2027 ICD-10-CM Diagnosis Code G21.3Postencephalitic parkinsonism
ICD-10-CM Codes›G00–G99›G20-G26›G21
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 199
- Chronic Condition
G21.3 is a billable ICD-10-CM diagnosis code for postencephalitic parkinsonism. 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. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other nervous system disorders (often hereditary or degenerative).
For Medicare Advantage risk adjustment, G21.3 maps to CMS-HCC Category 199 (Parkinson and Other Degenerative Disease of Basal Ganglia) under the V28 model, adding a risk factor of about 0.615 for a community, non-dual, aged beneficiary in payment year 2027.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
G21.3 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2027.
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.
- Postencephalitic parkinsonism
- Post-encephalitic syndrome
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to G21.3: 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.
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.
- 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)
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Referenced in Other NotesGuidance
Instructional notes printed at other codes that name G21.3, its category, or a range that includes it.
Excludes1 2
These codes carry an Excludes1 note naming G21.3: they are not reported together with it, unless the two conditions are unrelated.
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.
Clinical InformationClinical
Parkinson Disease, Postencephalitic
parkinsonism following encephalitis, historically seen as a sequella of encephalitis lethargica (von economo encephalitis). the early age of onset, the rapid progression of symptoms followed by stabilization, and the presence of a variety of other neurological disorders (e.g., sociopathic behavior; tics; muscle spasms; oculogyric crises; hyperphagia; and bizarre movements) distinguish this condition from primary parkinson disease. pathologic features include neuronal loss and gliosis concentrated in the mesencephalon; subthalamus; and hypothalamus. (from adams et al., principles of neurology, 6th ed, p754)
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 G21.3 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About G21.3Overview
What is the ICD-10 code for postencephalitic parkinsonism?
The ICD-10-CM code for postencephalitic parkinsonism is G21.3 (sometimes written as G213). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is G21.3 (Secondary parkinsonism) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report postencephalitic parkinsonism on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What MS-DRG does G21.3 group to?
When postencephalitic parkinsonism 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 G21.3 a CC or MCC?
CMS lists G21.3 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 G21.3 be reported with G10?
Not as a rule. The Excludes1 note at G21, which applies to G21.3, 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).
What HCC is G21.3?
G21.3 (postencephalitic parkinsonism) maps to CMS-HCC Category 199 (Parkinson and Other Degenerative Disease of Basal Ganglia), commonly written as HCC 199, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2027. It mapped to HCC 78 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 161.
Does G21.3 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, G21.3 adds a risk adjustment factor of about 0.615 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.219 to 0.634 depending on the payment segment). HCC 199 sits at the top of its hierarchy, so no other condition category supersedes it. See the full factor table on the HCC 199 category page.
