🧬 ICD-10 CM G35.C1 — Active Secondary Progressive Multiple Sclerosis
Billable Code Confirmed
ICD-10 CM G35.C1 is a valid, billable 5-character ICD-10-CM code for FY2026. All required characters are present:
G35(category) +.C(secondary progressive phenotype) +1(active / with disease activity).
Non-Billable Parent Codes — Never Submit These
- ❌
G35— 3-character header — missing phenotype and activity status- ❌
G35.C— 4-character header — missing activity statusAlways submit G35.C1 (all 5 characters) when SPMS is documented with current disease activity (a clinical relapse and/or new MRI lesions).
Clinical Context: "Secondary Progressive" vs. "Active"
ICD-10 CM G35.C1 identifies Secondary Progressive Multiple Sclerosis (SPMS) that is currently active. SPMS begins as an initial relapsing-remitting course (RRMS) and later transitions to a phase of steady, gradual accumulation of disability. The
1indicates that this steady progression is accompanied by superimposed inflammatory activity — a documented clinical relapse and/or new lesions on MRI. The physician must document the activity; coders cannot infer it from a radiology report alone.
Code Classification
ICD-10-CM Diagnosis Code — Fields for wRVU, assistant payable, and global period are not applicable. For associated inpatient profee and facility procedure coding, see the CPT Procedural Crosswalk and ICD-10-PCS Crosswalk sections below.
🔍 Code Description
ICD-10 CM G35.C1 classifies Active secondary progressive multiple sclerosis. It denotes an immune-mediated demyelinating disease of the central nervous system that followed an initial relapsing-remitting course, transitioned into a phase of steady functional decline, and is currently complicated by superimposed inflammatory disease activity.
In patients with SPMS, the underlying pathology is dominated by chronic neurodegeneration and “smoldering” progression. When the disease is active, this baseline progression is accompanied by acute focal inflammation — the influx of active immune cells (T-cells, B-cells, macrophages) across the blood-brain barrier producing new demyelinating plaques.
The qualifier “active” signifies that, in addition to the steady progressive course, the patient has documented evidence of current inflammatory activity. Per FY2026 conventions, activity is defined by one or more of the following:
- New gadolinium-enhancing (Gd+) lesion(s) on MRI;
- New or unequivocally enlarging T2/FLAIR lesion(s) on MRI compared to a prior study; and/or
- A documented clinical relapse (new or worsening neurologic deficits, typically lasting ≥24 hours).
Activity Must Be Physician-Documented
“Active” is a clinical determination. A coder may not assign G35.C1 based on a radiology report describing enhancing or new lesions alone — the treating provider must document that the SPMS is active (relapse and/or new MRI activity). Absent that documentation, default to G35.C0 (unspecified) or G35.C2 (non-active), as supported.
🌳 Code Tree / Hierarchy
G35 Multiple Sclerosis ❌ Non-billable
│
├── G35.A Relapsing-remitting multiple sclerosis (RRMS) ✅ Billable
│
├── G35.B Primary progressive multiple sclerosis (PPMS) ❌ Non-billable
│ │
│ ├── G35.B0 PPMS, unspecified ✅ Billable
│ ├── G35.B1 Active PPMS ✅ Billable
│ └── G35.B2 Non-active PPMS ✅ Billable
│
├── G35.C Secondary progressive multiple sclerosis (SPMS) ❌ Non-billable
│ │
│ ├── G35.C0 SPMS, unspecified ✅ Billable
│ ├── G35.C1 ACTIVE SPMS ◀ THIS CODE ✅ Billable
│ └── G35.C2 Non-active SPMS ✅ Billable
│
└── G35.D Multiple sclerosis, unspecified ✅ Billable
Specificity is Key for Proper Care Management
Documenting activity status is critical. A patient coded with G35.C2 (non-active) or G35.C0 (unspecified) is typically managed with maintenance Disease-Modifying Therapies (DMTs) and rehabilitation. A patient coded with G35.C1 has documented inflammatory activity that may justify the medical necessity for urgent MRIs, hospital admissions, high-dose IV steroid infusions, or escalation of DMT.
✅ Includes
The following clinical terms and scenarios map to G35.C1 when the provider documents current disease activity:
- Active secondary progressive MS (SPMS)
- Secondary progressive MS with a documented relapse
- Secondary progressive MS with new gadolinium-enhancing or new T2/FLAIR lesion(s)
- SPMS with progression plus superimposed inflammatory activity
❌ Excludes
Excludes 1 — Cannot Be Coded Simultaneously with G35.C1
| Code | Description | Note |
|---|---|---|
| G36.0 | Neuromyelitis optica [Devic] | Mutually exclusive — NMO involves antibodies targeting aquaporin-4 (AQP4); pathophysiology is distinct from MS. |
| G36.9 | Acute disseminated demyelination | ADEM is typically a monophasic post-infectious process, unlike chronic MS. |
| G37.0 | Schilder’s disease | A rare progressive demyelinating disorder distinct from MS. |
| G37.81 | MOGAD | Myelin oligodendrocyte glycoprotein antibody-associated disease is a distinct demyelinating syndrome. |
Mutual Exclusivity Within G35.C
The activity-status codes G35.C0 (unspecified), G35.C1 (active), and G35.C2 (non-active) are mutually exclusive (Excludes1-style). Only one may be assigned per encounter to describe the SPMS. You also cannot assign MS codes concurrently with other specified central demyelinating diseases like NMO or MOGAD. If a patient’s diagnosis is revised from SPMS activity to an NMO relapse based on positive AQP4 autoantibody testing, use the NMO code exclusively.
Excludes 2 — May Be Coded in Addition if Separately Present
| Code | Description | Note |
|---|---|---|
| G37.- | Other demyelinating diseases of central nervous system | May be coded additionally if a completely distinct demyelinating condition is confirmed alongside MS. |
📋 Clinical Overview
Disease Activity vs. Pseudoexacerbation
A critical clinical and documentation distinction is separating true disease activity (which supports “active” SPMS) from a pseudoexacerbation (which does not).
| Feature | Disease Activity (Relapse / New MRI Lesions) | Pseudoexacerbation |
|---|---|---|
| Pathophysiology | New area of active CNS inflammation and demyelination. | Temporary unmasking or worsening of old symptoms due to physiological stress. |
| Common Triggers | Autoimmune activity; sometimes preceded by viral illness. | Heat (Uhthoff’s phenomenon), UTI, pneumonia, systemic infection, fever, extreme stress. |
| Duration | Relapse lasts >24 hours; new MRI lesions reflect interval activity. | Typically resolves completely once the underlying trigger (e.g., fever, infection) is treated or subsides. |
| MRI Findings | New gadolinium-enhancing lesion(s) and/or new/enlarging T2/FLAIR lesion(s) vs prior. | Usually no new enhancing or new T2/FLAIR lesions. |
| Code Assignment | G35.C1 (This Code) — when provider documents activity | Usually G35.C2 (non-active) or G35.C0, plus the code for the trigger (e.g., N39.0 for UTI). |
CDI Query Trigger — "Activity Status Unclear"
If the documentation supports SPMS but does not clearly state whether the disease is active, or describes worsening entirely secondary to a UTI or fever with no relapse and no new MRI activity, a CDI query is warranted to establish activity status. Remember that “active” requires physician documentation of a relapse and/or new MRI lesions — a radiologist’s lesion description alone is insufficient.
Clinical Presentation of Active SPMS
Active SPMS combines a gradually progressive baseline (e.g., slowly worsening gait, spasticity, fatigue) with superimposed relapse symptoms that present rapidly (over hours to days) depending on the location of new demyelinating plaques:
- Optic neuritis: Unilateral eye pain and acute vision loss.
- Acute transverse myelitis symptoms: Sudden severe weakness or paralysis in the legs.
- Sensory changes: Ascending numbness or severe paresthesia.
- Brainstem/Cerebellar signs: Acute severe vertigo, ataxia, or diplopia (double vision).
Coding Manifestations
💰 HCC Risk Adjustment (CMS-HCC v28)
| Field | Detail |
|---|---|
| CMS-HCC Model Version | v28 (2024-2025 Implementation) |
| HCC Assignment | ✅ Mapped |
| HCC Category | HCC 198 — Multiple Sclerosis (verify against current CMS crosswalk; this replaces the legacy v24 HCC 77) |
| RAF Coefficient | ~0.45 - 0.65 (varies by demographic/status) |
G35.C1 maps directly to an HCC and contributes significantly to the RAF score. Capturing the active status also provides a clear narrative in the patient’s claims history regarding inflammatory activity superimposed on progressive disease and the associated resource utilization (inpatient stays, biologic/DMT therapies, intensive rehab).
HCC Crosswalk Note
Earlier CMS-HCC v24 mapped MS to HCC 77. Under v28, MS maps to HCC 198. Always validate the exact HCC against the CMS mapping for the applicable payment year.
🏥 MS-DRG Assignment
MDC 01 — Diseases and Disorders of the Nervous System
| DRG | Title | Est. Relative Weight* |
|---|---|---|
| DRG 058 | Multiple Sclerosis & Cerebellar Ataxia with MCC | ~1.30 - 1.50 |
| DRG 059 | Multiple Sclerosis & Cerebellar Ataxia with CC | ~0.90 - 1.10 |
| DRG 060 | Multiple Sclerosis & Cerebellar Ataxia without CC/MCC | ~0.65 - 0.80 |
Approximate. Verify against IPPS FY2026 Final Rule tables.
Sequencing Active Disease
When a patient is admitted primarily for management of active SPMS (e.g., for 5 days of IV Solu-Medrol due to an acute relapse with paraparesis), G35.C1 is sequenced as the principal diagnosis. It will group directly to MDC 01.
🔗 Related ICD-10-CM Codes
Activity Status Variants (Mutually Exclusive)
| Code | Description |
|---|---|
| G35.C1 | Active secondary progressive MS ← This Code |
| G35.C0 | Secondary progressive MS, unspecified |
| G35.C2 | Non-active secondary progressive MS |
Phenotype Variants
| Code | Description |
|---|---|
| G35.A | Relapsing-remitting multiple sclerosis (RRMS) |
| G35.B1 | Active primary progressive multiple sclerosis (PPMS) |
| G35.D | Multiple sclerosis, unspecified |
🛠️ Commonly Associated CPT Codes (Neurology / PM&R)
Outpatient and Profee Setting Context
The CPT codes below are associated with the diagnostic workup and acute management of active SPMS (a superimposed relapse) in profee and outpatient facility settings.
| CPT Code | Description | Profee Coding Notes (Modifier 26) |
|---|---|---|
| 70553 | MRI brain without contrast, followed by with contrast | Contrast is crucial to identify active (gadolinium-enhancing) lesions. Append -26 for profee interpretation only. |
| 72156 | MRI cervical spine w/ and w/o contrast | |
| 96365 | Intravenous infusion, for therapy, prophylaxis, or diagnosis; initial, up to 1 hour | Used for administering high-dose IV methylprednisolone (Solu-Medrol). |
| 96366 | Intravenous infusion, each additional hour | |
| 99222 | Initial hospital inpatient or observation care, moderate complexity | Typical E/M for acute admission during a relapse. |
NCCI Bundling Considerations
- Infusion services (96365) billed on the same day as an E/M visit require the E/M to be significant, separately identifiable, and above/beyond the standard work of the infusion. If supported by documentation, append Modifier -25 to the E/M code.
🔬 ICD-10-PCS Crosswalk (Inpatient Procedures)
When G35.C1 is an inpatient principal diagnosis for an active relapse, the following therapeutic ICD-10-PCS sections and root operations are highly relevant for acute management.
| PCS Section | Body System | Root Operation | Clinical Application |
|---|---|---|---|
| 3 (Administration) | E (Physiological Systems) | 0 (Introduction) | High-dose IV Corticosteroid therapy. Example: 3E033VZ (Introduction of Hormone into Peripheral Vein, Percutaneous). |
| 6 (Extracorporeal Therapies) | A (Physiological Systems) | 5 (Pheresis) | Therapeutic plasma exchange (plasmapheresis) for severe relapses unresponsive to steroids. Example: 6A550Z3 (Pheresis of Plasma, Single). |
💊 Coding Scenarios and Examples
Scenario 1 — Inpatient Admission for Active SPMS Relapse
Clinical Vignette: A 52-year-old female with a known history of secondary progressive MS — initially diagnosed with RRMS 18 years ago, now with steadily progressive gait decline — presents to the ER with sudden onset of severe right eye pain and profound vision loss over 24 hours. MRI of the brain and orbits with contrast reveals a new, strongly enhancing lesion on the right optic nerve. The neurologist documents “active secondary progressive MS with a clinical relapse and new enhancing lesion.” She is started on 1,000 mg IV methylprednisolone daily for 5 days.
Principal Diagnosis:
- G35.C1 — Active secondary progressive multiple sclerosis (Drives the MDC 01 DRG grouping)
Secondary Diagnoses:
- H46.9 — Unspecified optic neuritis, right eye (Captures the specific acute manifestation)
Inpatient Procedures (PCS):
- 3E033VZ — Introduction of Hormone into Peripheral Vein, Percutaneous Approach (Captures the IV steroid therapy)
Scenario 2 — Outpatient Infusion Center for Relapse Management
Clinical Vignette: A 55-year-old male with SPMS calls his neurologist reporting acute, severe worsening of lower extremity weakness over the last two days, making him entirely unable to bear weight (a significant decline from his baseline of walking with a cane). The neurologist evaluates him in the clinic, documents an acute relapse confirming active SPMS, and sends him to the outpatient infusion center for 3 days of IV Solu-Medrol.
CPT / HCPCS (Profee/Outpatient):
- 99215-25 — Office/outpatient visit, high complexity, established patient (Modifier 25 denotes the separate evaluation leading to the decision to treat)
- 96365 — IV infusion, initial, up to 1 hour
- J2920 — Injection, methylprednisolone sodium succinate, up to 40 mg (Billed based on total units administered)
ICD-10-CM:
- G35.C1 — Active secondary progressive multiple sclerosis
- G82.20 — Paraplegia, unspecified (To capture the acute weakness)
Scenario 3 — CDI Query: Activity Status Unclear
Clinical Vignette: A patient with SPMS is admitted for urosepsis. The H&P states: “Patient admitted for severe sepsis secondary to UTI. Also experiencing worsening of her baseline spasticity and fatigue today.” The patient is treated with IV Rocephin and fluids. No IV steroids or acute MS-specific therapies are ordered, and no new MRI is performed. As the infection clears on day 3, the physician notes, “MS symptoms returning to baseline.”
Action / Outcome: The clinical picture — symptoms resolving upon treatment of an underlying infection without steroid therapy and without documented relapse or new MRI lesions — points to a pseudoexacerbation (Uhthoff’s phenomenon / infection-related worsening) rather than true disease activity. There is no provider documentation of activity, so G35.C1 is not supported.
CDI Query: The coder queries the provider to clarify the SPMS activity status — active (documented relapse and/or new MRI lesions) versus non-active.
Query Response: The physician clarifies it was a pseudoexacerbation driven by the UTI, with no active MS disease activity.
Corrected ICD-10-CM Coding:
- A41.9 — Sepsis, unspecified organism (Principal Diagnosis)
- N39.0 — Urinary tract infection, site not specified
- G35.C2 — Non-active secondary progressive multiple sclerosis (Activity status clarified — C1 not supported)
⚠️ Coding Pitfalls and Tips
| Pitfall or Tip | |
|---|---|
| ❌ | Do not assign G35.C1 from a radiology report alone. “Active” requires the treating provider to document a clinical relapse and/or new MRI activity. Without that documentation, use G35.C0 or G35.C2 as supported. |
| ❌ | Do not confuse a pseudoexacerbation with true activity. Worsening solely due to heat, fever, or systemic infection (which resolves when the trigger resolves), with no relapse and no new MRI lesions, is not active disease — code G35.C2 or G35.C0, not G35.C1. |
| ❌ | Do not report more than one G35.C activity code per encounter. G35.C0, G35.C1, and G35.C2 are mutually exclusive. |
| ❌ | Do not confuse SPMS (G35.C) with PPMS (G35.B). SPMS follows an initial relapsing-remitting course; PPMS is progressive from onset. |
| ✅ | Sequence properly for admissions. If the patient is admitted specifically to treat an active SPMS relapse (e.g., IV steroids, plasmapheresis), sequence G35.C1 as the principal diagnosis. |
| ✅ | Code the acute deficits. Optic neuritis, acute paraparesis, or severe ataxia that prompted the encounter should be coded secondarily to paint a full picture of the severity of illness. |
| ✅ | Look for contrast MRI and relapse language. New gadolinium-enhancing or new T2/FLAIR lesions plus a provider-documented relapse support “active” status. |
📚 Sources
- CMS/NCHS. ICD-10-CM Official Guidelines for Coding and Reporting, FY2026.
- National Multiple Sclerosis Society. Types of MS: Secondary Progressive MS (SPMS) and Disease Activity.
- AAPC. ICD-10-CM Code G35.C1 — Active secondary progressive multiple sclerosis (FY2026).
- CMS. 2025-2026 Medicare Advantage Risk Adjustment — CMS-HCC Model v28 ICD-10-CM Mappings.
- CMS. IPPS Final Rule FY2026 — MS-DRG Definitions Manual v43. MDC 01 logic tables.
- CMS. ICD-10-PCS Reference Manual FY2026. Section 3 (Administration), Section 6 (Extracorporeal Therapies).
- AMA. CPT Professional Edition 2026. Neurology and Medicine subsections.