T66.XXXA — Radiation sickness, unspecified, initial encounter
T66.XXXA is a billable, specific ICD-10-CM code for radiation sickness, unspecified, initial encounter. It is valid for submission on a claim.
7th character "A" — initial encounter
The appropriate 7th character is to be added to code T66
- A
- initial encounter
- D
- subsequent encounter
- S
- sequela
Excludes1 — never code together
These conditions cannot occur together. Billing both on the same encounter is a denial.
Excludes1 inherited from the categories above T66.XXXA
These are not written on T66.XXXA, but they bind it: a note on a category applies to every code beneath it. Billing T66.XXXA with anything listed here is a denial.
Risk adjustment
T66.XXXA does not map to an HCC and does not risk-adjust under CMS-HCC V28. That is normal — most ICD-10 codes do not. It still needs to be coded correctly; it just will not move a RAF score.
Medicare coverage
T66.XXXA is named in 11 Medicare coverage policies — 11 listing it as supporting medical necessity. Which of them applies to you depends on your Medicare contractor, and coverage genuinely differs by state.
- A52370Billing and Coding: Bevacizumab and biosimilars
- A55315Billing and Coding: Proton Beam Therapy
- A56612Billing and Coding: CT of the Head
- A56625Billing and Coding: Echocardiography
Code history
T66.XXXA has not changed since FY2024 — no addition, revision or deletion across the three most recent ICD-10-CM releases.
Related codes at this level
Codes that share T66. If T66.XXXA is not quite right, the correct code is usually one of these.
Questions about T66.XXXA
- Is T66.XXXA a billable ICD-10-CM code?
- Yes. T66.XXXA is a billable, specific ICD-10-CM code and is valid for submission on a claim.
- Where does T66.XXXA sit in the tabular list?
- Injury, poisoning and certain other consequences of external causes (S00-T88), in the block Other and unspecified effects of external causes (T66-T78).
- What cannot be coded together with T66.XXXA?
- T66.XXXA carries Excludes1 notes, which means the conditions they list can never be coded together with it on the same encounter. Some are written on the categories above T66.XXXA (T66) and apply to it just the same.