ICD-10 Code R91.1: Solitary Pulmonary Nodule

Key Takeaways

R91.1 is a billable ICD-10-CM code for a solitary pulmonary nodule, valid for FY2026 (October 1, 2025 through September 30, 2026).

Use R91.1 only when imaging identifies a single lung nodule without a confirmed malignancy or other established diagnosis.

R91.8 covers multiple or other nonspecific lung field findings; selecting the wrong code between R91.1 and R91.8 is a common denial trigger.

Pabau’s claims management software helps pulmonology and radiology practices reduce coding errors and streamline R91.1 documentation workflows.

ICD-10 Code R91.1: Definition and Clinical Description

ICD-10 Code R91.1 identifies a solitary pulmonary nodule, one of the most common incidental findings in chest imaging. Clinically, a solitary pulmonary nodule (SPN) is defined as a single round or oval lesion measuring less than 3 centimeters in diameter, completely surrounded by lung parenchyma and not associated with lymphadenopathy, atelectasis, or pleural effusion. The finding is most often discovered on a chest X-ray (CXR) or low-dose CT (LDCT) scan ordered for an unrelated reason.

R91.1 sits within ICD-10-CM Chapter 18 (R00-R99), which covers symptoms, signs, and abnormal clinical and laboratory findings not classified elsewhere. More specifically, it falls under subcategory R90-R94: Abnormal findings on diagnostic imaging and in function studies, without diagnosis. The parent category R91 groups all abnormal imaging findings of the lung. According to the Centers for Medicare and Medicaid Services (CMS), R91.1 is classified as a billable and specific code, meaning it carries sufficient clinical detail to be submitted on a HIPAA-covered claim transaction without a more specific underlying diagnosis.

Two included terms appear in the official tabular list under R91.1:

• Coin lesion lung – a historical radiologic descriptor for a well-circumscribed round opacity, still encountered in older records and referral letters
• Solitary pulmonary nodule, subsegmental branch of the bronchial tree – a more precise anatomical descriptor when imaging localizes the nodule to a subsegmental bronchial territory

Both terms map directly to R91.1 and may be used as documentation anchors when a radiologist or pulmonologist reports either descriptor in their findings. On the historical side, R91.1 carries a direct General Equivalence Mapping from ICD-9-CM code 793.11 (Solitary pulmonary nodule), making conversion straightforward for practices migrating legacy records or comparing historical billing data.

Billable Status and Reimbursement Guidance

R91.1 holds active billable status for the current fiscal year. According to ICD List, the code is valid for HIPAA-covered claim submissions from October 1, 2025 through September 30, 2026. No additional specificity is required; the code stands alone as a diagnosis when imaging supports a solitary nodule without a confirmed histological or etiological diagnosis.

Reimbursement depends on the payer’s local coverage determination (LCD) and the procedure codes submitted alongside R91.1. The code itself does not trigger automatic reimbursement; it supports medical necessity for the associated imaging or evaluation service. Practices billing low-dose CT lung cancer screening should note that Medicare’s National Coverage Determination for lung cancer screening (LDCT) applies to a defined high-risk population and is coded separately from an incidental nodule finding. An incidental SPN discovered on a non-screening scan is correctly coded with R91.1, not with the lung cancer screening benefit codes.

For practices managing high volumes of pulmonary imaging claims, Pabau’s claims management software supports accurate code capture and helps reduce denial rates linked to incorrect diagnosis sequencing or missing specificity.

R91.1 vs R91.8: Selecting the Correct Code

The R91 category contains two billable codes, and choosing between them is a common source of claim errors in pulmonary billing. The distinction is straightforward once the clinical picture is clear.

• R91.1 – Solitary pulmonary nodule: Use when imaging identifies exactly one discrete nodule in the lung, with no confirmed underlying diagnosis to code instead.
• R91.8 – Other nonspecific abnormal finding of lung field: Use when imaging reveals multiple pulmonary nodules, diffuse opacities, or other nonspecific lung field abnormalities that do not meet the criteria for a single solitary nodule.

The key clinical trigger for R91.1 is singularity. If a radiology report describes two or more distinct nodules, R91.8 is the appropriate code, not R91.1. If the report identifies one nodule but also describes a confirmed diagnosis (for example, a biopsy-proven adenocarcinoma), the confirmed malignancy code takes precedence over R91.1. The Chapter 18 coding guideline is explicit: codes from R00-R99 are used when no more specific diagnosis has been established by the end of an encounter.

Practices managing lung nodule follow-up programs benefit from systematic code review at each encounter. When a previously documented SPN is re-imaged for surveillance, coders should assess whether the nodule’s status has changed. If a biopsy or PET scan now supports a specific diagnosis, the R91.1 code should be replaced accordingly. Pabau’s client record tools allow clinicians to track imaging findings longitudinally, supporting accurate coding at each follow-up visit.

Documentation Requirements for ICD-10 Code R91.1

Claim denials for R91.1 most often trace back to insufficient documentation rather than incorrect code selection. The code can stand on imaging evidence alone; no biopsy or histological confirmation is required. What the documentation must support is the absence of a more specific established diagnosis at the time of the encounter.

Required Documentation Elements

A complete clinical record for an R91.1 encounter should contain the following elements to withstand payer review:

• Imaging report: Radiology report explicitly describing a solitary, round or oval pulmonary opacity less than 3 cm in diameter, without evidence of mediastinal involvement or pleural effusion
• Nodule characteristics: Size in millimeters, location (lobe and segment), density (solid, part-solid, ground-glass), and margin characteristics (smooth, lobulated, spiculated)
• Clinical context: Whether the nodule was incidental (found during imaging for another indication) or symptomatic (the primary reason for imaging)
• Absence statement: A notation that no definitive diagnosis has been established and that further workup is pending or that the finding warrants surveillance
• Provider attestation: The ordering or treating clinician’s interpretation or acknowledgment of the radiology finding in the clinical note

Lung-RADS and Fleischner Society Context

Many radiology departments now use the ACR Lung-RADS scoring system or the Fleischner Society guidelines to categorize pulmonary nodules and recommend follow-up intervals. These structured reporting frameworks do not replace the ICD-10 code but they provide the clinical rationale that supports R91.1 when a nodule is categorized as Lung-RADS 2 (benign appearance, routine surveillance) or Lung-RADS 3 (probably benign, short-interval follow-up). A Lung-RADS 4 or 5 finding may prompt biopsy or PET scan, at which point the diagnosis could shift away from R91.1 to a malignancy code if confirmed.

Documenting the Lung-RADS category or Fleischner guideline recommendation within the clinical note strengthens the medical necessity argument for follow-up imaging billed under R91.1. Pabau’s Echo AI clinical documentation tools can assist in capturing structured findings from radiology reports directly into the patient record, reducing transcription errors and supporting coding accuracy at the point of care.

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CPT Codes Commonly Used with R91.1

R91.1 functions as the supporting diagnosis for several procedure codes used in the evaluation and surveillance of solitary pulmonary nodules. The CPT code selected depends on the clinical setting, the type of imaging, and whether the encounter involves evaluation and management services or a procedure alone.

When a pulmonologist conducts an office visit specifically to evaluate an SPN finding from a prior imaging report, the E&M code (99213-99215) is supported by R91.1 as the primary diagnosis. The complexity of medical decision-making (MDM) in that encounter is generally moderate to high, given the risk implications of an uncharacterized lung lesion. According to the AAPC’s ICD-10 coding resources, R91.1 is consistently paired with chest CT codes in radiology billing and with E&M codes in pulmonology and primary care encounters.

For practices using integrated EMR and billing platforms, linking the diagnosis code to the imaging order at the point of scheduling reduces downstream coding lag. Pabau’s digital forms and pre-visit documentation workflows can capture the clinical indication for imaging before the patient arrives, keeping the R91.1 diagnosis tied correctly to the procedure from the outset.

Related ICD-10 Codes and Coding Transitions

Understanding the codes adjacent to R91.1 helps coders navigate encounters where the clinical picture is evolving or where multiple findings are present on the same imaging study.

• R91.8 – Other nonspecific abnormal finding of lung field: Use for multiple nodules or nonspecific opacities, as discussed above
• R04.2 – Hemoptysis: May accompany an SPN encounter as a secondary code if the patient presents with coughing blood alongside the nodule finding
• Z12.2 – Encounter for screening for malignant neoplasm of respiratory organs: Distinct from R91.1; used for the lung cancer screening encounter itself, not for an incidental finding
• C34.xx – Malignant neoplasm of bronchus and lung: Replaces R91.1 once a pulmonary malignancy is confirmed; never code both R91.1 and a confirmed lung malignancy for the same nodule
• D14.3 – Benign neoplasm of bronchus and lung: Appropriate after a benign lesion such as a hamartoma is confirmed pathologically
• Z80.1 – Family history of malignant neoplasm of trachea, bronchus and lung: Commonly coded as a secondary code alongside R91.1 when family history informs the clinical decision to pursue surveillance

The ICD List’s free ICD-10-CM lookup provides current code descriptions and includes the Chapter 18 hierarchical context for R91.1, which is useful for coders verifying the correct subcategory before submission. For practices managing dermatology or skin clinic patients who also present with incidental pulmonary findings, the Pabau skin clinic software supports multi-specialty documentation in a single patient record.

The WHO classifies R91.1 under the international ICD-10 framework, and the U.S. clinical modification (ICD-10-CM) preserves this classification with added specificity. Coders in practices that also serve international patients or operate across jurisdictions should note that the WHO’s ICD-10 browser reflects the parent code structure, while CMS governs the U.S.-specific tabular list updates annually.

Coding Workflow Best Practices for R91.1

Consistent, accurate R91.1 capture requires a structured workflow across the radiology, ordering clinician, and coding teams. Incidental findings are particularly prone to coding gaps because the nodule is not the primary reason for the encounter and can be overlooked during the billing abstraction process.

Workflow Steps for Accurate Capture

1. Radiology report flagging: Radiologists should use structured reporting templates that flag solitary nodules explicitly, including size, location, and Lung-RADS category, to ensure coders have unambiguous documentation
2. Ordering clinician review: The ordering provider should acknowledge the incidental finding in a follow-up note or addendum, confirming that the SPN has been communicated to the patient and that a management plan is in place
3. Diagnosis code assignment: Coders assign R91.1 at the time of the imaging encounter and at each subsequent surveillance encounter until a more specific diagnosis is confirmed
4. Surveillance tracking: A separate tracking mechanism (such as a recall workflow or task flag in the practice management system) ensures the follow-up imaging is ordered and coded correctly at the appropriate interval
5. Transition coding: When pathology or advanced imaging changes the diagnosis, the coder updates the primary diagnosis code and retires R91.1 for that encounter series

Practices managing high volumes of incidental lung findings benefit from recall automation. Pabau’s automated workflow tools allow teams to build recall triggers tied to specific diagnosis codes, ensuring that patients coded with R91.1 receive timely follow-up scheduling without manual intervention. This reduces both clinical risk and the billing exposure that comes from missed surveillance encounters.

For multi-location practices, consistent R91.1 documentation standards across sites reduce inter-coder variability. Pabau’s multi-location management features support standardized templates and coding rules across all practice locations from a single platform.

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Conclusion

Incidental lung findings create a specific coding challenge: the diagnosis is real and clinically significant, but it sits in a holding pattern until workup confirms or rules out a more serious condition. Getting R91.1 right from the first encounter protects both revenue and patient safety, because missed or miscoded findings can disrupt the surveillance workflow that clinical guidelines require.

Pabau’s automated recall workflows and claims management tools give practices the infrastructure to track R91.1 encounters systematically, capture accurate documentation at each follow-up visit, and transition codes cleanly when a confirmed diagnosis arrives. To see how Pabau handles complex diagnostic coding workflows, book a demo with the team.

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