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I48.91ICD-10-CM

Unspecified atrial fibrillation

BillableMaps to an HCCSpecificity does not change the risk scoreNo instructional notes at the category

How the code is built

I

Chapter

Diseases of the circulatory system

48

Category

Atrial fibrillation and flutter

9

Char 4

Unspecified atrial fibrillation and atrial flutter

1

Char 5

Fibrillation rather than flutter

Chapter

I00 to I99

Diseases of the circulatory system

Block

I30 to I5A

Other forms of heart disease

Category

I48

Atrial fibrillation and flutter

Subcategory

I48.9

Unspecified atrial fibrillation and atrial flutter, not billable

Billable

Yes

Complete at 5 characters

Instructional notes

None at the category

Only a chapter-level Excludes2 reaches this code

Risk adjustment

HCC mapped

The specified heart arrhythmias category under CMS-HCC V28

DefinitionCode structureClinical pictureDocumentationDecision guideTabular listComparisonClaim contextRisk adjustmentDenialsSources

Key takeaways

  • I48.91 codes atrial fibrillation when the pattern is not documented
  • Rapid ventricular response has no code of its own and does not change the selection
  • Every atrial fibrillation and flutter code maps to the same risk adjustment category at the same weight
  • Specificity here protects audit defense rather than the risk score, which reverses the usual argument
  • It is billable, and the category carries no instructional notes at all
1Definition

What ICD-10 code I48.91 means

I48.91 is the ICD-10-CM code for unspecified atrial fibrillation. It applies when atrial fibrillation is confirmed, and the record does not identify which pattern it follows.

The Alphabetic Index reaches this code through fibrillation, atrial or auricular, with established as a parenthetical, non-essential modifier. So a note reading established atrial fibrillation still classifies here. The word adds nothing.

Code

I48.91

Descriptor

Unspecified atrial fibrillation

Chapter

Diseases of the circulatory system

Block

Other forms of heart disease

Category

I48, atrial fibrillation and flutter

Subcategory

I48.9, unspecified atrial fibrillation and atrial flutter

Billable

Yes

Instructional notes at I48

None

Atrial fibrillation with rapid ventricular response classifies here too, and that catches people out. RVR appears as an approximate synonym at this code, and ICD-10-CM has no combination code for fibrillation with rapid ventricular response. RVR is a rate description, not a pattern, so it does not select a code. If the pattern is documented, the pattern code applies, and the rapid response changes nothing. If the pattern is not documented, I48.91 applies whether the rate is controlled or not.

2Code structure

A pattern axis, and two headers that reject

I48.91 has no subdivisions. The fifth character separates fibrillation from flutter within the unspecified subcategory, and the fourth character carries the pattern.

The pattern axis

Not documented

I48.91

This code

Paroxysmal

I48.0

Longstanding persistent

I48.11

Other persistent

I48.19

Chronic, no further detail

I48.20

Permanent

I48.21

Two of the parents are not billable. I48.1 and I48.2 are headers, and a four-character submission trips a specificity edit before a payer looks at the claim. That trap did not exist in the earliest years of ICD-10-CM, when both were billable at four characters, so encounter forms built before the restructure still offer them.

Flutter runs alongside fibrillation in the same category

Typical atrial flutter

I48.3

Atypical atrial flutter

I48.4

Flutter, type not stated

I48.92

A patient documented with both fibrillation and flutter carries a code from each. Nothing in the category prohibits it.

What the code does not record

Rapid, controlled, or slow ventricular response

None

New onset

None

Valvular or non-valvular

None directly, though valve disease is separately coded

Anticoagulation status

None, but report the long-term drug therapy code

Stroke risk score

None

3Clinical picture

Clinical findings documented under I48.91

A record supporting this code confirms fibrillation without classifying its pattern.

Rhythm confirmation

Electrocardiogram, telemetry, ambulatory monitor, or implanted device interrogation

Rate

Ventricular response, including whether rapid or controlled

Symptoms

Palpitations, dyspnea, fatigue, dizziness, chest discomfort, or none

Duration and course

Onset, whether episodes self-terminate, prior cardioversion, prior ablation

Structural context

Valve disease, heart failure, cardiomyopathy, prior myocardial infarction

Risk factors

Hypertension, diabetes, prior stroke, age, vascular disease

Management

Rate control, rhythm control, anticoagulation, and the decision behind each

Duration and course are the row that resolves the code. Episodes that start and stop on their own point to paroxysmal. Continuous rhythm for more than a week points to persistent, and beyond a year to longstanding persistent. A documented decision to abandon rhythm control points to permanent. Each of those distinctions is a sentence the cardiologist has usually already written.

4Documentation

What the chart must show

  • A provider-stated diagnosis of atrial fibrillation, with the rhythm confirmed
  • The pattern, meaning paroxysmal, persistent, longstanding, chronic, or permanent
  • Whether atrial flutter is also present, since that is a separate code
  • Ventricular response, recognising it does not change the code
  • Prior cardioversion or ablation, and whether rhythm control is still being attempted
  • Any valve disease, since that is separately reportable
  • Anticoagulation status, so the long-term drug therapy code can be reported
  • An assessment and plan showing the condition was evaluated, addressed, or treated at this encounter

The pattern is usually documented somewhere other than the assessment line. A note describing episodes that self-terminate has documented paroxysmal fibrillation, and a note recording that rate control was chosen after rhythm control failed has documented permanent. The information exists; it does not reach the code.

One nuance on established patients. A patient with known chronic or paroxysmal fibrillation presenting with an acute episode is coded to the documented pattern, not to the unspecified code, unless the provider states that the classification has changed.

5Decision guide

When to assign I48.91, and when not to

Atrial fibrillation, pattern not stated

I48.91

Atrial fibrillation with rapid ventricular response, pattern not stated

I48.91, since RVR is not a code axis

Established atrial fibrillation

I48.91, since established is a non-essential modifier

New-onset atrial fibrillation, no pattern stated

I48.91, and query

Paroxysmal atrial fibrillation

I48.0

Longstanding persistent atrial fibrillation

I48.11

Other persistent atrial fibrillation

I48.19

Chronic atrial fibrillation, no further detail

I48.20

Permanent atrial fibrillation

I48.21

Typical or atypical atrial flutter

I48.3 or I48.4

Atrial flutter, type not stated

I48.92

Both fibrillation and flutter documented

A code from each

Fibrillation with rheumatic or structural valve disease

The fibrillation code plus the valve disease code

Ventricular fibrillation

I49.01, which is a different arrhythmia entirely

There is no sequencing rule attached to I48.91 and no instructional note anywhere in the category. The only question is which pattern the record supports.

Query rather than default when the note describes self-terminating episodes, records a cardioversion or ablation history, or states that rhythm control has been abandoned.

6Tabular list

Instructional notes at I48 and above

Instructional notes at I48.91

None

Instructional notes at I48.9

None

Instructional notes at I48

None

Excludes2 at I00 to I99

Certain conditions originating in the perinatal period, certain infectious and parasitic diseases, complications of pregnancy, childbirth and the puerperium, congenital malformations, endocrine, nutritional and metabolic diseases, injury and poisoning, neoplasms, symptoms and abnormal findings, systemic connective tissue disorders, and transient cerebral ischemic attacks

This is the only code in this glossary with no instructional notes at all. No Excludes1 anywhere in the path, no Excludes2 at the category, no Includes list, no use additional, no code first. The single annotation reaching I48.91 is the chapter-level Excludes2, and every entry on it is a permission rather than a prohibition.

That has a clear consequence. Nothing in the tabular list constrains this code. There is no prohibited pair to trip over and no companion code the classification demands. The entire constraint on I48.91 is documentation specificity, which is unusual and worth knowing before spending time hunting for notes that are not there.

It also means the pairings that matter here are clinical rather than instructional. Valve disease, heart failure, and anticoagulation status are all separately reportable, and none of them is prompted by a note.

7Comparison

Codes frequently confused with I48.91

I48.0

Descriptor

Paroxysmal atrial fibrillation

Use instead when

Episodes start and stop on their own

I48.11

Descriptor

Longstanding persistent atrial fibrillation

Use instead when

Continuous rhythm with rhythm control still being attempted

I48.19

Descriptor

Other persistent atrial fibrillation

Use instead when

Continuous rhythm that has not reached the longstanding threshold

I48.20

Descriptor

Chronic atrial fibrillation, unspecified

Use instead when

Documented as chronic with no further detail

I48.21

Descriptor

Permanent atrial fibrillation

Use instead when

Rhythm control has been abandoned by shared decision

I48.1, I48.2

Descriptor

Persistent and chronic atrial fibrillation

Use instead when

Never, since both are non-billable headers

I48.3, I48.4

Descriptor

Typical and atypical atrial flutter

Use instead when

Flutter is documented, reportable alongside fibrillation

I48.92

Descriptor

Unspecified atrial flutter

Use instead when

Flutter is documented without a type

I49.01, I49.02

Descriptor

Ventricular fibrillation and ventricular flutter

Use instead when

The arrhythmia is ventricular rather than atrial

I49.5

Descriptor

Sick sinus syndrome

Use instead when

Sinus node dysfunction is documented, reportable alongside

I47.-

Descriptor

Paroxysmal tachycardia

Use instead when

The arrhythmia is a paroxysmal tachycardia

I49.8, I49.9

Descriptor

Other specified and unspecified cardiac arrhythmias

Use instead when

Another arrhythmia is documented

I05.- to I08.-

Descriptor

Rheumatic valve disorders

Use instead when

Rheumatic valve disease is documented, reportable alongside

I34.- to I37.-

Descriptor

Nonrheumatic valve disorders

Use instead when

Structural valve disease is documented, reportable alongside

I50.-

Descriptor

Heart failure

Use instead when

Heart failure is documented, reportable alongside

Z79.01

Descriptor

Long term current use of anticoagulants

Use instead when

The patient is on anticoagulation, reported additionally

The I48.1 and I48.2 row is on this table because those codes still get submitted. Both were billable in the first years of ICD-10-CM, and both became headers when the pattern codes were expanded.

8Claim context

Services commonly reported with I48.91

I48.91 supports primary care and cardiology evaluation and management, electrocardiography, ambulatory rhythm monitoring, echocardiography, anticoagulation management, cardioversion, and catheter ablation.

Rhythm monitoring and echocardiography are governed by contractor policy rather than by any national laboratory determination, and the policies key on documented symptoms, on the clinical question the study is meant to answer, and on prior testing. Ablation and left atrial appendage occlusion carry their own coverage criteria, and those criteria look for the pattern, the failure of prior therapy, and the stroke risk assessment, none of which an unspecified code conveys.

On inpatient claims, atrial fibrillation is most valuable as a comorbidity rather than as the admission. It contributes to severity classification when it accompanies another principal diagnosis, and the codes reported alongside it carry the weight of the stay.

Anticoagulation management has its own documentation requirements, and the long-term drug therapy status code belongs on the claim for a patient on ongoing therapy.

9Risk adjustment

What I48.91 does to a risk score

The category pays, and specificity does not raise it

I48.91 maps to the specified heart arrhythmias category under the current model, which reached full phase-in for payment year 2026.

Every atrial fibrillation and flutter code maps to that same category at the same weight.

Paroxysmal, persistent, longstanding persistent, chronic, permanent, unspecified, and all three flutter codes land together. Specificity does not raise the risk score for this condition. That reverses the argument almost every coding guide makes, and it is worth stating plainly because chart review effort is finite and better spent where specificity does change the category.

Specificity still matters, for two reasons that are not the risk score.

A claim carrying the unspecified code beside a note documenting permanent fibrillation is an unsupported submission in a risk adjustment validation review, and that mismatch is a recognised audit target. Separately, the pattern affects clinical care decisions that the record has to stand behind.

Report it every year.

Risk adjustment does not carry a condition forward. Atrial fibrillation is chronic and almost always present, which also makes it the condition most likely to be carried on a problem list without an assessment supporting it.

Check which model your reference is quoting.

The category number changed between model versions, and several current published guides still cite the retired category from the previous model. Payment year 2026 runs entirely on the current one, so a crosswalk built on the older number is describing a model that no longer carries payment weight.

10Denials

Coding errors that cause denials

Submitting I48.1 or I48.2 without a fifth character

Complete the code, since both are non-billable headers

Coding I48.91 when the note describes self-terminating episodes

Assign the paroxysmal code

Coding I48.91 when rhythm control has been abandoned

Assign the permanent code

Coding I48.91 for an established patient with a documented pattern

Assign the documented pattern unless the provider states it changed

Looking for a combination code for rapid ventricular response

Assign the pattern code, since RVR is not a code axis

Reporting only the fibrillation code when flutter is also documented

Report both

Omitting the valve disease code

Report it, since valve disease is separately reportable

Omitting the long-term anticoagulant status code

Report it for a patient on ongoing therapy

Carrying the diagnosis on a problem list with no assessment

Report only when the encounter evaluates, addresses, or treats it

Relying on a guide that cites the retired risk adjustment category

Verify against the current payment year model file

11Sources

Where to verify I48.91

ICD-10-CM Official Guidelines for Coding and Reporting

The conventions on unspecified codes, non-essential modifiers, and the reporting of secondary diagnoses

ICD-10-CM Tabular List at I48 and the chapter head

The absence of instructional notes at the category level, and the chapter-level Excludes2 that is the only annotation reaching this code

CMS ICD-10 code page and the FY 2026 files

Current code files, the annual addenda, and the present structure of I48.1- and I48.2-

CMS Medicare Advantage rates and statistics

The current payment year model file, and the category all the atrial fibrillation codes share

Medicare Coverage Database

Search your contractor's determinations for ambulatory rhythm monitoring, echocardiography, ablation, and left atrial appendage occlusion.

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Last reviewed August 2026

This page is a free reference for billing and coding teams and is not coding, legal, or clinical advice. Code selection depends on the documentation in the record and on the policy of the payer being billed. Verify every code, instructional note, and sequencing rule against the current ICD-10-CM files and the Official Guidelines for Coding and Reporting for the applicable date of service. ICD-10-CM is maintained by the CDC's National Center for Health Statistics and published by CMS. CPT is a registered trademark of the American Medical Association. TruCare is not affiliated with CMS, NCHS, the AHA, or the AMA.

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