2026 ICD-10-CM Diagnosis Code I48.91Unspecified atrial fibrillation

ICD-10-CM Codes›I00–I99›I30-I5A›I48

ICD-10-CM I48.91
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

I48.91 is a billable ICD-10-CM diagnosis code for unspecified atrial fibrillation. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 308 through 310. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Cardiac dysrhythmias.

This medical diagnosis code is frequently used in Family Practice, Internal Medicine, and Cardiology medical specialties to designate conditions such other forms of heart disease.

Code Identity

ICD-10-CM Code
I48.91
Billable Status
Yes — Valid for Submission
Code Describes
Unspecified atrial fibrillation
Short Description
Unspecified atrial fibrillation
Same as the full description in the CMS dataset.
Parent Code
I48.9
Unspecified atrial fibrillation and atrial flutter

Code Classification

ChapterI00–I99Diseases of the circulatory system
SectionI30-I5AOther forms of heart disease
CategoryI48Atrial fibrillation and flutter
This CodeI48.91Unspecified atrial fibrillation

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Accelerated atrioventricular conduction
  • Atrial fibrillation
  • Atrial fibrillation due to heart valve disorder
  • Atrial fibrillation with rapid ventricular response
  • Controlled atrial fibrillation
  • Familial atrial fibrillation
  • Hypercoagulability state
  • Hypercoagulable state due to atrial fibrillation
  • Lone atrial fibrillation
  • Preexcited atrial fibrillation
  • Rapid atrial fibrillation
  • Secondary hypercoagulability state
  • Transient cerebral ischemia due to atrial fibrillation

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Fibrillation
      • atrial or auricular (established)

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR CIR017
Cardiac dysrhythmias
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Atrial Fibrillation

    abnormal cardiac rhythm that is characterized by rapid, uncoordinated firing of electrical impulses in the upper chambers of the heart (heart atria). in such case, blood cannot be effectively pumped into the lower chambers of the heart (heart ventricles). it is caused by abnormal impulse generation.

Patient EducationClinical

Atrial Fibrillation

Atrial fibrillation, also known as AFib or AF, is one of the most common types of arrhythmias. Arrhythmias are problems with the rate or rhythm of your heartbeat. They can cause your heart to beat too slowly, too fast, or in an irregular way.

The full article covers:

  • What is atrial fibrillation (AFib)?
  • What causes atrial fibrillation (AFib)?
  • Who is more likely to develop atrial fibrillation (AFib)?
  • What are the symptoms of atrial fibrillation (AFib)?
  • What other problems can AFib cause?
  • How is atrial fibrillation (AFib) diagnosed?
  • What are the treatments for atrial fibrillation (AFib)?
  • Can atrial fibrillation (AFib) be prevented?

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert I48.91 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
427.31 Atrial fibrillation
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About I48.91Overview

Is I48.91 (Unspecified atrial fibrillation and atrial flutter) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report unspecified atrial fibrillation on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does I48.91 group to?

When unspecified atrial fibrillation is the principal diagnosis on an inpatient stay, it groups to MS-DRG 308, 309, 310, with relative weights from 0.5661 to 1.2040 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of I48.91?

Under the General Equivalence Mappings, unspecified atrial fibrillation converts to ICD-9-CM 427.31 (atrial fibrillation). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.