2027 ICD-10-CM Diagnosis Code T79.A21ATraumatic compartment syndrome of right lower extremity, initial encounter

ICD-10-CM Codes›S00–T88›T79›T79

ICD-10-CM T79.A21A
CMSSource: CMS FY 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

T79.A21A is a billable ICD-10-CM diagnosis code for traumatic compartment syndrome of right lower extremity, 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 922 through 923, 963 through 965. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 53 closely related codes. Coders also document this condition as anterior tibial compartment syndrome. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other specified injury.

T79.A21A no longer risk-adjusts for Medicare Advantage: it mapped to HCC 173 under the retired CMS-HCC V24 model through payment year 2025 but maps to no category in the live V28 model. It still risk-adjusts in the PACE (CMS-HCC V22) category 173, ESRD (V21) category 173, and ESRD (V24) category 173 for payment year 2027.

Code Identity

ICD-10-CM Code
T79.A21A
Billable Status
Yes — Valid for Submission
Code Describes
Traumatic compartment syndrome of right lower extremity, initial encounter
Short Description
Traumatic compartment syndrome of r low extrem, init
Parent Code
T79.A21
Traumatic compartment syndrome of right lower extremity

Code Classification

ChapterS00–T88Injury, poisoning and certain other consequences of external causes
SectionT79Certain early complications of trauma
CategoryT79Certain early complications of trauma, not elsewhere classified
This CodeT79.A21ATraumatic compartment syndrome of right lower extremity, initial encounter

Medicare Risk Adjustment (HCC)Billing

T79.A21A no longer risk-adjusts for Medicare Advantage: it maps to no payment category in the live CMS-HCC V28 model, although it still risk-adjusts in the other CMS models shown below.

CMS-HCC V28 (Medicare Advantage Payment Model)
Not mapped
Dropped in V28 see all codes that no longer risk-adjust
Prior Model (CMS-HCC V24)
HCC 173
V24 retired last contributed to a Medicare Advantage risk score in payment year 2025
Other CMS Models
PACE (CMS-HCC V22): HCC 173 · ESRD (V21): HCC 173 · ESRD (V24): HCC 173
ESRD V21 weights: 0.042 dialysis, 0.061–0.290 functioning graft · ESRD V24 weights: 0.050 dialysis, 0.000–0.341 functioning graft
Part D (RxHCC)
Not mapped
T79.A21A does not risk-adjust in the RxHCC prescription drug model

Source: CMS Payment Year 2027 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Anterior tibial compartment syndrome
  • Anterior tibial compartment syndrome due to traumatic injury
  • Anterior tibial compartment syndrome of left lower leg
  • Anterior tibial compartment syndrome of left lower leg due to traumatic injury
  • Anterior tibial compartment syndrome of right lower leg
  • Anterior tibial compartment syndrome of right lower leg due to traumatic injury
  • Bilateral anterior tibial compartment syndrome of lower legs due to traumatic injury
  • Compartment syndrome of left lower limb due to traumatic injury
  • Compartment syndrome of lower leg due to traumatic injury
  • Compartment syndrome of right lower limb due to traumatic injury

Coding GuidelinesGuidance

The appropriate 7th character is to be added to each code from block Certain early complications of trauma, not elsewhere classified (T79). 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 T79.A21A: its own notes plus those printed at T79.A, T79, 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 T79.A Traumatic compartment syndrome applies to 27 codes
  • fibromyalgia (M79.7)
  • nontraumatic compartment syndrome (M79.A-)

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 T79.A Traumatic compartment syndrome applies to 27 codes
  • traumatic ischemic infarction of muscle (T79.6)
From T79 Certain early complications of trauma, not elsewhere classified applies to 54 codes
  • acute respiratory distress syndrome (J80)
  • complications occurring during or following medical procedures (T80-T88)
  • complications of surgical and medical care NEC (T80-T88)
  • newborn respiratory distress syndrome (P22.0)
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.-)

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 T79 Certain early complications of trauma, not elsewhere classified applies to 54 codes
  • The appropriate 7th character is to be added to each code from category T79
Values for category T79 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 T79.A21A, its category, or a range that includes it.

Excludes1 3

These codes carry an Excludes1 note naming T79.A21A: they are not reported together with it, unless the two conditions are unrelated.

  • M62.2 Nontraumatic ischemic infarction of muscle
    compartment syndrome (traumatic) (T79.A-) names T79.A-, which includes this code
  • M79.A Nontraumatic compartment syndrome
    compartment syndrome NOS (T79.A-) names T79.A-, which includes this code
  • M79.A Nontraumatic compartment syndrome
    traumatic compartment syndrome (T79.A-) names T79.A-, which includes this code

Excludes2 2

These codes carry an Excludes2 note naming T79.A21A: its condition is not part of those codes, and both may be reported when the patient has both.

  • Chapter 13 (M00-M99) Diseases of the musculoskeletal system and connective tissue
    compartment syndrome (traumatic) (T79.A-) names T79.A-, which includes this code
  • T79.6 Traumatic ischemia of muscle
    compartment syndrome (traumatic) (T79.A-) names T79.A-, which includes this code

Clinical ClassificationClinical

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

CCSR INJ026
Other specified injury
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Muscle Disorders

Your muscles help you move and help your body work. Different types of muscles have different jobs. There are many problems that can affect muscles. Muscle disorders can cause weakness, pain or even paralysis.

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert T79.A21A to ICD-9-CMHistory

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

ICD-9-CM
958.92 Trauma comp synd low ext
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 T79.A21AOverview

Is T79.A21A (Traumatic compartment syndrome of right lower extremity) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report traumatic compartment syndrome of right lower extremity, initial encounter on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

What does the 7th character A in T79.A21A mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for traumatic compartment syndrome of right lower extremity, such as an emergency visit or first evaluation.

What MS-DRG does T79.A21A group to?

When traumatic compartment syndrome of right lower extremity, initial encounter is the principal diagnosis on an inpatient stay, it groups to MS-DRG 964, 923, 965, 922, 963, with relative weights from 0.9980 to 2.6361 depending on complications. Higher weights mean higher Medicare reimbursement.

Is T79.A21A a CC or MCC?

CMS lists T79.A21A as a CC (complication or comorbidity) for FY 2027. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 53 closely related codes in its exclusion list.

Can T79.A21A be reported with M79.7?

Not as a rule. The Excludes1 note at T79.A, which applies to T79.A21A, lists "fibromyalgia (M79.7)". 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).

Does T79.A21A risk-adjust for Medicare Advantage payment?

Not for Medicare Advantage. T79.A21A mapped to HCC 173 in the retired CMS-HCC V24 model, which last determined payment in 2025, but it maps to no category in the live V28 model; see all codes that no longer risk-adjust. It still risk-adjusts in the PACE (CMS-HCC V22) category 173 (Traumatic Amputations and Complications), ESRD (V21) category 173 (Traumatic Amputations and Complications), and ESRD (V24) category 173 (Traumatic Amputations and Complications).