Skip to main content
ICD-10/T78.40XA

T78.40XA ICD-10-CM Code: Allergy, unspecified, initial encounter

Billing Status: YES. This is a valid, specific, and billable ICD-10-CM reference.

Inpatient Payment Groups (MS-DRG)

MS-DRGs this diagnosis helps define, as principal or secondary, per the CMS ICD-10-CM/PCS MS-DRG Definitions Manual v43.0, Appendix B.

  • MS-DRG 915 — ALLERGIC REACTIONS WITH MCC (MDC 21)
  • MS-DRG 916 — ALLERGIC REACTIONS WITHOUT MCC (MDC 21)

A diagnosis appearing in a group's logic does not by itself determine the DRG assigned to a stay; the grouper uses the full claim.

Official Registry Overview & Definition

Allergy, unspecified, initial encounter is a billable ICD-10-CM diagnosis code (T78.40A).

Official Tabular Instructional Notes

Sequencing, inclusion, and exclusion notes published for T78.40XA in the official ICD-10-CM tabular list.

Inclusion Terms

Alternative terms the tabular list files under this code.

  • Allergic reaction NOS
  • Hypersensitivity NOS

Excludes1 — Not Coded Here

Conditions generally not reported together with this code (Excludes1) -- an error unless the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).

Excludes2 — Not Included Here

Conditions not covered by this code, but which may be reported alongside it when both are present.

Code Also

Additional codes that may be required to fully describe the encounter.

  • Code also, 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-)

Change history

No changes recorded for this code — unchanged since the FY2016 baseline of the change ledger.

Verify Before Coding

  • No Medicare Code Editor or MS-DRG Definitions Manual restrictions apply to this code.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim.

Contextual Map

Every relationship of T78.40XA in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Hierarchy

  • S00-T88 — Chapter 19: Injury, Poisoning and Certain Other Consequences of External Causes (S00-T88) (S00-T88) [Hierarchy]
  • T66-T78 — Other and unspecified effects of external causes [Hierarchy]

Referenced by Excludes1 notes

  • T75 — Other and unspecified effects of other external causes [Excludes1](via T78.-): “adverse effects NEC (T78.-)” · check together

Clinical classification (CCSR)

  • INJ031 — Allergic reactions [CCSR]

Potential MS-DRG

  • DRG 915 — ALLERGIC REACTIONS WITH MCC [MS-DRG]: “ALLERGIC REACTIONS WITH MCC (MDC 21)”
  • DRG 916 — ALLERGIC REACTIONS WITHOUT MCC [MS-DRG]: “ALLERGIC REACTIONS WITHOUT MCC (MDC 21)”

MDC crossing · procedures (32029)

  • MDC 21 — Injuries, Poisonings and Toxic Effects of Drugs [MDC crossing]: “Injuries, Poisonings and Toxic Effects of Drugs — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs.”
  • 0016070 — Bypass Cerebral Ventricle to Nasopharynx with Autologous Tissue Substitute, Open Approach [Same-MDC procedure]: “Bypass Cerebral Ventricle to Nasopharynx with Autologous Tissue Substitute, Open Approach — grouped in MDC 21, the procedure side of this code's crossing.” · check together
  • 0016071 — Bypass Cerebral Ventricle to Mastoid Sinus with Autologous Tissue Substitute, Open Approach [Same-MDC procedure]: “Bypass Cerebral Ventricle to Mastoid Sinus with Autologous Tissue Substitute, Open Approach — grouped in MDC 21, the procedure side of this code's crossing.” · check together
  • 0016072 — Bypass Cerebral Ventricle to Atrium with Autologous Tissue Substitute, Open Approach [Same-MDC procedure]: “Bypass Cerebral Ventricle to Atrium with Autologous Tissue Substitute, Open Approach — grouped in MDC 21, the procedure side of this code's crossing.” · check together
  • 0016073 — Bypass Cerebral Ventricle to Blood Vessel with Autologous Tissue Substitute, Open Approach [Same-MDC procedure]: “Bypass Cerebral Ventricle to Blood Vessel with Autologous Tissue Substitute, Open Approach — grouped in MDC 21, the procedure side of this code's crossing.” · check together
  • 0016074 — Bypass Cerebral Ventricle to Pleural Cavity with Autologous Tissue Substitute, Open Approach [Same-MDC procedure]: “Bypass Cerebral Ventricle to Pleural Cavity with Autologous Tissue Substitute, Open Approach — grouped in MDC 21, the procedure side of this code's crossing.” · check together
  • and 32024 more

Nearest codes (40)

  • T78 — Adverse effects, not elsewhere classified [Sibling]
  • T78.120 — Other adverse food reaction due to egg with tolerance to baked egg [Sibling]
  • T78.120A — Other adverse food reaction due to egg with tolerance to baked egg, initial encounter [Sibling]
  • T78.120D — Other adverse food reaction due to egg with tolerance to baked egg, subsequent encounter [Sibling]
  • T78.120S — Other adverse food reaction due to egg with tolerance to baked egg, sequela [Sibling]
  • T78.121 — Other adverse food reaction due to egg with reactivity to baked egg [Sibling]
  • T78.121A — Other adverse food reaction due to egg with reactivity to baked egg, initial encounter [Sibling]
  • T78.121D — Other adverse food reaction due to egg with reactivity to baked egg, subsequent encounter [Sibling]
  • and 32 more

Referenced by Other Codes

Clinical classification (AHRQ CCSR):INJ031 — Allergic reactions (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Potential MS-DRG Relationships (FY2026)

Not on the CMS CC/MCC list — as a secondary diagnosis this code does not change MS-DRG severity.

Named in the grouper logic of 2 MS-DRGs: DRG 915 (MDC 21), DRG 916 (MDC 21).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Official ICD-10-CM tabular notes on other codes that name T78.40XA or its code family.

1 Excludes1 notes: T75 (via T78.-).

Frequently Asked Questions (FAQ) & Clinical Guidance

Does T78.40XA require a 7th character?

Yes. The appropriate 7th character is to be added to each code from category T78.

Nearest Codes in This Family

Official ICD-10-CM classifications closest to T78.40XA in its code family, with their registry titles.

  • T78.3XXA — Angioneurotic edema, initial encounter
  • T78.3XXD — Angioneurotic edema, subsequent encounter
  • T78.3XXS — Angioneurotic edema, sequela
  • T78.4 — Other and unspecified allergy
  • T78.40 — Allergy, unspecified
  • T78.40XD — Allergy, unspecified, subsequent encounter
  • T78.40XS — Allergy, unspecified, sequela
  • T78.41 — Arthus phenomenon
  • T78.41XA — Arthus phenomenon, initial encounter
  • T78.41XD — Arthus phenomenon, subsequent encounter

View all codes in the T78 family