I48.91: Unspecified atrial fibrillation

The ICD-10-CM code for atrial fibrillation when the diagnosis is confirmed but the record does not specify whether the rhythm is paroxysmal, persistent, longstanding persistent or permanent.

Billable

FY2027

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At a glance

Code Family

I48: atrial fibrillation and flutter.

DRG Grouping

MDC 05, Diseases and Disorders of the Circulatory System, under the FY2027 MS-DRG v44 grouper, DRG 308 with MCC, DRG 309 with CC, or DRG 310 without CC or MCC.

HCC Mapping (v28)

HCC 238: Specified Heart Arrhythmias.

Condition

Atrial fibrillation

Family Specificity

I48.91 is the unspecified code within the I48 family. The specificity ladder runs from paroxysmal I48.0 through persistent I48.19 and longstanding persistent I48.11 to permanent I48.21.

Which atrial fibrillation code do I assign?

I48.91 is the not-otherwise-specified code in the I48 family, and it belongs only where the record confirms atrial fibrillation but never names the rhythm pattern. When documentation supports a subtype, the corresponding code from I48.0 through I48.21 is assigned in preference to I48.91.

When documentation says…

Code

Self-terminating atrial fibrillation documented as paroxysmal

I48.0

Assign when the provider documents paroxysmal atrial fibrillation. The Index sends paroxysmal atrial fibrillation to I48.0.

Persistent atrial fibrillation, not documented as longstanding

I48.19

Other persistent atrial fibrillation. Its inclusion terms are chronic persistent atrial fibrillation and persistent atrial fibrillation, NOS.

Longstanding persistent atrial fibrillation

I48.11

Longstanding persistent atrial fibrillation. Assign when the provider documents the persistent atrial fibrillation as longstanding.

Permanent atrial fibrillation

I48.21

Permanent atrial fibrillation. Assign when the provider documents the atrial fibrillation as permanent.

Chronic atrial fibrillation documented without a stated pattern

I48.20

Chronic atrial fibrillation, unspecified. Use when the provider characterizes the arrhythmia as chronic but does not identify it as permanent or persistent. I48.20 carries an Excludes1 note for chronic persistent atrial fibrillation (I48.19). The two codes are not reported together for the same condition.

Typical atrial flutter, type I

I48.3

Assign when the documented arrhythmia is typical (type I) atrial flutter rather than fibrillation. Flutter and fibrillation are separately classified.

Atypical atrial flutter, type II

I48.4

Assign when atypical flutter is documented. As with typical flutter, the record must distinguish flutter from fibrillation.

Atrial flutter documented without a stated type

I48.92

Unspecified atrial flutter. Use when flutter is confirmed but the record does not identify it as typical or atypical.

Atrial fibrillation confirmed, but the record never states the subtype

I48.91

The unspecified code. Correct only when the provider documents atrial fibrillation as a confirmed diagnosis and the record gives no paroxysmal, persistent, longstanding persistent, permanent or chronic descriptor that maps to a more precise I48 subcode.

Tabular notes

EXCLUDES NOTES AT THE LEAF CODE LEVEL

No Excludes1 and no Excludes2 notes appear at the I48.91 leaf code level in the FY2027 tabular list. I48.91 may be used alongside other cardiac and non-cardiac codes when both conditions are independently documented.

PARENT CATEGORY NOTES

Category I48 and subcategory I48.9 carry no Excludes1, Excludes2, Use additional, Code first or Code also notes in the FY2027 tabular. The I30-I5A block carries none either. The Chapter 9 Excludes2 list applies to every code in the chapter, including I48.91.

NO USE ADDITIONAL, CODE FIRST OR CODE ALSO

I48.91 carries no Use Additional, Code First or Code Also instruction at the leaf code level in FY2027. No note at I48 or I48.9 calls for one either. A documented coexisting condition is coded in its own right when it meets Section III or Section IV.J of the guidelines.

GUIDELINE SECTION II: SEQUENCING

Section II assigns the principal diagnosis to the condition established after study to be chiefly responsible for occasioning the admission. When atrial fibrillation is that condition, I48.91 or the documented subtype is sequenced first. Category I48 carries no Code first note. Etiology and manifestation sequencing does not apply to it.

ATRIAL FIBRILLATION WITH RAPID VENTRICULAR RESPONSE

When atrial fibrillation is documented as the cause of rapid ventricular response, assign I48.91, or the specific subtype where it is documented. Do not assign R00.0, tachycardia unspecified, as the principal diagnosis when atrial fibrillation is the documented underlying cause. The atrial fibrillation code is the principal condition and the tachycardia is a manifestation that is subsumed.

FY2027 CODE CHANGES

The FY2027 codes addenda make no addition, deletion or revision in category I48. The FY2027 tabular addenda make no change to the I48 notes. I48.91 is unchanged in the FY2027 update.

Operational & financial impact

Why specificity matters beyond the chart: risk adjustment, DRG assignment and denial exposure all follow from which code is selected.

CMS-HCC v28 (PY2024+)

HCC 238: Specified Heart Arrhythmias. I48.91 maps to this category in CMS-HCC Model Version 28.

Under CMS-HCC v24, I48.91 mapped to HCC 96, also named Specified Heart Arrhythmias. CMS renumbered the category to HCC 238 in v28. HCC 96 is retired in v28. Risk adjustment analytics should use HCC 238 for payment year 2026.

Capture is annual. I48.91 must appear in provider documentation during the data collection year to contribute to the HCC 238 factor for the Medicare Advantage payment year. Wellness and chronic care visits are encounters where recapture can be missed.

MS-DRG v44 (FY2027)

When atrial fibrillation is the principal condition for which the patient was admitted, I48.91 is sequenced as the principal diagnosis and groups to MDC 05 under the FY2027 MS-DRG v44 grouper. The case groups to DRG 308, cardiac arrhythmia and conduction disorders with MCC; DRG 309, cardiac arrhythmia and conduction disorders with CC; or DRG 310, cardiac arrhythmia and conduction disorders without CC/MCC. The specific assignment is determined by the presence and level of complications and comorbidities documented.

Medicare and commercial payers

There is no National Coverage Determination specific to the I48.91 diagnosis code.

Documentation checklist to support I48.91

Chart elements that support code selection and reduce specificity-driven denials.

Atrial fibrillation stated as a confirmed diagnosis, not as a symptom or a finding still under evaluation.

The subtype whenever the provider can state one: paroxysmal, persistent, longstanding persistent, permanent or chronic.

The rate control versus rhythm control strategy where applicable, because that context supports the subtype distinction.

Any identified underlying cause: thyrotoxicosis, mitral stenosis or recent cardiac surgery, documented separately so it can be coded in its own right.

Re-documentation of the atrial fibrillation at an encounter in each data collection year, since HCC 238 capture depends on the code appearing in provider documentation that year.

Concurrent stroke risk conditions such as hypertension, diabetes or prior stroke, recorded through their own diagnosis codes. CHA2DS2-VASc is a clinical stroke risk stratification tool and has no ICD-10-CM code.

FAQ

When is I48.91 the right code rather than a specific atrial fibrillation subtype?

I48.91 is correct when atrial fibrillation is a confirmed diagnosis. It applies when the record gives no paroxysmal, persistent, longstanding persistent, permanent or chronic descriptor that maps to a more precise I48 subcode. In practice that covers three situations: the provider documents atrial fibrillation without qualifying the episode, the record cannot distinguish a new-onset episode from a first-detected episode or a recurrence, or the subtype is indeterminate at the time of coding. Once later documentation such as an electrophysiology study or a cardioversion note establishes the subtype, the specific code from I48.0 through I48.21 should be used going forward.

Does I48.91 carry risk-adjustment weight?

Yes. I48.91 maps to HCC 238, Specified Heart Arrhythmias, in CMS-HCC v28. Under CMS-HCC v24 it mapped to HCC 96, Specified Heart Arrhythmias.

A patient is admitted with atrial fibrillation with rapid ventricular response. Should the tachycardia be coded as the principal diagnosis?

No. When atrial fibrillation is documented as the cause of the rapid ventricular response, the atrial fibrillation code, I48.91, or the specific subtype where documented, is the principal condition. R00.0, tachycardia unspecified, should not be assigned as the principal diagnosis in that scenario, because the tachycardia is a manifestation of the arrhythmia and is subsumed by the atrial fibrillation code.

How should atrial flutter be coded?

Atrial flutter is separately classified and should never be coded to I48.91. Assign I48.3 for typical flutter, type I, I48.4 for atypical flutter, type II, or I48.92 when flutter is confirmed but the type is not documented.

Can I48.91 be assigned when atrial fibrillation is only suspected?

It depends on the setting. In outpatient records, ICD-10-CM Guidelines Section IV require coding to the highest degree of certainty. When atrial fibrillation is documented as possible or suspected, code the presenting sign or symptom, such as R00.0 tachycardia, R00.1 bradycardia, or R00.8 other abnormalities of heart beat, until the diagnosis is confirmed. In inpatient records, Section II.H permits a diagnosis documented as possible or probable at the time of discharge to be coded as though confirmed.

What else should be coded alongside I48.91?

Long-term anticoagulant use is reported with Z79.01 when documented, under Guideline I.C.21.c.3. No tabular note at I48, I48.9 or I48.91 calls for another code. A documented coexisting condition, such as thyrotoxicosis or a cerebral infarction, is coded in its own right under Section III or Section IV.J of the guidelines.

Related codes

Codes in the same family as I48.91. Only codes with a published page render as links.

Sibling codes in the I48 family

I48.0: Paroxysmal atrial fibrillation.
I48.11: Longstanding persistent atrial fibrillation.
I48.19: Other persistent atrial fibrillation.
I48.20: Chronic atrial fibrillation, unspecified.
I48.21: Permanent atrial fibrillation.
I48.3: Typical atrial flutter (type I).
I48.4: Atypical atrial flutter (type II).
I48.91: Unspecified atrial fibrillation.
I48.92: Unspecified atrial flutter.

Additional codes to consider

Codes outside the I48 family considered alongside I48.91.

Companion: code alongside when documented

Z79.01: Long term (current) use of anticoagulants, when the provider documents long-term anticoagulant therapy (Guideline I.C.21.c.3).

Consider instead

I48.0: Paroxysmal atrial fibrillation. Use when the paroxysmal subtype is documented and confirmed.
I48.19: Other persistent atrial fibrillation. Use when the episode lasted more than 7 days or required cardioversion but is not longstanding.
I48.11: Longstanding persistent atrial fibrillation. Use when the rhythm is documented as continuous for more than 12 months.
I48.21: Permanent atrial fibrillation. Use when the arrhythmia is accepted and cardioversion is no longer planned.
I48.3, I48.4 or I48.92: Atrial flutter. Use when flutter rather than fibrillation is documented.
I47.10: Supraventricular tachycardia, unspecified. Use when supraventricular tachycardia is documented without atrial fibrillation and no more specific type is stated.
I49.01: Ventricular fibrillation. Use when the documented arrhythmia is ventricular rather than atrial.
I49.5: Sick sinus syndrome. A distinct arrhythmia that also maps to HCC 238 in v28.
R00.0: Tachycardia, unspecified. Use for tachycardia as an isolated symptom, but not when atrial fibrillation is the documented cause.
Z86.79: Personal history of other diseases of the circulatory system. Use when prior atrial fibrillation is documented as resolved.

Sources

All claims on this page derive from government and professional-standards sources.

National Center for Health Statistics. (2026). International classification of diseases, tenth revision, clinical modification (ICD-10-CM) FY2027 files. https://www.cdc.gov/nchs/icd/icd-10-cm/files.html
Centers for Medicare & Medicaid Services and National Center for Health Statistics. (2026). ICD-10-CM official guidelines for coding and reporting FY2027. https://ftp.cdc.gov/pub/Health_Statistics/NCHS/Publications/ICD10CM/2027/ICD-10-CM-October-1-2026-FY27-Guidelines.pdf
Centers for Medicare & Medicaid Services. (2025). 2026 model software/ICD-10 mappings. https://www.cms.gov/medicare/payment/medicare-advantage-rates-statistics/risk-adjustment/2026-model-software-icd-10-mappings
Centers for Medicare & Medicaid Services. (2026). MS-DRG classifications and software, version 44. https://www.cms.gov/medicare/payment/prospective-payment-systems/acute-inpatient-pps/ms-drg-classifications-and-software