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  6. 2026 ICD-10-CM Code T78.121S

2026 ICD-10-CM Diagnosis Code T78.121S

Other adverse food reaction due to egg with reactivity to baked egg, sequela

ICD-10-CM Code:
T78.121S
ICD-10 Code for:
Other advrs food react d/t egg with rct to baked egg, sqla
Is Billable?
Yes - Valid for Submission
Code Navigator:

T78.121S is a billable diagnosis code used to specify a medical diagnosis of other adverse food reaction due to egg with reactivity to baked egg, sequela. The code is valid during the current fiscal year for the submission of HIPAA-covered transactions from October 01, 2025 through September 30, 2026. The code is exempt from present on admission (POA) reporting for inpatient admissions to general acute care hospitals.

This code describes a circumstance which influences the patient's health status but not a current illness or injury. The code is unacceptable as a principal diagnosis.

T78.121S is a sequela code, includes a 7th character and should be used for complications that arise as a direct result of a condition like other adverse food reaction due to egg with reactivity to baked egg. According to ICD-10-CM Guidelines a "sequela" code should be used for chronic or residual conditions that are complications of an initial acute disease, illness or injury. The most common sequela is pain. Usually, two diagnosis codes are needed when reporting sequela. The first code describes the nature of the sequela while the second code describes the sequela or late effect.

Code Classification

  • Injury, poisoning and certain other consequences of external causes
    S00–T88
    • Other and unspecified effects of external causes
      T66-T78
      • Adverse effects, not elsewhere classified
        T78

Coding Guidelines

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 aplicable episode of care:

  • A - initial encounter
  • D - subsequent encounter
  • S - sequela

New 2026 ICD-10-CM Code

T78.121S is new to ICD-10-CM code set for the FY 2026, effective October 1, 2025. The National Center for Health Statistics (NCHS) has published an update to the ICD-10-CM diagnosis codes which became effective October 1, 2025. This is a new and revised code for the FY 2026 (October 1, 2025 - September 30, 2026).

Code Edits

The Medicare Code Editor (MCE) detects errors and inconsistencies in ICD-10-CM diagnosis coding that can affect Medicare claim validity. These Medicare code edits help medical coders and billing professionals determine when a diagnosis code is not appropriate as a principal diagnosis, does not meet coverage criteria. Use this list to verify whether a code is valid for Medicare billing and to avoid claim rejections or denials due to diagnosis coding issues.

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.

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Present on Admission (POA)

T78.121S is exempt from POA reporting - The Present on Admission (POA) indicator is used for diagnosis codes included in claims involving inpatient admissions to general acute care hospitals. POA indicators must be reported to CMS on each claim to facilitate the grouping of diagnoses codes into the proper Diagnostic Related Groups (DRG). CMS publishes a listing of specific diagnosis codes that are exempt from the POA reporting requirement. Review other POA exempt codes here.

CMS POA Indicator Options and Definitions

POA Indicator: Y

Reason: Diagnosis was present at time of inpatient admission.

CMS Pays CC/MCC DRG? YES

POA Indicator: N

Reason: Diagnosis was not present at time of inpatient admission.

CMS Pays CC/MCC DRG? NO

POA Indicator: U

Reason: Documentation insufficient to determine if the condition was present at the time of inpatient admission.

CMS Pays CC/MCC DRG? NO

POA Indicator: W

Reason: Clinically undetermined - unable to clinically determine whether the condition was present at the time of inpatient admission.

CMS Pays CC/MCC DRG? YES

POA Indicator: 1

Reason: Unreported/Not used - Exempt from POA reporting.

CMS Pays CC/MCC DRG? NO

Replacement Code

T78121S replaces the following previously assigned ICD-10-CM code(s):

  • T78.1XXS - Oth adverse food reactions, NEC, sequela

Code History

  • FY 2026 - Code Added, effective from 10/1/2025 through 9/30/2026