By Eman Zahra | Last updated August 20, 2026

This article is for general educational purposes only. It is not legal, financial, insurance, or coding-compliance advice. Coding decisions should be made by qualified, credentialed coders in consultation with treating providers, and payer-specific rules should always be verified directly with the payer before submitting or appealing a claim.

What Happened to ICD-10 Code G35?

If a multiple sclerosis claim submitted for a date of service in October 2025 or later came back denied, the code is almost certainly the problem — not the diagnosis itself. Effective October 1, 2025, the Centers for Medicare & Medicaid Services (CMS), the CDC’s National Center for Health Statistics, and the American Hospital Association’s coordinated ICD-10-CM update process retired G35 as a stand-alone billable code. For nearly a decade, G35 alone was enough to report any form of multiple sclerosis. As of the FY2026 code set, it isn’t anymore.

This wasn’t a small tweak buried in a footnote. It was part of a broader annual update that added 487 new diagnosis codes, revised 38 existing codes, and deleted 28 codes across the ICD-10-CM system — and MS coding was one of the most significant changes in the release. CMS’s FY2026 IPPS Final Rule explicitly lists G35 among the invalid diagnosis codes for the fiscal year, meaning payer systems that follow CMS specifications — which is effectively all of them, Medicare and commercial alike — are built to reject claims that still use it for dates of service on or after October 1, 2025.

Why CMS made this change. The old single-code system captured that a patient had MS but not what kind. Most MS treatment decisions — and most payer medical necessity policies for disease-modifying therapies — already hinge on the specific phenotype (relapsing-remitting vs. primary progressive vs. secondary progressive) and whether the disease is currently active. Coders and clinicians have effectively been working around this gap for years, documenting subtype in the chart even though the claim only carried the generic G35 code. The 2026 update closes that gap by building subtype and activity status directly into the code itself.

The New MS Code Family (Replacing G35)

G35 didn’t disappear from the code book — it became a parent category. The billable, claim-ready codes now sit one level deeper:

Old Code	New Code	Description	When to Use
G35	G35.A	Relapsing-remitting multiple sclerosis	Documented RRMS, regardless of activity (activity subcodes not currently split for this subtype)
G35	G35.B0	Primary progressive MS, unspecified activity	PPMS documented, activity status not specified
G35	G35.B1	Primary progressive MS, active	PPMS with documented relapse, new MRI activity, or progression
G35	G35.B2	Primary progressive MS, non-active	PPMS documented as clinically/radiographically stable
G35	G35.C0	Secondary progressive MS, unspecified activity	SPMS documented, activity status not specified
G35	G35.C1	Secondary progressive MS, active	SPMS with documented relapse, new MRI activity, or progression
G35	G35.C2	Secondary progressive MS, non-active	SPMS documented as clinically/radiographically stable
G35	G35.D	Multiple sclerosis, unspecified	Diagnosis of MS confirmed, but subtype not documented — a fallback, not a default

Two timing rules matter more than anything else in this table:

1. Old G35 is still correct for old dates of service. If a patient was seen or infused on September 30, 2025, that claim uses G35 — even if it’s being submitted, corrected, or resubmitted well after October 1. The rule follows the date of service, not the date of filing.
2. New subcodes are invalid before October 1, 2025. You can’t retroactively apply G35.A to a June 2025 visit even if it would be more accurate. Use the code set that was valid on the actual date of service.

Why Your Claim Was Denied — Common Scenarios

You billed G35 for a date of service on or after October 1, 2025. This is the single most common cause. Practice management systems, superbills, and even some EHR problem lists may still have G35 as the default MS diagnosis pick if they weren’t updated before the transition. The claim goes out, hits the payer’s edit for an invalid/deleted diagnosis code, and bounces.

Your prior authorization doesn’t match your claim code. Payers frequently require the diagnosis code on the claim to match the code on file from the prior authorization. If a prior auth was approved under the old G35 before October 1 but the treatment (an infusion, for example) occurs after that date, the claim’s new subcode may not match what’s on file — triggering a denial even though the treatment itself was approved.

The payer’s system hasn’t fully caught up. Even though CMS’s effective date is fixed, individual commercial payers and Medicaid managed-care plans update their claims-adjudication logic on their own timelines. Some may briefly reject correctly coded new subcodes, or their phone-support staff may not yet have benefit detail tied to the new codes. This is a real, if temporary, source of denials that has nothing to do with your coding accuracy.

Documentation doesn’t support subtype or activity specificity. If the chart says only “multiple sclerosis” without noting the phenotype, a coder may be forced into G35.D — and if the payer’s medical policy requires a more specific subtype code for the billed service (common for disease-modifying therapy authorizations), even the correctly assigned G35.D can trigger a medical necessity denial.

How to Fix a Denied Claim

Step 1 — Confirm the date of service. Before touching the code, verify which side of October 1, 2025 the service falls on. This determines whether G35 or a new subcode is even the right starting point.

Step 2 — Pull the clinical documentation. Locate the note supporting the diagnosis and look for explicit language about MS subtype (relapsing-remitting, primary progressive, secondary progressive) and activity status (active vs. non-active, recent relapse, new MRI lesions, progression).

Step 3 — Select the correct subtype/activity code. Match what’s documented to the table above. If the documentation is genuinely silent on subtype, G35.D is the appropriate fallback — but treat that as a signal to query the provider for more specific documentation going forward, not a permanent solution.

Step 4 — Correct and resubmit, or appeal. If the original claim used the deleted G35 for a post-October-1 date of service, this is typically a straightforward corrected-claim resubmission rather than a formal appeal — payers generally have a process for submitting a corrected claim with the accurate code rather than disputing the original decision. If the claim was already coded correctly and still denied, move to a formal appeal (see below).

Step 5 — Update prior authorizations. If treatment continues past the transition date, contact the payer to confirm whether the prior authorization needs to be reissued or amended to reflect the new subcode. Don’t assume an approval issued under old G35 will automatically carry over.

How to Appeal If the Claim Was Coded Correctly and Still Denied

If documentation, dates of service, and coding are all correct and the claim is still denied — most likely due to a payer system lagging on recognizing the new codes, or a medical-necessity dispute over the documented subtype — standard payer appeal processes apply. For Medicare fee-for-service claims, this generally means filing a redetermination request with the Medicare Administrative Contractor, typically within 120 days of the denial notice, followed by further appeal levels (reconsideration, administrative law judge hearing, and beyond) if needed. Commercial payers and Medicaid plans each have their own appeal windows and required forms — these vary by state and by plan, so the specific payer’s appeal process and deadlines should always be confirmed directly rather than assumed to match Medicare’s timeline.

When appealing, include: the clinical note documenting MS subtype and activity status, a copy of the prior authorization (if applicable), and a clear, brief statement noting that the diagnosis code used reflects the correct FY2026 ICD-10-CM code set effective for the date of service in question. Citing the CMS FY2026 code update directly in the appeal letter can help resolve disputes faster, especially with payer representatives unfamiliar with the change.

Preventing This Going Forward

• Audit EHR problem lists and superbills for any lingering G35 entries and replace them with the appropriate subtype-specific default, or at minimum flag them for provider review at the next visit.
• Run a report of all patients previously coded G35 and cross-reference documentation to proactively assign the correct new subcode before their next claim goes out, rather than reacting to denials one at a time.
• Build a documentation query process so that when a chart only says “MS” without subtype, coding staff have a fast way to ask the treating provider for clarification before the claim is submitted — not after it’s denied.
• Confirm system readiness with each payer you bill regularly, since rollout speed for recognizing the new codes has varied.

Methodology: This article is based on CMS’s FY2026 ICD-10-CM guidelines and IPPS Final Rule, cross-checked against multiple independent coding-industry and patient-advocacy sources. Payer-specific behavior (timing, grace periods, system readiness) varies and should be confirmed directly with each payer.

FAQs

Is G35 still valid for older claims? Yes, for dates of service before October 1, 2025. The new subtype-specific codes only apply to services on or after that date.

What code should I use if the MS subtype isn’t documented? G35.D (multiple sclerosis, unspecified) is the appropriate fallback, but it should prompt a documentation query to the provider rather than become a permanent default, since some payer medical policies require subtype-specific codes for certain treatments.

Will this affect my MS treatment approval? Not because of the disease itself — MS remains a covered diagnosis. But if a prior authorization or claim still references the deleted G35 for a service after October 1, 2025, it can create a processing delay until the correct subtype code is submitted.

Do all insurance companies apply this change the same way? The underlying ICD-10-CM code set is standardized, but how quickly each payer’s claims system was updated to recognize the new codes — and how strictly they enforce subtype-specific documentation — can vary by payer, plan, and sometimes by state. Confirm directly with the specific payer if a denial seems inconsistent with the rules described here.