I48.91 ICD-10-CM Code: Unspecified atrial fibrillation
HCC Buddy Code Card
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FY 2026 Apr update / Diseases of the circulatory system (I00-I99) / Other forms of heart disease (I30-I5A)
I48.91
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceUnspecified atrial fibrillation
An irregular heartbeat in the upper chambers of the heart that is not further specified or classified into a more specific category.
Buddy Insight
Unspecified atrial fibrillation represents a significant risk adjustment condition due to its association with stroke, heart failure, and increased mortality.
CMS-HCC V28
MappedHCC 238
RAF 0.299
CMS-HCC V24
MappedHCC 96
RAF 0.268
ACA/HHS
00
RAF 0
ESRD/PACE
MappedHCC 96
RAF 0.0
RXHCC
MappedHCC 193
RAF 0.0
Code Trumping
Basket needed
Code Book Path
Inclusion Terms
OfficialICD-10-CM does not list inclusion terms for I48.91 in this effective period.
Excludes 2
OfficialICD-10-CM does not list Excludes 2 notes for I48.91 in this effective period.
Related Child Codes
Includes
OfficialICD-10-CM does not list Includes notes for I48.91 in this effective period.
Excludes 1
OfficialICD-10-CM does not list Excludes 1 notes for I48.91 in this effective period.
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for I48.91 in this effective period.
Use Additional
OfficialICD-10-CM does not list Use Additional Code instructions for I48.91 in this effective period.
Code Also
OfficialICD-10-CM does not list Code Also instructions for I48.91 in this effective period.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.
Is I48.91 an HCC code?
Yes. I48.91 maps to Specified Heart Arrhythmias under the CMS-HCC V28 risk adjustment model (and Specified Heart Arrhythmias under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
Work I48.91 in the Code Book — tabular path, V28 RAF, and MEAT checklist →
MEAT Criteria for I48.91
For I48.91to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.
- MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
- EEvaluate: test results, medication response, or physical findings reviewed by the provider
- AAssess: explicit mention in the assessment or plan with acknowledgment of status
- TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed I48.91 during that encounter, not just copy-forwarded from a problem list.
Coder workflow notes
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What This Code Means
I48.91 is the ICD-10-CM diagnosis code for unspecified atrial fibrillation. An irregular heartbeat in the upper chambers of the heart that is not further specified or classified into a more specific category. I48.91 sits in the ICD-10-CM chapter for diseases of the circulatory system (i00-i99), within the section covering other forms of heart disease (i30-i5a).
Under the CMS-HCC V28 risk adjustment model, I48.91 maps to Specified Heart Arrhythmias (HCC 238) with a community, non-dual, aged base RAF weight of 0.299. Under the older V24 model, I48.91 mapped to the same category but with a base RAF weight of 0.268, V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
This is an unspecified code; use only when atrial fibrillation is documented but the type, duration, and persistence cannot be determined. Because I48.91 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for I48.91 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is an unspecified code; use only when atrial fibrillation is documented but the type, duration, and persistence cannot be determined
- •Query the provider for clarification on whether the atrial fibrillation is paroxysmal, persistent, or permanent
Clinical Significance
Unspecified atrial fibrillation represents a significant risk adjustment condition due to its association with stroke, heart failure, and increased mortality. It requires ongoing management including anticoagulation therapy, rate/rhythm control, and monitoring for thromboembolic events. Capturing this diagnosis is critical for ensuring appropriate resource allocation for these high-risk patients.
Documentation Requirements
- ✓Provider must explicitly document 'atrial fibrillation' as an active diagnosis assessed during the encounter
- ✓Current treatment plan including anticoagulation status, rate or rhythm control medications
- ✓Type should be specified (paroxysmal, persistent, long-standing persistent, or permanent) — if not specified, this unspecified code is used
- ✓Associated symptoms such as palpitations, shortness of breath, dizziness, or fatigue
- ✓CHA2DS2-VASc score or stroke risk documentation
- ✓Any monitoring such as electrocardiogram findings or Holter monitor results
Commonly Confused Codes
- •I48.0: Paroxysmal atrial fibrillation: use when atrial fibrillation is documented as paroxysmal (intermittent, self-terminating episodes)
- •I48.11: Longstanding persistent atrial fibrillation: use when documented as lasting more than 12 months continuously
- •I48.19: Other persistent atrial fibrillation: use when documented as persistent but not longstanding
- •I48.20: Chronic atrial fibrillation, unspecified: use when documented as chronic without further specificity
- •I48.92: Unspecified atrial flutter: flutter has a different mechanism (rapid organized circuit) vs fibrillation (chaotic electrical activity)
- •R00.1: Bradycardia: a slow heart rate, not an arrhythmia classification