A laboratory internal audit is usually a systems audit or a technical audit, or possibly a combination of both.
Systems audit

Directed at management or administration activities such as reception of samples or preparation and issue of test reports.

Directed at adherence to the documented procedures - ’are you doing exactly what you say you are doing in your procedure?’

Technical audit

Directed at technical correctness of each activity such as the actual performance of tests or laboratory quality control.

The auditor conducting a technical audit must have technical knowledge of the process or test method to be able to understand the implications of incorrectly performing the activity.

Combination audits

Audits which involve the whole laboratory or all processes within a particular department i.e. vertical audit.