2027 ICD-10-CM Diagnosis Code G21.2Secondary parkinsonism due to other external agents

ICD-10-CM Codes›G00–G99›G20-G26›G21

ICD-10-CM G21.2
CMSSource: CMS FY 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

G21.2 is a billable ICD-10-CM diagnosis code for secondary parkinsonism due to other external agents. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 56 through 57. 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 12 closely related codes. Coders also document this condition as carbon monoxide encephalopathy. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other nervous system disorders (often hereditary or degenerative).

G21.2 no longer risk-adjusts for Medicare Advantage: it mapped to HCC 78 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 78, ESRD (V21) category 78, ESRD (V24) category 78, and RxHCC Part D (V08) category 161 for payment year 2027.

Code Identity

ICD-10-CM Code
G21.2
Billable Status
Yes — Valid for Submission
Code Describes
Secondary parkinsonism due to other external agents
Short Description
Secondary parkinsonism due to other external agents
Same as the full description in the CMS dataset.
Parent Code
G21
Secondary parkinsonism

Code Classification

ChapterG00–G99Diseases of the nervous system
SectionG20-G26Extrapyramidal and movement disorders
CategoryG21Secondary parkinsonism
This CodeG21.2Secondary parkinsonism due to other external agents

Medicare Risk Adjustment (HCC)Billing

G21.2 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 78
V24 retired last contributed to a Medicare Advantage risk score in payment year 2025
Other CMS Models
PACE (CMS-HCC V22): HCC 78 · ESRD (V21): HCC 78 · ESRD (V24): HCC 78
ESRD V21 weights: 0.065 dialysis, 0.141–0.583 functioning graft · ESRD V24 weights: 0.079 dialysis, 0.206–0.608 functioning graft
Part D (RxHCC)
RxHCC 161 — Parkinson Disease
also risk-adjusts in the Part D prescription drug model (V08)

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.

  • Carbon monoxide encephalopathy
  • Carbon monoxide-induced parkinsonism
  • MPTP-induced parkinsonism
  • Parkinsonism caused by carbon disulfide
  • Parkinsonism caused by cyanide
  • Pesticide-induced Parkinsonism
  • Toxin-induced parkinsonism

Instructional NotesGuidance

Instructions from the official ICD-10-CM Tabular List that apply to G21.2: its own notes plus those printed at G21 and Chapter 6. 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 G21 Secondary parkinsonism applies to 8 codes
  • Huntington's disease (G10)
  • neurocognitive disorder with Lewy bodies (G31.83)
  • Shy-Drager syndrome (G90.3)
  • syphilitic Parkinsonism (A52.19)

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 Chapter 6 (G00-G99) Diseases of the nervous system applies to 700 codes
  • certain conditions originating in the perinatal period (P04-P96)
  • certain infectious and parasitic diseases (A00-B99)
  • complications of pregnancy, childbirth and the puerperium (O00-O9A)
  • congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)
  • endocrine, nutritional and metabolic diseases (E00-E88)
  • injury, poisoning and certain other consequences of external causes (S00-T88)
  • neoplasms (C00-D49)
  • symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)

Code First

The underlying condition named in the note is sequenced before this code.

  • (T51-T65) to identify external agent

Source: CMS ICD-10-CM Tabular List. How to read instructional notes.

Referenced in Other NotesGuidance

Instructional notes printed at other codes that name G21.2, its category, or a range that includes it.

Excludes1 2

These codes carry an Excludes1 note naming G21.2: they are not reported together with it, unless the two conditions are unrelated.

  • G20.C Parkinsonism, unspecified
    secondary parkinsonism (G21.-) names G21.-, which includes this code
  • R25 Abnormal involuntary movements
    specific movement disorders (G20-G26) names G20-G26, which includes this code

Excludes2 1

These codes carry an Excludes2 note naming G21.2: its condition is not part of those codes, and both may be reported when the patient has both.

  • F98.4 Stereotyped movement disorders
    movement disorders of organic origin (G20-G25) names G20-G25, which includes this code

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

Clinical ClassificationClinical

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

CCSR NVS006
Other nervous system disorders (often hereditary or degenerative)
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Parkinson Disease, Secondary

    conditions which feature clinical manifestations resembling primary parkinson disease that are caused by a known or suspected condition. examples include parkinsonism caused by vascular injury, drugs, trauma, toxin exposure, neoplasms, infections and degenerative or hereditary conditions. clinical features may include bradykinesia, rigidity, parkinsonian gait, and masked facies. in general, tremor is less prominent in secondary parkinsonism than in the primary form. (from joynt, clinical neurology, 1998, ch38, pp39-42)

Patient EducationClinical

Parkinson's Disease

Parkinson's disease (PD) is a type of movement disorder. It happens when nerve cells in the brain don't produce enough of a brain chemical called dopamine. Sometimes it is genetic, but most cases do not seem to run in families. Exposure to chemicals in the environment might play a role.

Read the full article at MedlinePlus

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

Convert G21.2 to ICD-9-CMHistory

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

ICD-9-CM
332.1 Secondary parkinsonism
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 G21.2Overview

What is the ICD-10 code for secondary parkinsonism due to other external agents?

The ICD-10-CM code for secondary parkinsonism due to other external agents is G21.2 (sometimes written as G212). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

Is G21.2 (Secondary parkinsonism) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report secondary parkinsonism due to other external agents on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

What MS-DRG does G21.2 group to?

When secondary parkinsonism due to other external agents is the principal diagnosis on an inpatient stay, it groups to MS-DRG 57, 56, with relative weights from 1.2835 to 2.2649 depending on complications. Higher weights mean higher Medicare reimbursement.

Is G21.2 a CC or MCC?

CMS lists G21.2 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 12 closely related codes in its exclusion list.

Can G21.2 be reported with G10?

Not as a rule. The Excludes1 note at G21, which applies to G21.2, lists "Huntington's disease (G10)". 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 G21.2 risk-adjust for Medicare Advantage payment?

Not for Medicare Advantage. G21.2 mapped to HCC 78 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 78 (Parkinsons and Huntingtons Diseases), ESRD (V21) category 78 (Parkinsons and Huntingtons Diseases), ESRD (V24) category 78 (Parkinsons and Huntingtons Diseases), and RxHCC Part D (V08) category 161 (Parkinson Disease).