The Quality management system requirements for a laboratory must be based on the General requirements of ISO 17025 plus any sector-specific regulations and accreditation body program requirements.
In some situations the Scope of tests is regulated, otherwise it is the laboratory’s decision which test methods will be accredited and which not. The internal audit purpose is to assess whether the management system conforms to all the identified requirements (ISO 17025 and others) and if it is implemented and maintained effectively. The audit criteria are then the requirements. An audit program (schedule) must be established and the Scope of the various audits over a period would change, depending on what test method or area of work is being audited.
For more information, have a look at the ISO 17025 toolkit at https://advisera.co/ISO17025Toolkit, where there are there is a procedure, audit program format, an audit checklist, and report forms to guide you.
Also have a look at the articles ISO 9001 Horizontal audit vs. vertical audit at https://advisera.co/HorizontalVsVerticalAudit and ISO 17025 Technical internal audit: The basics at https://advisera.co/17025TechnicalAudit