ICD-10-CM Code B35.1: Tinea Unguium (Onychomycosis)

ICD-10-CM code B35.1 is used to document tinea unguium, commonly known as onychomycosis. This condition is a fungal infection of the nails, most often affecting toenails but occasionally fingernails. Accurate coding of B35.1 is essential for proper documentation, treatment planning, and reimbursement.

This guide explains what B35.1 represents, when to use it, related ICD-10 codes, coding guidelines, and common pitfalls, making it a practical reference for healthcare providers and medical coders.

What Is ICD-10-CM Code B35.1?

B35.1 identifies a confirmed diagnosis of tinea unguium (onychomycosis)—a fungal infection involving the nail plate, nail bed, or both.

Use this code when:

• The provider documents onychomycosis or tinea unguium
• The diagnosis is confirmed, not suspected
• The condition impacts patient care or treatment decisions

Important: If the diagnosis is only suspected and not confirmed, report signs and symptoms codes instead of B35.1.

Detailed Description of Tinea Unguium (B35.1)

Tinea unguium is caused by dermatophytes, yeasts, or non-dermatophyte molds. The infection leads to progressive nail changes that can impair function and quality of life.

If left untreated, onychomycosis may:

• Cause pain and difficulty walking or using hands
• Lead to nail deformity
• Increase the risk of secondary bacterial infections, especially in patients with diabetes or compromised immunity

Because the condition is often chronic, accurate diagnosis and documentation are critical for ongoing care.

Common Symptoms Associated With B35.1

Patients with tinea unguium may present with one or more of the following symptoms:

• Nail discoloration (white, yellow, or brown)
• Thickened nail plate
• Brittle, crumbling, or ragged nail edges
• Distorted or misshapen nails
• Separation of the nail from the nail bed (onycholysis)
• Foul odor from the affected nail

Symptoms may worsen over time, reinforcing the need for early diagnosis and correct coding.

Related and Similar ICD-10-CM Codes

When documenting fungal infections, specificity is essential. Related ICD-10 codes include:

ICD-10 Code	Description
B35.0	Tinea barbae and tinea capitis
B35.2	Tinea manuum
B35.3	Tinea pedis
B35.4	Tinea corporis
B35.5	Tinea imbricata
B35.6	Tinea cruris
B35.8	Other specified dermatophytoses
B35.9	Dermatophytosis, unspecified

Select B35.1 only when the infection specifically involves the nails.

Appropriate Use of B35.1 for Billing

ICD-10-CM code B35.1 should be reported when:

• The provider confirms tinea unguium or onychomycosis
• The diagnosis is clearly documented in the assessment or plan of care
• Treatment or monitoring is provided for the condition

Accurate use supports medical necessity, reduces claim denials, and improves data integrity.

Official Coding Guidelines and Instructional Notes for B35.1

Follow these important coding guidelines when assigning B35.1:

Inclusion Terms

B35.1 includes:

• Dermatophytic onychia
• Dermatophytosis of nail
• Onychomycosis
• Ringworm of nails

Category Notes

• Category B35 (Dermatophytosis) includes favus infections caused by Epidermophyton, Microsporum, and Trichophyton species
• Tinea infections are classified under B35, except those reported in B36.-

Documentation Guidance

• Do not assign B35.1 for suspected or rule-out diagnoses
• Ensure the fungal infection is clearly identified as affecting the nails

Common Coding Pitfalls With B35.1

Avoid these frequent errors to ensure accurate billing:

• Incomplete documentation of the confirmed diagnosis
• Coding B35.1 for suspected onychomycosis
• Using unspecified dermatophytosis codes (B35.9) when nail involvement is documented
• Failing to stay current with ICD-10-CM updates

Consistent documentation review and coder-provider communication help prevent these issues.

Key Resources for Accurate B35.1 Coding

Use authoritative resources to maintain compliance and accuracy:

• ICD-10-CM Official Guidelines for Coding and Reporting
• American Academy of Dermatology (AAD) clinical resources
• Centers for Medicare & Medicaid Services (CMS) coding updates
• Clinical Documentation Improvement (CDI) programs

These tools support accurate diagnosis capture and coding consistency.

Frequently Asked Questions

What does ICD-10-CM code B35.1 represent?

B35.1 represents tinea unguium (onychomycosis), a fungal infection of the nails.

Can B35.1 be used for suspected onychomycosis?

No. B35.1 should only be used for confirmed diagnoses. Suspected cases require symptom-based coding.

Is B35.1 a billable ICD-10 code?

Yes. B35.1 is a billable and specific ICD-10-CM code.

Does B35.1 apply to fingernail infections?

Yes. While more common in toenails, B35.1 also applies to fungal fingernail infections.

When should B35.9 be used instead of B35.1?

Use B35.9 only when documentation does not specify the site of dermatophytosis.

Conclusion:

ICD-10-CM code B35.1 is essential for documenting tinea unguium, a common but often chronic fungal nail infection. Accurate coding ensures:

• Proper clinical documentation
• Appropriate reimbursement
• Reduced claim denials
• Better continuity of patient care

By understanding the symptoms, related codes, instructional notes, and common pitfalls, healthcare professionals can confidently apply B35.1.