🧬 ICD-10 CM G04.01 — Postinfectious Acute Disseminated Encephalitis And Encephalomyelitis (Postinfectious ADEM)
Billable Code Confirmed
ICD-10 CM G04.01 is a complete, fully-specified five-character code requiring no further extension, making it valid for direct claim submission.¹ It captures ADEM occurring as an autoimmune sequela following a documented antecedent infection, distinguishing it from the unspecified and postimmunization variants at the same subcategory level.
Non-Billable Parent Codes
G04.0 — Acute disseminated encephalitis and encephalomyelitis (ADEM), unspecified etiology subtype — non-billable header requiring a fourth character to specify postinfectious, postimmunization, or unspecified trigger.² G04 — Encephalitis, myelitis and encephalomyelitis, the category-level parent — non-billable, requires full subcategory specificity before use on any claim.
Clinical Context
Code Classification
ICD-10 CM G04.01 is a diagnosis code only; it carries no procedural component and must always be paired with applicable CPT/HCPCS codes for any diagnostic workup or therapeutic intervention performed during the encounter.
🔍 Code Description
Postinfectious ADEM is an acute, typically monophasic autoimmune demyelinating disorder of the central nervous system that develops days to weeks after a documented infectious illness, most classically a viral prodrome. The presumed mechanism is molecular mimicry or bystander immune activation directed against CNS myelin, producing multifocal white matter lesions distinct from the chronic relapsing course seen in G35.D-type demyelinating disease. Clinically it presents with rapidly evolving encephalopathy, multifocal neurologic deficits, seizures, or altered consciousness, which distinguishes it diagnostically from isolated G37.3 transverse myelitis confined to the spinal cord.
Coding G04.01 correctly requires explicit provider documentation linking the current neurologic presentation to a prior infection, whether the organism is named or only the infectious context is described. When the causative infection remains active and separately reportable, an additional code from Chapter 1 may be assigned per the excludes2 instruction. This code sits alongside its sibling G04.02, the postimmunization variant, within the same fourth-character subcategory, and both are distinguished from G04.81, which captures noninfectious ADEM without any infectious or immunization trigger identified.
🌳 Code Tree / Hierarchy
G04 Encephalitis, myelitis and encephalomyelitis ❌ Non-billable
│
├── G04.0 Acute disseminated encephalitis and encephalomyelitis (ADEM) ❌ Non-billable
│ │
│ ├── G04.00 ADEM, unspecified ✅ Billable
│ ├── G04.01 Postinfectious ADEM ◀ THIS CODE ✅ Billable
│ └── G04.02 Postimmunization ADEM ✅ Billable
│
├── G04.1 Tropical spastic paraplegia ✅ Billable
├── G04.2 Bacterial meningoencephalitis and meningomyelitis, not elsewhere classified ❌ Non-billable (requires further specificity)
│
└── G04.8 Other encephalitis, myelitis and encephalomyelitis ❌ Non-billable
│
└── G04.81 Other encephalitis and encephalomyelitis (noninfectious ADEM) ✅ BillableTrigger-Specificity Drives Code Selection
Tip
Query the provider whenever the chart mentions “post-viral” or “post-infectious” encephalomyelitis without naming or describing the antecedent illness, since CDI clarification here directly determines whether G04.01 or G04.00 is the compliant code choice.
✅ Includes
- Postinfectious ADEM occurring after a clinically or serologically documented viral or bacterial illness, coded here regardless of whether the specific organism is identified in the record.
- Acute monophasic disseminated demyelination of the CNS temporally linked to a resolved or resolving infection, as opposed to a chronic relapsing course captured under G35.D-family codes.
❌ Excludes
Excludes 1
- B01.1 — Post chickenpox (varicella) encephalitis: use this code instead of G04.01 whenever varicella is the specifically documented preceding illness, since it is a more precise combination code for that etiology.
- B05.0 — Post measles encephalitis: reported instead of G04.01 when measles is the documented antecedent infection, as ICD-10-CM provides a dedicated measles-specific combination code.
- B05.1 — Documented in the Excludes1 note under this subcategory as the measles-related myelitis exclusion; verify the exact current manual descriptor before use, as some references list this as post-measles meningitis rather than myelitis.
Danger
The most common Excludes1 error is defaulting to G04.01 whenever any viral prodrome is documented, without checking whether the specific virus (measles or varicella) has its own dedicated combination code that must be used instead. This is a mutually exclusive pairing edit that will trigger a claim edit if both codes are submitted together.
Excludes 2
- G37.3 — Acute transverse myelitis in demyelinating disease of the CNS: may be coded together with G04.01 when the record documents both encephalitic/encephalomyelitic involvement and a distinct transverse myelitis presentation.
- A00-B99 range codes — The causative infectious/parasitic disease itself may be coded in addition to G04.01 when it remains clinically active and separately significant to the encounter, rather than merely a resolved historical trigger.
📋 Clinical Overview
Trigger-Based Subtype Differentiation
The three billable children of G04.0x are differentiated solely by the documented precipitating event, which is the single most important CDI focus point for this code family.
| Feature | G04.01 | G04.00 | G04.02 |
|---|---|---|---|
| Trigger documentation | Documented antecedent infection (viral or bacterial) | Trigger not specified or undetermined in the record | Documented preceding immunization/vaccination |
| Typical onset window | Days to ~4 weeks post-infection | Not clinically established | 1–3 weeks post-vaccination |
| Coder action if trigger unclear | Query provider before assigning; do not assume | Default only after querying yields no further clarification | Query provider to confirm vaccine name and administration date |
Important
A CDI query should be triggered any time “ADEM” or “acute disseminated encephalomyelitis” appears in documentation without an explicit statement of infectious versus immunization versus unknown etiology, since the DRG and quality-reporting implications differ by subtype.
Manifestations & Symptom Burden
- Rapidly progressive encephalopathy with altered mental status, often the presenting and most clinically significant finding.
- Multifocal neurologic deficits reflecting the widespread, non-localized nature of the demyelinating lesions on MRI.
- New-onset seizures, which when documented may independently support MCC-level severity and should be captured with a separate code.
- Fever and antecedent constitutional symptoms consistent with the preceding infectious illness driving the diagnosis.
Tip
Code each clinically significant manifestation (seizure, respiratory failure, etc.) separately in addition to G04.01 rather than assuming it is bundled into the ADEM diagnosis, since these frequently drive CC/MCC assignment.
💰 HCC Risk Adjustment
ICD-10 CM G04.01 does not currently map to a CMS-HCC category under V28, V24, ESRD, or RxHCC models.¹ Because this is a relatively rare, acute, typically non-recurring diagnosis, this is consistent with the general absence of HCC weighting for acute monophasic neurologic events versus chronic degenerative conditions. You as the coder should independently verify this against the live CMS-HCC V28 mapping file at each reporting cycle, since risk-adjustment models are updated annually and coverage for previously non-mapped codes can change.
🏥 MS-DRG Assignment
DRG assignment when G04.01 is principal diagnosis typically falls within the nervous system infection/inflammation DRG family; exact DRG number and weight must be verified against your facility’s current encoder, as these vary by grouper version and are not derivable from the project’s fee schedule files. Sequencing should reflect whether ADEM was the reason for admission (principal) or a secondary complication of an inpatient stay initiated for another reason. CC/MCC capture is driven by concurrent seizure disorder, respiratory failure, or other qualifying secondary diagnoses rather than by G04.01 itself, which carries no inherent CC/MCC designation.
- NCD/LCD: No national coverage determination or local coverage determination specifically governs diagnosis code assignment for G04.01, as NCD/LCD policies generally attach to procedures, drugs, or DME rather than to standalone ICD-10-CM diagnosis codes; however, any MRI, lumbar puncture, or immunotherapy (e.g., IVIG, plasmapheresis) ordered to work up or treat ADEM should be checked against the applicable MAC’s LCD for medical necessity documentation requirements before claim submission.
🔗 Related ICD-10-CM Codes
Same-subcategory ADEM variants:
- G04.00 — ADEM, unspecified trigger
- G04.02 — Postimmunization ADEM
- G04.81 — Noninfectious ADEM (other encephalitis/encephalomyelitis)
Related demyelinating and infectious-trigger codes:
- B01.1 — Post chickenpox encephalitis (excludes1 alternative)
- B05.0 — Post measles encephalitis (excludes1 alternative)
- G37.3 — Acute transverse myelitis in CNS demyelinating disease
- R56.9 — Unspecified convulsions, when seizure activity is documented but not further characterized
🛠️ Commonly Associated CPT Codes
- 70553 — MRI brain without and with contrast, the primary imaging study used to identify multifocal demyelinating lesions supporting the ADEM diagnosis.
- 62270 — Lumbar puncture, diagnostic, frequently performed to obtain CSF for inflammatory marker analysis and to rule out infectious meningoencephalitis.
- 96365 — IV infusion, therapeutic, for administration, commonly reported when IVIG is given as first-line ADEM therapy.
- 36514 — Therapeutic apheresis for plasma exchange, reported when plasmapheresis is used for steroid-refractory presentations.
- 95816 — EEG, when seizure activity or altered consciousness prompts electroencephalographic monitoring.
🏷️ Modifier Reference
| Modifier | Name | When to Apply |
|---|---|---|
| -25 | Significant, Separately Identifiable E/M | Apply when a significant, separately identifiable E/M service is performed on the same day as a minor procedure related to ADEM workup or treatment. |
| -59 | Distinct Procedural Service | Apply when two procedurally distinct services (e.g., lumbar puncture and imaging) performed the same encounter would otherwise be bundled under NCCI edits. |
| -52 | Reduced Services | Apply if a diagnostic or therapeutic procedure related to ADEM management is reduced in scope from its standard description. |
NCCI Bundling Considerations
Diagnostic lumbar puncture and MRI brain imaging are not typically bundled under NCCI edits when medically necessary and separately documented, but therapeutic infusion codes should be reviewed against same-day E/M and injection administration edits to confirm appropriate modifier use.
🔬 ICD-10-PCS Crosswalk
No procedure is inherent to the diagnosis code itself; PCS assignment depends entirely on the specific inpatient procedures performed. Diagnostic lumbar puncture typically crosswalks to a root operation “Drainage” of the spinal canal (character-level selection requires independent verification against the current PCS tables). Therapeutic plasma exchange crosswalks to the Extracorporeal or Percutaneous Therapies section under “Pheresis” of physiological systems. Both crosswalks must be independently verified against the current-year ICD-10-PCS code book, as character selection depends on approach and body part detail not derivable from documentation summaries alone.
💊 Coding Scenarios and Examples
Example 1
Clinical Scenario:
A patient is admitted three weeks after a documented viral upper respiratory illness with new-onset encephalopathy, multifocal weakness, and MRI findings of multifocal white matter demyelinating lesions. Provider documentation explicitly links the presentation to the recent viral infection.
| Field | Code | Rationale |
|---|---|---|
| CPT | 70553 | MRI brain with and without contrast performed to establish the demyelinating lesion pattern supporting the ADEM diagnosis. |
| PDx | G04.01 | Postinfectious ADEM is the principal diagnosis, as the documented viral prodrome directly precedes and is linked to the current encephalomyelitis presentation. |
Tip
Confirm the provider explicitly documents the temporal and causal link to the prior infection in the final progress note or discharge summary, not just in a single early note, to withstand audit scrutiny.
Example 2
Clinical Scenario:
A patient with postinfectious ADEM develops new-onset generalized tonic-clonic seizures during the inpatient stay, requiring EEG monitoring and antiepileptic therapy in addition to IVIG for the underlying ADEM.
| Field | Code | Rationale |
|---|---|---|
| CPT | 96365 | IV infusion administration for IVIG therapy directed at the underlying postinfectious ADEM. |
| CPT 2 | 95816 | EEG performed to characterize and monitor the new-onset seizure activity. |
| PDx | G04.01 | Postinfectious ADEM remains principal diagnosis as the underlying condition driving admission and treatment. |
Tip
Code the seizure activity as a secondary diagnosis in addition to G04.01, as it may independently qualify as an MCC and materially affects DRG weight; do not assume it is bundled into the ADEM code.
Example 3
Clinical Scenario:
A patient presents with encephalopathy following documented varicella infection, with the provider explicitly diagnosing “post-varicella encephalitis.”
| Field | Code | Rationale |
|---|---|---|
| CPT | 62270 | Diagnostic lumbar puncture performed to evaluate CSF for inflammatory markers and rule out concurrent infectious meningoencephalitis. |
| PDx | B01.1 | Post chickenpox (varicella) encephalitis is reported instead of G04.01 per the Excludes1 instruction, since varicella is a specifically documented and separately classified trigger; note B01.1 is not wikilinked here as it falls outside this note’s primary specialty scope. |
Tip
This is a classic Excludes1 trap: because the trigger organism (varicella) has its own dedicated combination code, G04.01 must not be assigned even though the clinical presentation is otherwise identical to postinfectious ADEM.
⚠️ Coding Pitfalls and Tips
- Pitfall 1: Assigning G04.01 whenever any preceding illness is mentioned, without confirming the specific organism is not measles or varicella; Tips: Always cross-check documented triggers against the Excludes1 list before finalizing the code.
- Pitfall 2: Defaulting to G04.00 out of convenience when the trigger is actually documented elsewhere in the chart (e.g., an earlier ED note); Tips: Review the full encounter, including prior notes, before concluding the trigger is truly unspecified.
- Pitfall 3: Failing to separately code seizure activity, respiratory failure, or other complications alongside G04.01; Tips: Systematically review the clinical course for CC/MCC-qualifying secondary diagnoses at discharge.
- Pitfall 4: Coding the resolved antecedent infection as an active concurrent diagnosis when it is purely historical; Tips: Use a personal history Z-code rather than an active infectious disease code once the trigger infection has fully resolved, per current coding guidelines.
- Pitfall 5: Assuming G04.01 carries HCC weight because it is a serious neurologic diagnosis; Tips: Verify HCC status against the current CMS-HCC model file rather than assuming acuity implies risk-adjustment mapping.
- Pitfall 6: Omitting NCD/LCD medical necessity documentation for high-cost interventions like IVIG or plasmapheresis ordered for ADEM; Tips: Confirm supporting documentation meets the applicable MAC’s LCD criteria before submitting therapy-related claims.
📚 Sources
1. Centers for Medicare & Medicaid Services and National Center for Health Statistics. *ICD-10-CM FY2026 Code Tables and Index.* CMS/NCHS; 2026. 2. AAPC. *ICD-10-CM Code G04.01 — Coding Guidelines and Excludes Notes.* AAPC; 2026. https://www.aapc.com/codes/icd-10-codes/G04.01 3. ICD-10 Codes Reference. *G04.0, G04.01, G04.81 — Billability Status and Parent/Child Hierarchy, FY2026.* icd-10codes.com; 2026.Sources listed above correspond to superscript citations throughout this note. Verify all Medicare payment figures against your current CMS PFS Lookup tool and applicable MAC LCD prior to claim submission. Please use the latest AAPC/AHIMA Coding Books to verify each code within this note.