Ill-defined and unknown cause of mortality (R99) ICD-10-CM
The R99 code range covers ill-defined and unknown cause of mortality with 1 ICD-10-CM diagnosis codes. 1 of them are billable and valid for claim submission in fiscal year 2026, and the category headers group them but cannot themselves be billed.
Inclusion Terms
These terms are the conditions for which that code is to be used. The terms may be synonyms of the code title, or, in the case of "other specified" codes, the terms are a list of the various conditions assigned to that code. The inclusion terms are not necessarily exhaustive. Additional terms found only in the Alphabetic Index may also be assigned to a code.
- Death (unexplained) NOS
- Unspecified cause of mortality
Codes in the R99 Range 1 codes · 1 billable
- R99 Ill-defined and unknown cause of mortality
Clinical Terms in This Code Range
Definitions from the National Library of Medicine for conditions coded in the R99 range.
Infant
A child between 1 and 23 months of age.
Infant Death
The death of a live-born INFANT within its first year of life.
Maternal Death
The death of the female parent.
Perinatal Death
The death of a FETUS of GESTATIONAL AGE 28 weeks or more, or the death of a live-born INFANT less than 28 days of age.
Sudden Infant Death
The abrupt and unexplained death of an apparently healthy infant under one year of age, remaining unexplained after a thorough case investigation, including performance of a complete autopsy, examination of the death scene, and review of the clinical history. (Pediatr Pathol 1991 Sep-Oct;11(5):677-84)
Sudden Unexpected Death in Epilepsy
Sudden death in a patient with EPILEPSY associated with SEIZURES and seizure-related symptoms (e.g., APNEA; HYPOXEMIA) without other identifiable accidental causes (e.g., DROWNING; WOUNDS AND INJURIES).
About the R99 Code Range
The ICD-10 code R99 is specifically used to classify deaths where the cause is ill-defined or unknown. This code covers a wide range of situations including sudden infant death, unattended death, and unexpected or unexplained mortality cases where no clear cause has been identified.
R99 helps healthcare providers and medical coders record deaths that cannot be attributed to a specific diagnosis or condition. Synonyms such as “sudden infant death syndrome,” “unattended death,” “death during anesthetic induction,” and “natural death with unknown cause” highlight the variety of scenarios included. This code applies in contexts ranging from hospital inpatients found deceased to deaths occurring outside medical supervision, including perinatal, maternal, and accidental deaths without a definitive cause. By capturing these cases under a single code, R99 facilitates consistent documentation and statistical reporting when postmortem investigations fail to determine an exact cause, ensuring that all deaths with uncertain causes are accurately represented in medical records and mortality data.
Questions About This Page
How many billable codes are in the R99 range?
Of the 1 codes in this range, 1 are billable and valid for claim submission from October 1, 2025 through September 30, 2026. Category header codes group them but cannot be reported on claims.
What does the R99 range classify?
The range classifies ill-defined and unknown cause of mortality. Each code links to its own reference page with billing status, MS-DRG grouping, coding notes, and clinical information.
Related References
Source: CMS FY 2026 ICD-10-CM Tabular List and order file, effective October 1, 2025 through September 30, 2026.
