Onychomycosis is a fungal infection of the fingernails or toenails that causes nails to become thick, brittle, discolored, or painful. It commonly affects toenails and can interfere with walking, footwear, and daily activities.
Onychomycosis ICD 10 code B35.1 is used to bill fungal infections of the fingernails or toenails. Insurance pays only when the infection causes pain, problems walking, or other health risks. Nail color or appearance changes alone are not covered.
| Element | Requirement |
|---|---|
| Primary ICD-10 | B35.1 (Onychomycosis – fungal nail infection) |
| Billable | Yes |
| Medical Necessity Required | Yes |
| Cosmetic Coverage | No |
| Common CPT | 11720, 11721 |
| Medicare Risk Modifiers | Q7, Q8, Q9 |
The most important billing rule for onychomycosis is that B35.1 must be supported by symptoms or risk factors. In 2026, using the code alone is no longer enough to pass payer review.
In 2026, simply using the code isn't enough. Payers now use advanced AI to scan for "cosmetic" red flags. This guide ensures your claims meet the latest medical necessity and specificity rules.
🛑 2026 Regulatory Alert: The "Specificity" Mandate
As of the 2026 fiscal year, CMS and commercial payers have implemented stricter filters. Claims for B35.1 are now being auto-denied if they lack "laterality" (specifying which side of the body) or fail to reflect new diabetes status codes.
While B35.1 does not include laterality, payers now require laterality-specific symptom codes (such as toe pain) to establish medical necessity for treatment.
Key 2026 Code Updates for Podiatry
| Condition | New/Updated 2026 Code | Billing Impact |
|---|---|---|
| Type 2 DM in Remission | E11.A | Use for patients with HbA1c <6.5% for 3+ months without medications |
| Right Toe Pain | M79.674 | Specificity is mandatory; using unspecified codes triggers denials |
| Left Toe Pain | M79.675 | Specificity for the left side is required to avoid payer rejection |
| Deep Foot Ulcers | L97.515 / L97.525 | New codes for ulcers reaching muscle layer without necrosis; accurate documentation is crucial |
🚀 2026 Pro Tip: Avoid T-Modifier Errors
Do not add T-modifiers (T1–T9) to CPT 11721. CPT 11721 is a "global" code for the feet. Adding specific toe modifiers to a global code will cause an automatic denial in the new 2026 clearinghouse systems.
What is the Main ICD-10 Code for Onychomycosis?
For most cases, you will use B35.1. This primary Onychomycosis ICD 10 code covers fungal infections of both fingernails and toenails caused by Dermatophytes (skin-eating fungi).
| Condition | ICD-10 Variation | Clinical Use |
|---|---|---|
| Nail fungus | B35.1 | Main Onychomycosis ICD-10 code |
| Dermatophyte skin infection | B35.3 | Use for Athlete's foot (Tinea Pedis) |
| Mycosis nail fungus | B37.2 | Use specifically for yeast/Candida infections |
| Hand fungus | B35.2 | Use for fungal infections of the hand |
🚀 2026 Pro Tip: The "Pain-Function" Documentation Hack
Payers deny "yellow nails" but pay for "functional deficits." Your notes must state how the fungus hurts the patient.
- Weak Note: "Patient has thick, yellow nails."
- Strong Note: "Nails are 4mm thick, causing 7/10 pain while walking. Patient cannot wear standard shoes."
How to Prove Medical Necessity (CMS Standards)
The CMS will not pay to make nails look pretty. They only pay if the fungus causes a real health problem. You must document these signs clearly:
- Pain: Does the patient hurt when they wear shoes or walk?
- Difficulty walking: Does the thick nail cause a limp?
- Infection risk: Is there redness, swelling, or pus near the nail?
- Testing: Did a KOH prep or PAS stain prove the fungus is present?
🚀 2026 Pro Tip: The 60-Day Medicare Frequency Rule
Medicare typically covers nail trimming once every 60 days. If a patient returns at day 45 due to a new infection or sudden pain, you must add Modifier 25 to the office visit. You must also explain why the visit was urgent.
Which Comorbidities Support Your Claims?
For patients with systemic (body-wide) issues, nail fungus is a major risk. Listing these "comorbidities" shows the insurance company why the treatment is a medical "must."
| Underlying Health Issue | ICD-10 Code | Connection to Toenail Fungus |
|---|---|---|
| Type 2 Diabetes | E11.9 / E11.A | High risk of foot ulcers and secondary infection |
| Poor Circulation (PVD) | I73.9 | Slower healing and increased risk of limb complications |
| Foot Pain | M79.674 / M79.675 | Supports that the fungal infection is causing physical discomfort |
| Nail Deformity | L60.2 | Shows nail is physically damaged and clinically significant |
🚀 2026 Pro Tip: Deploy the "Q" Modifiers
For Medicare, the Onychomycosis ICD 10 code alone is rarely enough. You must prove the patient is "At-Risk" by adding a modifier:
- Q7: One pulse in the foot is missing.
- Q8: Two smaller findings (like thick nails AND skin color changes).
- Q9: One major and two minor findings (like a missing pulse plus swelling).
Linking Your CPT Codes to Onychomycosis
You must match your CPT code (what you did) to your ICD-10 code (why you did it).
- CPT 11720: Trimming 1 to 5 nails.
- CPT 11721: Trimming 6 or more nails.
Important:
If you prescribe strong pills like Terbinafine, 2026 guidelines suggest having a lab report (KOH/Culture) on file from the last 12 months. This proves the medicine is necessary.
Summary: Winning Your Onychomycosis ICD 10 Claims
Success with Onychomycosis ICD 10 billing comes down to being specific. When you use the right code, provide proof of medical need, and follow the 2026 laterality rules, you get paid faster. Don't let your hard work go unpaid because of a simple coding error.
Frequently Asked Questions
Can I bill B35.1 without a lab report in 2026?
Yes, but claims are stronger when laboratory confirmation is available. Many payers in 2026 expect a recent KOH prep or fungal culture when oral antifungals are prescribed to support medical necessity.
What is the best secondary code to support B35.1?
For painful fungal nail infections, B35.1 is commonly supported by laterality-specific symptom codes such as M79.674 (Right toe pain) or M79.675 (Left toe pain) to satisfy 2026 payer edits.
How do I bill for skin infections, like Athlete's foot, that occur at the same time?
If a patient has both nail fungus and a skin infection, you must report them as two separate issues. Use Onychomycosis ICD-10 code B35.1 for the nails and code B35.3 for Tinea Pedis (Athlete's foot).
In 2026, insurance companies use automated software to "cross-check" your claims. They want to make sure that a skin cream you prescribed isn't accidentally linked to the nail code. If the diagnosis and the treatment site don't match perfectly, it could trigger a denial for an "incorrect site" error.
Is lab confirmation mandatory to use the Onychomycosis ICD-10 code in 2026?
Technically, you can still bill B35.1 based on what you see during a physical exam. However, 2026 Medicare guidelines strongly prefer that you have lab proof—like a KOH prep or a fungal culture—before you start a patient on expensive oral drugs like Terbinafine.
Insurance auditors look for specific phrases in your notes. Writing "confirmed via PAS stain" next to your Onychomycosis ICD-10 diagnosis is considered the "gold standard." It provides a solid defense if the insurance company ever audits your office to see if the treatment was actually necessary.
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