From the DOT Publication Best Practices for Random Drug and Alcohol Testing.

When an employee is notified, he or she must proceed immediately to the collection site. Contrary to the urban legends circulating among some employees, immediately does not mean two hours. Immediately means that after notification, all the employee’s actions must lead to an immediate specimen collection.

Why? For the integrity of the testing process. Best Practice: Many employers develop random testing procedures or policies that clearly state what activities are acceptable after notification: for instance, which safety-sensitive duties Agency regulations permit them to complete. If an employee is notified of a random test while working “off site” or “on the road,” the company’s policies should spell-out exactly what the employee must do before resuming safety-sensitive functions. That way there is no misunderstanding among employees about what is expected. Note to Service Agents and Consortia/Third Party Administrators: Owner-operators and other employers who themselves perform safety-sensitive duties present a special notification challenge. So, if you are a service agent or C/TPA providing random selections and notices to an owner-operator, you should have written procedures on how they are notified and instructed after notification on when to report to a specific collection site. You must also have a written policy about what constitutes a refusal to test if they fail to appear for a test when notified. You should also provide these written items to these owner-operators and self-employed safety-sensitive employees.