2026 ICD-10-CM Diagnosis Code I40.1Isolated myocarditis
ICD-10-CM Codes›I00–I99›I30-I5A›I40
- Billable — Valid for Submission
- MCC — Major Complication or Comorbidity
- Not Chronic
I40.1 is a billable ICD-10-CM diagnosis code for isolated myocarditis. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 314 through 316, 974 through 976. As a secondary diagnosis, it counts as a major complication or comorbidity (MCC) and places an inpatient stay in the highest severity level of its MS-DRG family. It does not count, however, when the principal diagnosis is one of 22 closely related codes. Coders also document this condition as eosinophilic myocarditis. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Myocarditis and cardiomyopathy.
Code Identity
Code Classification
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Eosinophilic myocarditis
- Giant cell myocarditis
- Idiopathic myocarditis
- Isolated myocarditis
- Isolated diffuse granulomatous myocarditis
Tabular List NotesGuidance
Coding notes and annotation back-references applicable to this code.
Inclusion Terms
- Fiedler's myocarditis
- Giant cell myocarditis
- Idiopathic myocarditis
These terms are the conditions for which that code is to be used. The terms may be synonyms of the code title, or, in the case of "other specified" codes, the terms are a list of the various conditions assigned to that code. The inclusion terms are not necessarily exhaustive. Additional terms found only in the Alphabetic Index may also be assigned to a code.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
myocarditis (acute) I40.1
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Giant Cell Myocarditis
an often fatal inflammatory disorder that affects the myocardium. morphologically, it is characterized by the presence of an inflammatory infiltrate in the myocardial tissue that includes multinucleated giant cells. it is often associated with the presence of an autoimmune disorder. patients present with arrhythmias and/or heart failure. heart transplantation is the only treatment option available.
Patient EducationClinical
Cardiomyopathy
Cardiomyopathy is the name for diseases of the heart muscle. These diseases enlarge your heart muscle or make it thicker and more rigid than normal. In rare cases, scar tissue replaces the muscle tissue.
Read the full article at MedlinePlus
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Convert I40.1 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About I40.1Overview
What is the ICD-10 code for isolated myocarditis?
The ICD-10-CM code for isolated myocarditis is I40.1 (sometimes written as I401). It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
Is I40.1 (Acute myocarditis) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report isolated myocarditis on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What MS-DRG does I40.1 group to?
When isolated myocarditis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 314, 315, 316, 974, 975, 976, with relative weights from 0.6821 to 2.8860 depending on complications. Higher weights mean higher Medicare reimbursement.
Is I40.1 a CC or MCC?
CMS lists I40.1 as an MCC (major complication or comorbidity) for FY 2026. Reported as a secondary diagnosis, it places the inpatient stay in the highest-weighted DRG of its severity family. It does not count when the principal diagnosis is one of the 22 closely related codes in its exclusion list.
What is the ICD-9 equivalent of I40.1?
Under the General Equivalence Mappings, isolated myocarditis converts to ICD-9-CM 422.91 (idiopathic myocarditis). The mapping is approximate, so confirm the match fits the documentation.
