Appeal from the Circuit Court of Cook County. Honorable John K. Madden, Judge Presiding.
 The opinion of the court was delivered by: Justice South
 This appeal arises from an order of the circuit court of Cook County which denied a petition for administrative review of a decision of the Cook County Sheriff's Merit Board (Merit Board) to discharge petitioner, Charmaine Garrido, after it found that she was in violation of the sheriff's office drug-free workplace policy
 Petitioner was hired by the Cook County sheriff's office (Sheriff) in January 1990 and was continuously employed there as a deputy sheriff until her termination in January 2001. Since April 26, 1993, the sheriff's office has had in effect a drug-free workplace policy. General Order No. 1002 (Sheriff's Drug-Free Workplace Policy) provides, in relevant part:
 The purpose of this order is to disseminate and implement the Sheriff's Drug-Free Workplace Policy (Appendix A), and the Mandatory Guidelines for Drug Testing (Appendix B), for all sworn Department Personnel.
 A. It is the policy of this Department to take all reasonable measures to provide and maintain a work environment free of unlawful use of drugs or controlled substances.
 B. The unlawful involvement with drugs; the presence in an employee's system of drugs or controlled substances, or their metabolites, the use of cannabis or non-prescribed controlled substances by sworn personnel, at any time, while on or off-duty are strictly prohibited.
 B. 'Drugs or controlled substances' include, but are not limited to, the following substances and their respective metabolites:
 (2) Controlled substance as defined in Chapter 56½, Section 1100, et seq., Illinois Revised Statutes, or as amended. IV. Responsibilities [of] Sworn Personnel
 It is the responsibility of all sworn Department personnel to acquaint themselves with, and abide fully by, the provisions of the Sheriff's Drug-Free Workplace Police (Appendix A) and the Mandatory Guidelines for Drug Testing (Appendix B).
 VI. Disciplinary Action
 Violation of this policy, substantiated by a confirmed positive drug test, will result in disciplinary action leading to *** the referral of charges to the Merit Board, by the Sheriff or his designee, seeking the discharge of a sworn merit employee."
 An employee found in violation of General Order No. 1002, substantiated by a confirmed positive test for an illegal substance or the metabolites thereof, is referred to the Merit Board for the purpose of seeking his or her discharge. All parties agree that this is a zero-tolerance policy.
 On October 5, 2000, as part of the drug-free workplace policy, petitioner was randomly selected by computer to undergo drug testing. Following the October 5 test, the sheriff's office was notified by Quest Diagnostic Laboratory, which performed the test and analysis, that petitioner's urine sample had tested positive for the presence of cocaine metabolites. After being informed of those positive test results, petitioner requested that the specimen be retested by a different laboratory. That laboratory, the University of Missouri-Columbia Toxicology Laboratory, confirmed the original findings of Quest.
 On January 16, 2001, the sheriff's office filed a complaint with the Merit Board, pursuant to section 3-7012 of the Counties Code (55 ILCS 5/3-7012 (West 2000)) maintaining that petitioner had violated the sheriff's office drug-free workplace ...