ICD code S91.255A – Open bite of left lesser toe(s) with damage to nail

Found our content helpful?

Key takeaways

ICD-10 Code S91.255A covers an open bite of the left lesser toe(s) with damage to nail, initial encounter. It is billable and valid for HIPAA-covered transactions under the 2026 edition.

The 7th character ‘A’ designates an initial encounter, meaning active treatment. Use ‘D’ for subsequent encounters and ‘S’ for sequela. Choosing the wrong character is the most common error on this code.

Documentation must state laterality, wound mechanism, nail involvement, and encounter type. Missing any one of the four risks a claim denial.

Practice management software like Pabau attaches ICD-10 codes to the treatment record itself, so the code reaches the claim without a copy-paste step.

ICD-10 Code S91.255A: Definition and clinical description

S91.255A describes an open bite of the left lesser toe(s) with damage to nail, at the initial encounter. The CDC/NCHS ICD-10-CM lookup tool confirms it is billable and specific, so it can go on a claim without further specification.

An open bite in ICD-10-CM means a wound caused by a bite, animal or human, that breaks the skin and leaves it open. A puncture wound codes to the S91.235A family instead, and a laceration to S91.215A. The “lesser toe(s)” qualifier covers toes 2 through 5, and the great toe has its own code family.

Nail damage must be present and documented for this code to apply. Without it, the correct code is S91.155A, an open bite of the left lesser toe(s) without damage to nail.

S91.255A 7th character extensions: A, D, and S

S91.255 is the base code, and it is non-billable on its own. A 7th character must be appended to every claim submission. Each character signals a distinct clinical scenario. Using the wrong one is the most common reason this code family gets denied.

When to use S91.255A: Initial encounter criteria

Use S91.255A while the patient is still receiving active treatment for the wound. “Initial encounter” does not mean the patient’s first visit to any provider. Active treatment covers debridement, wound repair, antibiotic initiation, and initial assessment. A patient seen by their primary care physician three days after an ER visit still takes the A suffix.

• Active wound cleaning or debridement: Use A
• Suture or closure at first repair visit: Use A
• Follow-up after sutures placed, wound healing normally: Switch to D
• Nail deformity developing months after original injury: Use S
• Multiple providers treating the same active injury: Each may use A independently

Code hierarchy: Where S91.255A sits in ICD-10-CM

Reading the ICD-10 classification hierarchy prevents the most common structural error in this family. Coders pick a parent category code, which is not billable, instead of the fully specified child. S91.255A sits at the end of a six-level chain, and every level above it is non-billable.

The same logic runs through every injury chapter in the coder’s ICD-10-CM reference. The further down the tree you go, the more specific the code, and only the deepest level is billable. Submitting S91.255 or S91.25 without a 7th character triggers an automatic rejection for HIPAA-covered transactions.

Approximate synonyms and index entries for S91.255A

Clinical documentation rarely uses the exact ICD-10-CM phrase. The index entries below are the terms that map to S91.255A in the ICD-10-CM Alphabetic Index. Any of these descriptions in a chart note should route the coder to this code.

• Open bite, left lesser toe(s), with nail damage
• Bite wound, left lesser toe(s), with toenail damage, initial encounter
• Animal bite, left lesser toe, with nail injury, initial encounter
• Human bite, left lesser toe(s), nail involved, initial encounter
• Left toe open bite wound with toenail damage
• Bite, open, left toes (2-5), nail damage, initial encounter

In ICD-10-CM, “open bite” means a bite wound that remains open. A closed bite injury, such as bruising with no break in the skin, codes elsewhere. The note must say the wound is open, not only that a bite occurred.

Documentation requirements for S91.255A

Every element of this code must be traceable in the medical record. Missing a single qualifier leaves the code and the note saying different things, which is what an audit looks for. Run this checklist against the encounter note before you code.

• Laterality confirmed as left: The note must state “left toe” or “left lesser toe”. “Toe wound” alone is not enough.
• Wound mechanism documented as open bite: Specify whether the bite was animal or human, and note that the skin is broken.
• Nail damage explicitly stated: “Nail involvement”, “damage to toenail”, or “toenail lacerated”. Implied damage does not count.
• Toes identified as lesser (2-5): Name the specific toe or toes where you can. If the great toe is involved, a different code family applies.
• Encounter type matches the 7th character: Active treatment takes A, routine healing takes D, and a late effect takes S.
• Provider type and setting documented: The initial treating provider should be identifiable in the record.

Laterality and encounter type are the two fields most often missing from emergency department and urgent care notes. Two templated prompts in the clinical note fix most of that: “Laterality: ___” and “Nail involved: Y/N”. Both take a second to complete and save the coder a query later.

S91.255A coding guidelines and common errors

The AAPC ICD-10-CM code guidelines and the CMS official coding guidelines flag several recurring mistakes in the S91 family. Each one below has a documentation fix behind it.

• Using the base code S91.255 without a 7th character: This is non-billable. Every claim must carry S91.255A, S91.255D, or S91.255S, never the base code alone.
• Applying “A” to a routine follow-up: If the patient returns for a dressing change and the wound is healing normally, the correct code is S91.255D. Continuing to use “A” misrepresents the encounter type.
• Wrong laterality: S91.255A is left side only. A right lesser toe open bite with nail damage codes to S91.254A. Laterality is one of the most frequently mis-documented fields in injury coding.
• Confusing open bite with puncture or laceration: An animal bite can also produce a puncture. If the wound is puncture-type, with a small entry point and a deep track, code the puncture family instead. S91.235A covers a left lesser toe puncture with nail damage and no foreign body.
• Failing to code the external cause: S91.255A identifies the injury but not the mechanism. Add a secondary code from the W50-W64 range, such as W50.3 for a human bite, to satisfy payer external-cause requirements.

Correcting a code after a denial costs more than getting it right the first time. The claim has to be reworked, resubmitted, and tracked again. Every error above is catchable with the documentation checklist, before the claim leaves the practice.

Related ICD-10-CM codes to know

S91.255A sits in a grid of adjacent codes that each differ from it by one field. Change the laterality, the nail damage status, or the wound mechanism, and a different code applies.

The same laterality and encounter-type logic runs across the whole S00-T88 injury block. A coder who can read one of these code families can read the rest.

CPT codes commonly used with S91.255A

S91.255A is a diagnosis code. A claim also needs a paired CPT code describing what was done. Simple closure of a small bite wound reports under 12001, and the table below lists the other CPT codes commonly submitted alongside S91.255A. Verify payer medical-necessity requirements before submission, because coverage policies vary.

For a bite wound, the CPT code chosen has to match the wound severity described in the note. Debridement, repair, and nail avulsion are not interchangeable, and a mismatch between the note and the procedure billed is a denial waiting to happen.

ICD-9-CM crosswalk for S91.255A

Practices auditing legacy claims, or working with data from before October 1, 2015, may need to cross-reference S91.255A against the predecessor ICD-9-CM system. The CMS General Equivalence Mappings (GEMs) provide the official crosswalk. ICD-9-CM open wound codes carried no laterality and no nail-damage qualifier, so a one-to-one mapping does not exist.

The crosswalk is approximate because ICD-9-CM code 893.1 covered open toe wounds without specifying laterality, nail involvement, or bite mechanism. For historical billing reconciliation, record the mapping basis and note the difference in specificity. It matters most for payers still processing legacy claims from encounters before 2015.

How Pabau handles S91.255A

A lookup tool gives you the code. Getting that code onto a clean claim is a separate job. Transcription slips, wrong 7th characters, and missing external-cause codes all creep in when someone copies a code by hand into a separate billing system.

Practice management software like Pabau closes that step. Pabau’s claims software for coders lets clinicians and coders search ICD-10-CM codes inside the clinical record. The code attaches to the encounter, drives the invoice, and flows into the claim with no copy-paste step.

For S91.255A, laterality, nail status, and encounter type all have to agree between the note and the claim. Every manual transfer you remove is one fewer place for them to disagree.

Claims submitted through Pabau’s Claim.MD clearinghouse integration are validated against payer edits before they transmit.

A wrong laterality, a non-billable parent code, or a missing external-cause pairing gets flagged on screen, not in a remittance report three weeks later. For a practice coding wound care at volume, that check removes most of the rework before it starts.

Reduce coding errors at the point of care

Pabau connects ICD-10-CM codes directly to treatment records and invoices, so your team spends less time correcting denials and more time delivering care. See how integrated claims management works for your practice.

Book a demo

Conclusion

Four fields decide this code, and the note has to carry all four. Most denials on S91.255A trace back to the 7th character, then laterality, then nail damage the clinician saw but never wrote down. Fix the note template once and the code follows.

If your coders are still retyping codes from a lookup tool into a billing system, that is the step to close first. Book a demo to see how Pabau carries an ICD-10 code from the treatment note to the submitted claim.

Continue your research

Need help with claim submission and clearinghouse integration? Medical claims clearinghouse guide covers how electronic claim submission works and what to look for in a clearinghouse partner.

Want to understand denial patterns for injury codes? Denial codes in medical billing explains the most common CARC and RARC codes your team will encounter on rejected wound care claims.

Looking for guidance on revenue cycle compliance? Medical billing compliance outlines the documentation and submission standards that protect against audits on injury coding.

Frequently asked questions