Fentanyl is a synthetic opioid that does not share the chemical structure traditional opiate immunoassays target, so a standard 5- or 10-panel screen will not reliably flag it — testing requires a specific fentanyl/norfentanyl assay. As fentanyl became the dominant driver of overdose deaths, regulators moved to close the gap: HHS proposed adding fentanyl to the federal workplace testing panel, and DOT adoption follows the HHS mandatory guidelines once finalized. In the meantime, many employers — especially in healthcare, where diversion risk is real, and in non-DOT safety-sensitive roles — add fentanyl to their company-authority panels. MRO review still applies: legitimately prescribed fentanyl (patches, for example) verifies as negative.