Onychomycosis ICD-10 Coding Guide

Onychomycosis, commonly known as nail fungus, affects millions of people worldwide. This condition causes nails to become thickened, discolored, and sometimes painful, often requiring medical diagnosis and treatment to prevent complications. 

Accurate ICD-10 coding for onychomycosis and related nail infections is essential for healthcare practitioners, medical coders, and billers. Precise coding facilitates effective communication between providers and insurers, ensures proper insurance reimbursement, and helps track the prevalence and trends of these infections. 

This guide reviews the relevant ICD-10 codes for onychomycosis and other fungal nail conditions, assisting professionals in navigating the documentation and billing processes more efficiently.

Onychomycosis ICD-10: What It Means

ICD-10, the International Classification of Diseases, 10th Revision, is an international coding system used by healthcare professionals to categorize and describe illnesses, symptoms, and procedures. In this system, onychomycosis—a fungal infection of the nail—receives specific codes that indicate the infection’s location and severity.

The primary ICD-10 code for onychomycosis is:

  • B35.1 — tinea unguium (Onychomycosis)

Providers use the ICD-10 codes mentioned above when diagnosing fungal nail infections caused by dermatophytes, yeasts, or non-dermatophyte molds. Unless additional documentation specifies otherwise, these codes apply to both fingernail and toenail infections. Correct coding of onychomycosis ensures clear communication among providers and helps prevent claim denials caused by vague or incomplete diagnoses.

Definition and Scope

Onychomycosis is classified under the ICD-10 code B35.1. This code identifies dermatophytic infections of the nail. Other fungal nail infections caused by yeast or non-dermatophyte molds may be coded differently, but B35.1 is the primary code used for onychomycosis caused by dermatophytes. The coding allows for documentation of infections affecting one or multiple nails.

ICD-10 classifies onychomycosis as a superficial mycosis (codes B35–B49). Medical coders specifically identify it with code B35.1, which refers to tinea unguium—the clinical term for fungal nail infection.

This code applies to all age groups and is relevant in outpatient and inpatient settings.

It covers:

  • Toenail and finger nail infection
  • Chronic or recurrent fungal nail infections
  • Various fungal organisms cause nail fungus.

Understanding and using the correct ICD-10 codes helps identify diagnoses and bill the fungal nail services accurately.  

Code Specifics

Coders use the specific code B35.1 for onychomycosis. 

B35.1 – Tinea unguium

Medical professionals use this code to certify a fungal nail infection, regardless of whether it affects the fingernails or toenails. The broader category, B35 (Dermatophytosis), encompasses various fungal skin and nail diseases.

Key Points About B35.1:

  • It does not distinguish between toenails and fingernails. The documents, not the ICD-10 code, should contain any necessary site-specific information.
  • Providers use code B35.1 only when a clinical assessment confirms the diagnosis and illness becomes a medical necessity/ or billable.
  • Onychomycosis is coded as ICD-10 B35.1, which is specific to fungal nail infections; non-fungal conditions like psoriasis or trauma must not use this code to avoid inaccurate records and billing errors.

Common Coding Combinations

Depending on the situation, providers may list onychomycosis alongside related codes in a patient’s chart.

For example:

E11.42 – Type 2 diabetes mellitus with diabetic polyneuropathy:
This code applies to patients with type 2 diabetes complicated by diabetic nerve damage. Since diabetic neuropathy can increase the risk of foot infections and delayed wound healing, onychomycosis may be a contributing factor in these patients, potentially worsening their foot care complications.

L60.0 – Ingrowing nail:
This code is used for ingrown nails. Onychomycosis can sometimes contribute to developing or worsening ingrown nails by thickening or deforming the nail plate, which increases pressure on the surrounding skin.

Z20.828 – Contact with and (suspected) exposure to other communicable diseases:
In certain healthcare or care settings, this code is used when a patient is known to have or is suspected of being exposed to communicable diseases, including fungal infections like onychomycosis. It is not used for confirmed diagnosis but for exposure or risk documentation.

Ensure proper documentation supporting additional codes alongside B35.1 to show the clinical process.

Avoiding Common Coding Mistakes with Onychomycosis ICD-10

Accurate coding of onychomycosis may seem very easy, but mistakes can occur without proper documentation or a clear understanding of the ICD-10 guidelines. These errors can lead to claim denials, compliance issues, or miscommunication among healthcare providers.

As a result, practices and physicians may face a revenue loss. 

Here are some common coding mistakes to keep in mind and how to avoid them:

Use of Incorrect Codes:

A common mistake is coding onychomycosis without a verified diagnosis. Onychomycosis should only be coded with B35.1 when the doctor has verified and documented the condition, whether clinically or through lab tests.

Confusing Onychomycosis with Other Nail Disorders

Not all nail abnormalities result from fungal infections. Conditions like psoriasis, eczema, trauma, or lichen planus can mimic fungal nail infections. Assigning code B35.1 to a non-fungal nail condition creates a coding error that may lead to incorrect treatment or insurance claim denials.

Missing Combrid Conditions

Many patients with onychomycosis also have underlying conditions that affect nail health or healing, especially patients with diabetes, circulatory disorders, or immunosuppression.  Failing to code these comorbidities can impact reimbursement and clinical management.

Neglecting Document Severity and Laterality

While code B35.1 does not specify which nails are affected, clear documentation should identify the affected nails.

Failing to include this information doesn’t change the code but reduces clinical clarity, affecting future care decisions.

While documentation should provide

  • Which nails are involved (toenails, fingernails, both)
  • Number of nails affected
  • Severity and duration

Ignoring Follow-Up Coding

Onychomycosis requires long-term treatment and monitoring. Repeatedly using the exact code without reflecting ongoing care, changes in condition, or treatment response may lead to coding gaps.​

Confusing B35.1 with L60.x

A coding error is mistaking B35.1 (Tinea unguium) for codes in the L60.x series. These codes correspond to completely different diseases, even though the symptoms, like thickening, discoloration, or separation of the nails, may appear similar.

Here is the breakdown of B35.1– Tinea unguium (Onychomycosis)

  • Code B35.1 is used only when a fungal infection is confirmed.
  • Overs nail infections caused by dermatophytes, yeasts, or molds
  • Applies to both fingernails and toenails
  • The identification of affected nails should be based on clinical evaluation or laboratory confirmation.

L60.x – Nail Disorders (Non-Fungal)

Incorrectly using L60.x codes to report fungal nail infections, such as onychomycosis, leads to inaccurate documentation and billing. These codes describe nail disorders unrelated to infectious causes, so they fail to reflect the fungal etiology of onychomycosis. When providers classify symptoms like nail thickening or discoloration under non-fungal diagnoses, they risk misrepresenting the clinical condition. This misclassification compromises coding accuracy and increases the likelihood of denied claims or compliance issues.

Why the Confusion Happens

Nail disorders often share similar appearances, particularly in their early stages. Without lab testing or detailed medical notes, coders may become confused about L60.x for convenience or choose B35.1 based on an assumption.

How to Avoid Common Coding Mistakes with Onychomycosis ICD-10

Appropriate documentation and coding are critical for complete reimbursement, compliance, and patient care. Here is how to avoid common ICD-10 coding mistakes when dealing with onychomycosis:

Confirm the Diagnosis before Coding

  • ICD-10 code B35.1 (Tinea unguium) should only be used if a fungal infection has been confirmed by laboratory testing or clinical assessment. 
  • Do not use assumed coding based only on nail appearance. 

Differentiate Fungal from Non-Fungal Nail Disorders

  • Onychomycosis should not be confused with lichen planus, psoriasis, trauma, or eczema.
  • Use B35.1 only for fungal infections, and code L60.x is for non-fungal nail disorders. 

Code All Relevant Comorbidities

  • Document and code related conditions such as diabetes and peripheral vascular disease. Or immunosuppression that may impact treatment or healing.
  • These comorbidities can affect reimbursement and patient outcomes.

Document Nail Involvement Clearly

Specify the following while documenting

  • Whether toenails, fingernails, or both are involved
  • The total number of nails infected
  • Seriousness and duration of the condition

While documenting B35.1 does not require laterality, detailed documentation supports better care and claim justification.

Update Coding for Follow-up Visits

  • Avoid using the same code more than once.
  • To gauge correct and current coding, consider any changes in the diagnosis, therapeutic response, or ongoing treatment.

Avoid Code Confusion: B35.1 vs. L60.x

    • B35.1 is used for confirmed fungal infection
    • L60.x is used for non-infectious nail disorders
  • Avoid assigning assumption-based codes; use diagnostic evidence to support your code selection.

Missing Anatomical Details

When coding onychomycosis using ICD-10, it is crucial to note that the codes typically do not require a detailed anatomical specification of which nails are affected, specifically finger or toenails. The ICD-10 codes for onychomycosis, such as B35.1 for tinea unguium, do not differentiate based on the exact anatomical location of the nail infection.

However, missing anatomical details in documentation can lead to uncertainty in medical records. For example, understanding whether the infection affects the fingernails or toenails, treatment choices, and aftercare may be influenced. Therefore, Healthcare providers should record the precise location of the infection, even if the ICD-10 code does not require this level of detail. Partial anatomical information can create challenges during claims review or audits. Insurance companies may request details if the clinical notes do not adequately explain the condition’s severity.

To avoid such issues:

  • Ensure providers document the nail location—whether fingernail or toenail—when diagnosing onychomycosis.
  • Ensure the coding reflects the documented diagnosis without predicting missing notes.
  • Use appropriate codes for complications or secondary conditions, even if the anatomical location is unspecified.

Documentation Gap

Precise ICD-10 coding for onychomycosis depends intensely on complete and accurate clinical documentation. Documentation gaps can lead to incorrect or incomplete coding, which affects billing, patient care, and data reporting.

Each record should fulfill the following criteria:

  • Lab test results ( KOH preparation and culture results)
  • Intention of treatment (e.g., removal, oral antifungal).
  • Clinical instructions (e.g, nail thickness and discolouration).

Missing these in documentation can lead to rejections or penalties.

Related ICD-10 Codes for Onychomycosis ICD-10

When coding onychomycosis, it is crucial to understand the primary codes and related ICD-codes that cover different types of fungal nail infections and other nail disorders. Correct coding helps ensure accurate diagnosis representation and facilitates billing and clinical tracking.

ICD Codes for Nail Disorders

Here is a clear and precise table for ICD codes for nail disorders

ICD-10 Codes Description Use
L60.0 Ingrowing nail Use when nail curvature causes skin penetration, no fungal cause confirmed
L60.2 Onychogryphosis Thickened, claw-like nails typically appear in elderly or diabetic patients.
L60.3 Nail dystrophy General nail deformity not caused by a fungal infection.
L62 Nail disorders in systemic disease Use when the nail issue is secondary to conditions like psoriasis or lupus.

Skin and Soft Tissue Infections

Onychomycosis can be associated with or complicated by skin and soft tissue infections (SSTIs), particularly when the nail infection results in breaks in the skin or when patients have underlying conditions like diabetes. Accurate documentation and coding of these infections are crucial to capture the clinical situation and guide treatment.

According to ICD data, common skin and soft tissue infections related to nail fungus include:

ICD-10 Codes Description Condition
L03 Cellulitis Caused by bacteria. Presents with redness, swelling, warmth, and pain.
L03.0 Paronychia Caused by bacteria or fungi, it can be acute and coexist with onychomycosis
L02 Abscess An infection near the nail can cause it, requiring antibiotics.

ICD-10 Codes to Bill:

Here are the ICD-10 codes to use for reimbursement, as L03.0 is not used for the reimbursement process, and multiple codes below it contain a greater level of detail. 

B35.1 – Onychomycosis (primary condition)

L03.022 – Cellulitis of the left toe

L02.412 – Cutaneous abscess of left toe

Avoid using L03.0 or L02; directly select the most specific subcodes. 

Conclusion

Correct ICD-10 coding for onychomycosis and related nail conditions is crucial in effective patient care, accurate billing, and reliable healthcare reporting. Nail fungus can cause discomfort and serious complications if left untreated, so precise coding helps ensure patients receive appropriate care. Using the correct codes also streamlines communication between healthcare providers and payers. Healthcare teams can avoid claim denials and improve collaboration among clinicians, coders, and payers by addressing common pitfalls- such as missing anatomical details and incomplete documentation. This guide will help you identify and apply the correct ICD-10 codes for onychomycosis and related nail treatments.

Need help coding onychomycosis and related nail conditions accurately with ICD-10?

Utah Billing Services specializes in accurate medical coding and billing solutions, ensuring your dermatology claims, including nail fungal cases, are submitted accurately.

Let the experts handle the billing process so you can focus on patient care!