ab(p)-AgTSH-BtnAb(m) = Antigen-Antibodies Sandwich Complex
Ka = Rate Constant of Association
K-a = Rate Constant of Dissociation

Simultaneously, the complex is fixed to the well through the high affinity reaction of streptavidin and biotinylated antibody. This interaction is illustrated below: 

Streptavidin CW = Streptavidin immobolized on well.
Immobilized Complex = Antibodies-Antigen sandwich bound.

After equilibrium is attained, the antibody-bound fraction is separated from unbound antigen by aspiration. The native antigen concentration is directly proportional to the HRP activity in the antibody-bound fraction. The activity of the conjugated HRP is quantitated by reaction with TMB substrate to produce blue colour. The reaction is terminated by adding stop solution which turns the blue colour into yellow. The absorbance is measured on a plate reader.

Specific performance characteristics: 

Correlation with RIA performed on 36 samples is r = 0.99

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