T21.79 ICD-10-CM Code: Corrosion of third degree of other site of trunk

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for T21.79 in the official ICD-10-CM tabular list, quoted as published.

Notes without a marker are published on T21.79 itself; “inherited from” names the category or block whose note applies here.

7th Character Guide

"The appropriate 7th character is to be added to each code from category T21"

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

How the encounter character is assigned. “Initial encounter” means the patient is receiving active treatment for the condition — it is not limited to the first visit, and a different or new provider giving active treatment still assigns it. “Subsequent encounter” covers care during the healing or recovery phase after active treatment. “Sequela” is for complications or conditions that arise as a direct result of the original condition.

ICD-10-CM Official Guidelines for Coding and Reporting, Section I.C.19.a (paraphrased by MedCoder).

Variant codes in this family

T21.79XA, T21.79XD, T21.79XS

Character meanings are CMS's official 7th-character extensions for this family, as carried in each variant code's official description; variant codes are registry rows. The explanation of how the encounter character is assigned is MedCoder editorial, distinct from the official content above it.

Coder workflow for T21.79

MedCoder structured workflow — derived from this code’s own official record

Before you code T21.79

1. T21.79 is not reportable as written. Select the more specific code beneath it that the documentation supports. Codes are reported to the highest level of specificity the classification provides (Guidelines I.B.2).

   ReviewT21.79XA, T21.79XD

   See the hierarchy · Guide: How to choose an ICD-10-CM code →

2. A 7th character is required in this family. Confirm the documented encounter: initial (active treatment), subsequent (healing or recovery phase), or sequela. T21.79 is not valid without one. “Initial” means active treatment, not the first visit (Guidelines I.C.19.a).

   See the 7th Character Guide · Guide: 7th characters: initial, subsequent, sequela →

3. Confirm the injury type, the anatomical site at the most specific level documented, laterality where the family codes it, and the encounter for the 7th character. Each injury is coded separately unless the classification provides a combination code, and a superficial injury is not coded when a more severe injury of the same site is documented. Injury chapter guidelines (Guidelines I.C.19.b, I.C.19.b.1).

   Guide: 7th characters: initial, subsequent, sequela →

Choose the right path

1. Does the documentation support one of the more specific codes beneath T21.79?
   Yes → Select that code and continue the checks below on its own page.
   No → T21.79 cannot be reported as written; query for the specificity its subcategory needs.

   ReviewT21.79XA, T21.79XD

2. Is the underlying (etiologic) condition the Code First note names documented?
   Yes → Sequence the underlying condition first, then T21.79.
   No → Continue; do not add an underlying condition the record does not document.
3. Is the encounter phase documented — active treatment, healing or recovery, or a sequela?
   Yes → Assign the matching 7th character; the variant codes are in the 7th Character Guide.
   No → The code is invalid without its 7th character — query for the missing element.

Consider T21.79. Then work the Use Additional Code note, and confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement

Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).

Anatomical site

At the most specific level the record states; a site the classification separates cannot be assumed from a procedure or a measurement.

Encounter type

Active treatment, healing or recovery phase, or sequela — the basis of the 7th character (Guidelines I.C.19.a).

The underlying (etiologic) condition

Named in the Code First note; sequenced before this code when documented (Guidelines I.A.13).

The conditions the Use Additional Code note names

Reported with this code when documented; a conditional instruction (“if applicable”, “if known”) applies only when the record supports it.

External cause, place and activity

For the external cause codes reported with the injury or poisoning, where documented (Guidelines I.C.20.a).

Official instructions as workflow

• Excludes2 — not part of T21.79(3 notes)

  Coding workflow: The conditions named in this note are not included in T21.79. When the record documents both, both may be reported; the note is a boundary, not a prohibition.

  CompareT22

  See the official tabular notes · Guidelines I.A.12.b

• Code First — sequencing check(1 note)

  Coding workflow: Check whether the underlying or etiologic condition the note names is documented. When it is, sequence it before T21.79. Do not add an underlying condition the record does not document.

  See the official tabular notes · Guidelines I.A.13

• Use Additional Code — after identifying T21.79(2 notes)

  Coding workflow: Check whether the documentation supports the additional code(s) the note names, and report them with T21.79 when it does. Where the instruction is conditional (“if applicable”, “if known”), it applies only when the record documents the condition.

  ReviewY92, T31, T32

  See the official tabular notes · Guidelines I.A.13

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Indexed Clinical Terms (1)

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

• Corrosion (injury) (acid) (caustic) (chemical) (lime) (external) (internal), trunk, specified site NEC, third degree

Don’t confuse T21.79 with…

MedCoder analysis — pairs from this code’s official notes, distinctions written by MedCoder

The codes most likely to be picked instead of T21.79 — named in its own official notes, or sharing its laterality or encounter family — with what separates them and which documentation moves the case. Each links to the full comparison.

• T21.70— Specificity relationship (Review)

  Overlapping diagnoses in the same category.

  Overlapping codes in the same category — the documentation decides which is the more specific fit.

  What moves you there: A record that documents no more than “Corrosion of third degree of trunk, unspecified site” takes T21.70; specificity in the record moves it back here.

  Full comparison: T21.79 vs T21.70 →

• T21.79XA— Encounter relationship (Review)

  Same underlying diagnosis, different encounter phase.

  The same diagnosis at a different phase of care; one encounter takes one 7th character.

  What moves you there: Documentation of a initial encounter moves the case to T21.79XA.

  Full comparison: T21.79 vs T21.79XA →

• T21.79XD— Encounter relationship (Review)

  Same underlying diagnosis, different encounter phase.

  The same diagnosis at a different phase of care; one encounter takes one 7th character.

  What moves you there: Documentation of a subsequent encounter moves the case to T21.79XD.

  Full comparison: T21.79 vs T21.79XD →

• T22— Excludes2 relationship (Informational)

  Distinct conditions — both may be reported.

  A distinct condition — both may be reported when both are documented.

  What moves you there: Documentation of burns and corrosion of axilla adds T22; it is not part of this code and both may be reported when both are documented.

  Full comparison: T21.79 vs T22 →

Relationships are read from this code's own official notes, its laterality siblings and its encounter family; the wording of each distinction is MedCoder editorial, and the documentation named in each “what moves you there” line is the official note phrase or sibling title itself. Source: CMS ICD-10-CM tabular instructional notes; ICD-10-CM Official Guidelines, Section I.A.12.a.

Decision Points

The directives on this code's own record, as a pre-claim checklist.

1. A 7th character applies in this family. Confirm the documented encounter matches the character assigned. See the official 7th-character definitions and this code’s variants
2. Sequencing: 1 Code First instruction — the underlying condition is sequenced before this code when present. See the Code First notes
3. 2 Use Additional Code instructions — report the named additional code(s) when the documentation supports them. See the Use Additional Code notes
4. 3 Excludes2 entries — those conditions are not part of this code and may be reported additionally when documented. See the Excludes2 notes

Checklist rows are derived from this code's own official directives; the wording of each check is MedCoder editorial. The official notes themselves are in the sections each row links to.

Verify Before Coding

• Not billable as written — a more specific code is required: T21.79XA, T21.79XD, T21.79XS.
• Requires a 7th character. "The appropriate 7th character is to be added to each code from category T21" Options: T21.79XA, T21.79XD, T21.79XS

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

Relationships & Classification

Other codes that name T21.79 or its code family, from the CMS ICD-10-CM tabular instructional notes. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 2 Excludes1 notes: T07 — Unspecified multiple injuries (via T21.-), T66 — Radiation sickness, unspecified (via T21.-).

These codes’ tabular lists mark this diagnosis as mutually exclusive — not reported together unless the conditions are documented as unrelated.

Referenced by 123 Excludes2 notes: S00 — Superficial injury of head (via T21.-), S00-S09 — Injuries to the head (S00-S09) (via T21.-), S01 — Open wound of head (via T21.-), S02 — Fracture of skull and facial bones (via T21.-), S03 — Dislocation and sprain of joints and ligaments of head (via T21.-), S04 — Injury of cranial nerve (via T21.-), S05 — Injury of eye and orbit (via T21.-), S06 — Intracranial injury (via T21.-), S07 — Crushing injury of head (via T21.-), S08 — Avulsion and traumatic amputation of part of head (via T21.-), S09 — Other and unspecified injuries of head (via T21.-), S10 — Superficial injury of neck (via T21.-), S10-S19 — Injuries to the neck (S10-S19) (via T21.-), S11 — Open wound of neck (via T21.-), S12 — Fracture of cervical vertebra and other parts of neck (via T21.-), S13 — Dislocation and sprain of joints and ligaments at neck level (via T21.-), S14 — Injury of nerves and spinal cord at neck level (via T21.-), S15 — Injury of blood vessels at neck level (via T21.-), S16 — Injury of muscle, fascia and tendon at neck level (via T21.-), S17 — Crushing injury of neck (via T21.-), +103 more.

These codes’ tabular lists name this diagnosis as distinct — both may be reported when both are documented.

Contextual Map

Every relationship of T21.79 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.

Run T21.79 with these 10 related codes in Claim Check

Hierarchy

• S00-T88 — Chapter 19: Injury, Poisoning and Certain Other Consequences of External Causes (S00-T88) (S00-T88)[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026

Referenced by Excludes1 notes

• T07 — Unspecified multiple injuries[Excludes1](via T21.-): “burns and corrosions (T20-T32)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• T66 — Radiation sickness, unspecified[Excludes1](via T21.-): “burns (T20-T31)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Referenced by Excludes2 notes (123)

• S00 — Superficial injury of head[Excludes2](via T21.-): “burns and corrosions (T20-T32)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• S00-S09 — Injuries to the head (S00-S09)[Excludes2](via T21.-): “burns and corrosions (T20-T32)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• S01 — Open wound of head[Excludes2](via T21.-): “burns and corrosions (T20-T32)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• S02 — Fracture of skull and facial bones[Excludes2](via T21.-): “burns and corrosions (T20-T32)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• S03 — Dislocation and sprain of joints and ligaments of head[Excludes2](via T21.-): “burns and corrosions (T20-T32)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• S04 — Injury of cranial nerve[Excludes2](via T21.-): “burns and corrosions (T20-T32)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• S05 — Injury of eye and orbit[Excludes2](via T21.-): “burns and corrosions (T20-T32)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• S06 — Intracranial injury[Excludes2](via T21.-): “burns and corrosions (T20-T32)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• and 115 more

Index entries

• Corrosion (injury) (acid) (caustic) (chemical) (lime) (external) (internal), trunk, specified site NEC, third degree[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes (40)

• T21 — Burn and corrosion of trunk[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• T21.69 — Corrosion of second degree of other site of trunk[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• T21.69XA — Corrosion of second degree of other site of trunk, initial encounter[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• T21.69XD — Corrosion of second degree of other site of trunk, subsequent encounter[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• T21.69XS — Corrosion of second degree of other site of trunk, sequela[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• T21.7 — Corrosion of third degree of trunk[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• T21.70 — Corrosion of third degree of trunk, unspecified site[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• T21.70XA — Corrosion of third degree of trunk, unspecified site, initial encounter[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• and 32 more

Change history

• FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Common coding questions

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data

CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page

Claim edits Official source data

CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page

Change history and date-of-service validity Official source data

CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page

Comparisons, relationships and the contextual map MedCoder structured relationship

Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears

Summary and FAQ answers MedCoder explanation

Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Nearest Codes in This Family

Official ICD-10-CM classifications closest to T21.79 in its code family, with their registry titles.

• T21.76 — Corrosion of third degree of male genital region
• T21.76XA — Corrosion of third degree of male genital region, initial encounter
• T21.76XD — Corrosion of third degree of male genital region, subsequent encounter
• T21.76XS — Corrosion of third degree of male genital region, sequela
• T21.77 — Corrosion of third degree of female genital region
• T21.77XA — Corrosion of third degree of female genital region, initial encounter
• T21.77XD — Corrosion of third degree of female genital region, subsequent encounter
• T21.77XS — Corrosion of third degree of female genital region, sequela
• T21.79XA — Corrosion of third degree of other site of trunk, initial encounter
• T21.79XD — Corrosion of third degree of other site of trunk, subsequent encounter

View all codes in the T21 family