The applicable standard for all non-medical testing laboratories is ISO 17025:2017. Depending on the country and accreditation body, a specific program may be on offer. For example, under Life Sciences where the accreditation body will have mandatory additional criteria and recommendations for testing genetically modified (GM) material. An accreditation body could offer a flexible scope of accreditation for laboratories quantifying GMOs. In the European Union for example, there is a published guideline for such flexible scopes. You can go to https://ec.europa.eu/jrc/en/publications-list and search for the keywords “flexible scope GMO”.
In some cases the program would be under General Testing, where accreditation would be per test; for example Material type: Foods and Food Products – Rice: Detection of genetically modified organism; or under the Biological program, for example Screening for Genetically Modified Organisms (GMO) by Immunoassay. I suggest you contact your accreditation body to find out the specific program and additional criteria.