IV.A.2
Observation Stay
When a patient is admitted for observation for a medical condition, assign a code for the medical condition as the first-listed diagnosis. When a patient presents for outpatient surgery and develops complications requiring admission to observation, code the reason for the surgery as the first reported diagnosis (reason for the encounter), followed by codes for the complications as secondary diagnoses.
ICD-10-CM Official Guidelines for Coding and Reporting, FY 2026 (effective October 1, 2025). Published by CMS and NCHS. Reproduced for reference — always verify against the official CMS release.