2027 ICD-10-CM Diagnosis Code T78.41XAArthus phenomenon, initial encounter

ICD-10-CM Codes›S00–T88›T66-T78›T78

ICD-10-CM T78.41XA
CMSSource: CMS FY 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

T78.41XA is a billable ICD-10-CM diagnosis code for arthus phenomenon, 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 791, 793, 917 through 918. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Allergic reactions.

Code Identity

ICD-10-CM Code
T78.41XA
Billable Status
Yes — Valid for Submission
Code Describes
Arthus phenomenon, initial encounter
Short Description
Arthus phenomenon, initial encounter
Same as the full description in the CMS dataset.
Parent Code
T78.41
Arthus phenomenon

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 CodeT78.41XAArthus phenomenon, initial encounter

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 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 T78.41XA: its own notes plus those printed at T78.4, T78, 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.

From T78.4 Other and unspecified allergy applies to 9 codes
  • specified types of allergic reaction such as:
  • allergic diarrhea (K52.29)
  • allergic gastroenteritis and colitis (K52.29)
  • dermatitis (L23-L25, L27.-)
  • food protein-induced enterocolitis syndrome (K52.21)
  • food protein-induced enteropathy (K52.22)
  • hay fever (J30.1)

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.

From T78 Adverse effects, not elsewhere classified applies to 81 codes
  • complications of surgical and medical care NEC (T80-T88)
From Chapter 19 (S00-T88) Injury, poisoning and certain other consequences of external causes applies to 41,187 codes

Use Additional Code

Add a second code, after this one, to fully describe the condition.

From Chapter 19 (S00-T88) Injury, poisoning and certain other consequences of external causes applies to 41,187 codes
  • 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.

From Block T66-T78 Other and unspecified effects of external causes applies to 456 codes
  • 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.

From Chapter 19 (S00-T88) Injury, poisoning and certain other consequences of external causes applies to 41,187 codes
  • 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.

From T78 Adverse effects, not elsewhere classified applies to 81 codes
  • The appropriate 7th character is to be added to each code from category T78
Values for category T78 Encounter: Initial, Subsequent, Sequela
  • 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 T78.41XA, its category, or a range that includes it.

Excludes1 1

These codes carry an Excludes1 note naming T78.41XA: they are not reported together with it, unless the two conditions are unrelated.

  • T75 Other and unspecified effects of other external causes
    adverse effects NEC (T78.-) names T78.-, which includes this code

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR INJ031
Allergic reactions
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Arthus Reaction

    a dermal inflammatory reaction produced under conditions of antibody excess, when a second injection of antigen produces intravascular antigen-antibody complexes which bind complement, causing cell clumping, endothelial damage, and vascular necrosis.

Convert T78.41XA to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
995.21 Arthus phenomenon
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2026No changes
FY 2027CurrentCurrent code set, no changesEffective October 1, 2026 through September 30, 2027.

Questions About T78.41XAOverview

What is the ICD-10 code for arthus phenomenon, initial encounter?

The ICD-10-CM code for arthus phenomenon, initial encounter is T78.41XA (sometimes written as T7841XA). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

Is T78.41XA (Arthus phenomenon) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report arthus phenomenon, initial encounter on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

What does the 7th character A in T78.41XA mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for arthus phenomenon, such as an emergency visit or first evaluation.

What MS-DRG does T78.41XA group to?

When arthus phenomenon, initial encounter is the principal diagnosis on an inpatient stay, it groups to MS-DRG 793, 791, 918, 917, with relative weights from 0.8716 to 4.2192 depending on complications. Higher weights mean higher Medicare reimbursement.

Can T78.41XA be reported with K52.29?

Not as a rule. The Excludes1 note at T78.4, which applies to T78.41XA, lists "allergic diarrhea (K52.29)". 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).