2026 ICD-10-CM Diagnosis Code T78.121AOther adverse food reaction due to egg with reactivity to baked egg, initial encounter

ICD-10-CM Codes›S00–T88›T66-T78›T78

ICD-10-CM T78.121A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T78.121A is a billable ICD-10-CM diagnosis code for other adverse food reaction due to egg with reactivity to baked egg, 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 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 922 through 923. The code is not accepted as a principal diagnosis by the Medicare Code Editor.

Code Identity

ICD-10-CM Code
T78.121A
Billable Status
Yes — Valid for Submission
Code Describes
Other adverse food reaction due to egg with reactivity to baked egg, initial encounter
Short Description
Other advrs food react d/t egg with rct to baked egg, init
Parent Code
T78.121
Other adverse food reaction due to egg with reactivity to baked egg

Code Classification

ChapterS00–T88Injury, poisoning and certain other consequences of external causes
SectionT66-T78Other and unspecified effects of external causes
CategoryT78Adverse effects, not elsewhere classified
This CodeT78.121AOther adverse food reaction due to egg with reactivity to baked egg, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T78.121A.

Unacceptable principal diagnosis
There are selected codes that describe a circumstance which influences an individual's health status but not a current illness or injury, or codes that are not specific manifestations but may be due to an underlying cause. These codes are considered unacceptable as a principal diagnosis.

Coding GuidelinesGuidance

The appropriate 7th character is to be added to each code from block Adverse effects, not elsewhere classified (T78). 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 2026, published by CMS and the National Center for Health Statistics.

Code History & ChangesHistory

New Code T78.121A was added to the ICD-10-CM code set for FY 2026, effective October 1, 2025.

Replacement T78.121A replaces the following previously assigned code(s):

  • T78.1XXA - Oth adverse food reactions, not elsewhere classified, init
FY 2026AddedAdded to the ICD-10-CM code setEffective October 1, 2025.
FY 2026CurrentRevised in the current code setEffective October 1, 2025 through September 30, 2026.

Questions About T78.121AOverview

Is T78.121A a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report other adverse food reaction due to egg with reactivity to baked egg, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in T78.121A mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for other adverse food reaction due to egg with reactivity to baked egg, such as an emergency visit or first evaluation.

What MS-DRG does T78.121A group to?

On inpatient claims, other adverse food reaction due to egg with reactivity to baked egg, initial encounter maps to MS-DRG 922, 923, with relative weights from 1.0177 to 1.7494 depending on complications. Higher weights mean higher Medicare reimbursement.

Can T78.121A be a principal diagnosis?

No. The Medicare Code Editor rejects this code as a principal diagnosis because other adverse food reaction due to egg with reactivity to baked egg, initial encounter describes a circumstance that influences health status rather than a current illness. Report it as a secondary diagnosis.