2027 ICD-10-CM Diagnosis Code T66.XXXARadiation sickness, unspecified, initial encounter
T66.XXXA is a billable ICD-10-CM diagnosis code for radiation sickness, unspecified, initial encounter. The 7th character A marks it as an initial encounter code, used while the patient is receiving active treatment. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 922 through 923. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Effect of other external causes, initial encounter.
Code Identity
Code Classification
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Acquired pancytopenia
- Acute radiation disease
- Acute radiation nephritis
- Acute radiation otitis externa
- Acute radiation syndrome
- Acute vomiting
- Anemia caused by ionizing radiation
- Anemia caused by physical agent
- Aplastic anemia caused by ionizing radiation
- Cancer dataset administrative items
- Chronic radiation nephritis
- Chronic radiation sickness
- Disorder caused by radiation
- Disorder due to radiotherapy
- Disorder of bone marrow caused by ionizing radiation
- Disorder of conjunctiva caused by radiation
- Disorder of eyelid caused by radiation
- Disorder of macula of retina caused by radiation
- Disorder of optic nerve caused by radiation
- Disorder of retina caused by radiation
- Disorder of sclera caused by radiation
- Effect of ionizing radiation
- Effect of radiation
- Effects of occupational exposure to radiation
- Effects of overexposure to electromagnetic radiation
- Hematopoietic subsyndrome of acute radiation syndrome
- Infertility caused by radiation
- Male infertility caused by ionizing radiation
- Nausea and vomiting
- Nausea and vomiting caused by ionizing radiation
- Nephritis caused by ionizing radiation
- Occupational injury
- Radiation agranulocytosis
- Radiation sickness
- Radiation sickness following radiation therapy
- Radiotherapy course change due to acute radiotherapy toxicity
- Reason for change in radiotherapy course
- Subacute radiation sickness
- Toxicity due to radiotherapy
Coding GuidelinesGuidance
The appropriate 7th character is to be added to each code from block Radiation sickness, unspecified (T66). Use the following options for the applicable episode of care:
- A - initial encounter
- D - subsequent encounter
- S - sequela
Source: ICD-10-CM Official Guidelines for Coding and Reporting, FY 2027, published by CMS and the National Center for Health Statistics.
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to T66.XXXA: its own notes plus those printed at T66, block T66-T78, and Chapter 19. A note printed at a category, block or chapter applies to every code under it.
Excludes1
Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.
Excludes2
Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.
Use Additional Code
Add a second code, after this one, to fully describe the condition.
- code to identify any retained foreign body, if applicable (Z18.-)
Code Also
A second code may be needed to fully describe the condition; the note gives no sequencing direction.
- if applicable, place of occurrence, such as:
- highway rest stop (Y92.523)
- school (private) (public) (state) as the place of occurrence of the external cause (Y92.21-)
- service areas as the place of occurrence of the external cause (Y92.52-)
- single-family non-institutional (private) house as the place of occurrence of the external cause (Y92.01-)
- transport vehicle as the place of occurrence of the external cause (Y92.81-)
Notes
General instructions on how these codes are used.
- Use secondary code(s) from Chapter 20, External causes of morbidity, to indicate cause of injury. Codes within the T section that include the external cause do not require an additional external cause code
- The chapter uses the S-section for coding different types of injuries related to single body regions and the T-section to cover injuries to unspecified body regions as well as poisoning and certain other consequences of external causes.
7th Character
Every code in the category takes a 7th character from the listed values, with the placeholder X filling any empty positions before it.
- The appropriate 7th character is to be added to code T66
- Ainitial encounter this code
- Dsubsequent encounter
- Ssequela
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Referenced in Other NotesGuidance
Instructional notes printed at other codes that name T66.XXXA, its category, or a range that includes it.
Excludes2 1
These codes carry an Excludes2 note naming T66.XXXA: its condition is not part of those codes, and both may be reported when the patient has both.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Patient EducationClinical
Radiation Exposure
Radiation is energy. It travels in the form of energy waves or high-speed particles. Radiation can occur naturally or be man-made. There are two types:
The full article covers:
- What is radiation?
- What are the sources of radiation exposure?
- What are the health effects of radiation exposure?
- What are the treatments for acute radiation sickness?
- How can radiation exposure be prevented?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert T66.XXXA to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About T66.XXXAOverview
What is the ICD-10 code for radiation sickness, unspecified, initial encounter?
The ICD-10-CM code for radiation sickness, unspecified, initial encounter is T66.XXXA (sometimes written as T66XXXA). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is T66.XXXA (Radiation sickness, unspecified) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report radiation sickness, unspecified, initial encounter on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What does the 7th character A in T66.XXXA mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for radiation sickness, unspecified, such as an emergency visit or first evaluation.
What MS-DRG does T66.XXXA group to?
When radiation sickness, unspecified, initial encounter is the principal diagnosis on an inpatient stay, it groups to MS-DRG 923, 922, with relative weights from 0.9980 to 1.6248 depending on complications. Higher weights mean higher Medicare reimbursement.
Can T66.XXXA be reported with K52.0?
Not as a rule. The Excludes1 note at T66, which applies to T66.XXXA, lists "radiation gastroenteritis and colitis (K52.0)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).
