Assessing addiction after incident

Assumed facts:

1. An employee did not disclose addiction voluntarily to their employer.
2. The employer has a policy stating workers must be fit for duty, including not being under the influence of drugs.
3. The employee was involved in an incident or showed signs of impairment resulting in a “reasonable suspicion” report.
4. The employee was tested and had a positive drug test result.

In such cases, is it the responsibility of the employer to have the employee assessed by a Substance Abuse Expert to determine ‘addict’ vs ‘recreational user’ before determining disciplinary actions? Or, can the employer discipline based solely on the incident and the fit for duty policy infraction, considering the fact that the worker did not voluntarily report addiction before the incident?