I.C.18.h
Death NOS
Code R99, Ill-defined and unknown cause of mortality, is only for use in the very limited circumstance when a patient who has already died is brought into an emergency department or other healthcare facility and is pronounced dead upon arrival. It does not represent the discharge disposition of death.
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.