When drug panel testing is required for construction sites, industrial facilities, or specific careers or job roles, it is important to note that there is usually a specific validity period assigned to results. Many industrial plant sites, for instance, will require employees on-site to have been tested within the past 30, 60, or 90 days, and they will not accept test results older than that. If you are having employees tested on the request of a client such as this, make sure that you note the schedule according to which refresher testing will be required.