Immunoassays (the ELISA method)

Source obtained from: http://afic.org/Detecting%20Genetically%20Modified%20Foods%20-%20Easier%20said%20than%20done.htm

Rather than measuring the rDNA in a sample, immunoassays measure the levels of proteins expressed by DNA sequences inserted by genetic modification. The test works by using anti-bodies specific for proteins encoded by rDNA sequences. One example of a commonly used immunoassay is the Enzyme-Linked Immunosorbent Assay (or the ELISA).

The ELISA relies on a reaction between antibodies (soluble proteins that are produced by the immune system in response to a foreign substance) and a "foreign substance" (typically the inserted protein), called the "antigen". The reaction is detected by a colour change or a flourometric reaction that can be measured quantitatively.

Immunoassays are less sensitive than PCR methods, which means they are less susceptible to false positives caused by minor levels of contamination. However, careful validation of each food type is needed before the test can be performed because of the large diversity of foods. This means that new assays need to be continually developed as new products of biotechnology are developed. Many hundreds of different assays will eventually be required to enable accurate detection methods.

One of the key advantages of this method of analysis is that the results are generally available within minutes. The costs of immunoassays are also much lower than that for PCR techniques (about US$2-10 per sample) however the costs for the assay development and the generation of antibodies and protein standards mean that the up-front costs can be significant.

One problem with immunoassays is that the technique does not distinguish between different sources of biotechnology-derived rDNA that may express similar protein characteristics. For example, the technique won't detect if a specific protein, such as the Bt protein, is derived from corn or soy. In addition, proteins are denatured by many food processing methods so this technique is more useful for raw foods or food ingredients that have undergone minimal processing.

Sampling methods

The choice of technique largely depends on the type of product being analysed and the availability of equipment and funds for the analysis. Both techniques also share some common problems such as a lack of internationally recognised sampling methods or agreement on the number or size of samples required.