ICD-10 Code T23.421S: Corrosion of right finger sequela

Key takeaways

T23.421S is the ICD-10-CM code for corrosion of unspecified degree of single right finger (nail) except thumb, sequela.

It codes one finger, not the whole right hand. A whole-hand corrosion sequela takes T23.401S instead.

In T23.4 the fifth character carries the site and the sixth character carries laterality. The 2 means single finger except thumb.

The 7th character S means the acute chemical injury has resolved and the visit treats what it left behind.

The tabular entry says code first (T51-T65) to identify the chemical and the intent. There is no X40-X49 block in ICD-10-CM.

Y92 codes the place of occurrence and Y93 codes the activity. Both are reported once, at the initial encounter.

T23.421S is the ICD-10-CM code for corrosion of unspecified degree of single right finger (nail) except thumb, sequela. It codes one finger. It does not code the whole right hand, and it does not include the thumb.

The 2 in the middle of T23.421S is a site character, not a laterality character. A coder who reads it as laterality will reach for this code on a whole-hand injury that belongs to T23.401S.

Corrosion sequela visits usually land months or years after the chemical injury itself. The patient comes back for scar release, a nail deformity, or hand therapy. The coder still has to pin down the site and the encounter phase.

This reference covers the full descriptor and the character-position map for T23.4. It also covers the sequela rules, the sibling codes, the external cause chain, and the documentation that supports the claim.

ICD-10 Code T23.421S: definition and billable status

T23.421S is a billable, valid-for-submission ICD-10-CM diagnosis code for corrosion of unspecified degree of single right finger (nail) except thumb, sequela. It is effective for FY2025 (October 1, 2024) and FY2026 (October 1, 2025).

The code sits in Chapter 19, which covers injury, poisoning and certain other consequences of external causes (S00-T88). Report it when a patient is treated for a lasting effect of an earlier chemical injury to one right finger other than the thumb.

Code details at a glance

The table below summarizes the attributes coders check before they submit T23.421S. Note the code-first row, because the corrosion code is never complete on its own.

Full code description and meaning of T23.421S

Every element of T23.421S carries a specific clinical and coding meaning. Misreading any one of them produces the wrong code and a likely denial. Per the CMS ICD-10-CM official code listings, the description breaks down as follows.

• Corrosion: the injury came from a chemical agent rather than heat. That is what separates the T23 corrosion codes from the T23 burn codes. Chemical agents include acids, alkalis, caustic cleaning products, and corrosive organic compounds.
• Unspecified degree: the original record did not state whether the corrosion was first, second, or third degree. When the degree was documented, use T23.521S, T23.621S, or T23.721S for the same finger site instead.
• Single right finger (nail) except thumb: the injury involved one finger on the right hand, and the code covers that finger’s nail as well. The thumb is excluded. Left side is T23.422S and an undocumented side is T23.429S.
• Sequela (7th character S): the patient is being treated for a lasting consequence of the chemical injury, not for the injury itself.

How the fifth and sixth characters work in T23.4

This is the single most common source of error on T23.4 claims. The fifth character carries the anatomical site, and laterality arrives only at the sixth character. Reading T23.421S from left to right gives corrosion of unspecified degree of wrist and hand, then single finger except thumb, then right, then sequela.

The panel below maps that reading position by position, next to the whole-hand alternative.

Several approximate clinical synonyms route to this code. Examples include late effect of chemical burn of a right finger, and sequela of corrosive injury to the right index finger. Both index here only when the degree was never documented.

Understanding the 7th character ‘S’ in T23.421S

The 7th character decides which encounter phase you are billing, so payers watch it closely. Section I.C.19.a of the ICD-10-CM Official Guidelines for Coding and Reporting sets out three options for injury codes. A, D, and S map to different covered services, which is why the wrong character reads as a mismatch on the claim.

One practical test settles most cases. Do not use S if the wound is still open, still healing, or still under active care. Use S only once the corrosion has resolved and the visit treats what the injury left behind.

Clinical picture of a finger corrosion sequela

Corrosive agents keep damaging tissue until they are neutralized or diluted, so a single finger can end up with deep injury from a small exposure. Fingers also carry very little soft tissue over the tendon sheath and the nail bed. The residual findings that bring these patients back are therefore mechanical and sensory, not just cosmetic.

• Hypertrophic or contracted scar over the volar or dorsal surface of the finger, coded as L90.5 for scar conditions and fibrosis of skin.
• Flexion contracture that limits the finger’s range of motion and weakens grip and pinch.
• Nail dystrophy or nail loss where the corrosive reached the nail bed, coded as L60.3.
• Altered or reduced sensation, cold intolerance, and neuropathic pain from digital nerve injury.
• Pigment change and thin, fragile skin that breaks down again under repeated use.

Corrosion versus burn: why T23 splits the two

T23 covers both mechanisms, and the fourth character tells them apart. Thermal, electrical, friction, and radiation injuries are burns and take T23.0 through T23.3. Chemical injuries are corrosions and take T23.4 through T23.7.

The thermal equivalent of T23.421S is T23.021S, which covers a sequela of a thermal burn to the same finger site. Check the mechanism in the original note before you pick the fourth character.

ICD-10-CM code hierarchy and classification

T23.421S sits inside a clean parent-child chain under Chapter 19. Following that chain is the fastest way to confirm you are on the right site before you add laterality and the 7th character.

Sibling and related codes for T23.421S

The codes below are the ones most often confused with T23.421S. The first group varies the encounter phase and the side. The second group varies the site, and the third varies the documented degree. The AAPC ICD-10-CM code lookup carries the full T23 range for cross-referencing.

External cause coding requirements for a finger corrosion

The corrosion code is never complete on its own. The tabular entry at T23.4 carries a code-first instruction for the chemical agent, and Chapter 20 supplies the rest of the chain.

Section I.C.20 of the Official Guidelines requires external cause codes whenever the cause of the injury is documented. Missing companion codes are a recurring source of rejections, which our guide to denial codes works through.

One correction worth flagging: there is no X40-X49 block in ICD-10-CM. The X chapter jumps from X39 straight to X50, so X46 and X49 are not valid codes and never were. Any instruction to report them for a corrosive substance is wrong. Chemical identity and intent live in T51-T65.

• Chemical and intent (T51-T65), coded first: the tabular note at T23.4 reads code first (T51-T65) to identify chemical and intent. Corrosive acids use T54.2X-, and corrosive alkalis use T54.3X-. The intent character is 1 for accidental, 2 for intentional self-harm, 3 for assault, and 4 for undetermined.
• Match the 7th character on the chemical code: an accidental acid exposure is T54.2X1A at the initial encounter and T54.2X1S on a sequela visit. Keep it aligned with the corrosion code.
• Place of occurrence (Y92): Y92 codes say where the injury happened, such as Y92.010 for the kitchen of a single-family private house. Occupational exposures fall in the Y92.6- industrial and construction range.
• Activity (Y93): Y93 codes say what the patient was doing, for example Y93.89 for activity, other specified, or Y93.9 when the activity is unspecified. Y93 is activity, not place of occurrence.
• External cause status (Y99): Y99.0 marks a civilian activity done for income or pay, which matters on work-related corrosion claims.
• Extent of body surface (T32): category T32 reports how much surface the corrosion covered. A single finger falls under T32.0, less than 10 percent of body surface.

Y92, Y93, and Y99 are each reported once, at the initial encounter for the injury. They are not repeated on later visits, so a clean sequela claim usually carries the residual condition, the T51-T65 chemical code, and T23.421S.

Practice management software like Pabau keeps the note, the codes your coder assigns, and the invoice on one patient record. The original encounter’s detail is then still there when the sequela visit arrives.

Pabau’s claims management software submits and tracks claims through our Claim.MD integration. Your team sees where each sequela claim sits without logging into a separate portal. Coding decisions stay with your coder. The software carries them to the payer and brings the response back.

Sequela coding guidelines for T23.421S

Sequela encounters follow their own sequencing rules. Section I.B.10 covers sequelae in general and Section I.C.19.d covers burns and corrosions specifically. The CDC ICD-10-CM official tool is the place to confirm a descriptor before you submit.

1. Code the residual condition first. The condition treated at this visit is the first-listed diagnosis, for example L90.5 for a scar or L60.3 for nail dystrophy.
2. Follow the tabular code-first note. The T51-T65 chemical code is sequenced ahead of the corrosion code, so an accidental acid sequela reads T54.2X1S then T23.421S.
3. Add T23.421S to identify the original injury. It tells the payer which injury produced the residual condition being treated.
4. Do not repeat the Y92, Y93, and Y99 codes. Place, activity, and status were reported at the initial encounter and are not carried forward.
5. Keep A, D, and S apart for the same injury. One injury is either in active care, healing, or resolved. Guideline I.C.19.d does allow a current corrosion with A or D alongside a healed corrosion with S, because separate injuries heal at different rates.

The documentation problems that block clean claim submission on these encounters are consistent. Coders see notes that record a prior chemical burn without naming the residual condition. They see the whole hand documented when the treated site was one finger. They also see records with no chemical agent anywhere in the chart, which leaves the code-first instruction unmet.

Documentation requirements for T23.421S

T23.421S is only defensible when the chart supports each element of the descriptor. The list below is what an auditor looks for on a corrosion sequela encounter. Most of it comes from the original injury record rather than today’s note, which is why continuity of the chart matters so much.

• The specific finger and the side, stated as the right index, middle, ring, or little finger.
• A statement that the thumb was not involved, or a different code if it was.
• Confirmation that the corrosion has healed and active treatment has ended.
• The residual condition being treated at this visit, named rather than implied.
• A causal link between that residual condition and the earlier chemical injury.
• The chemical agent and the intent from the original encounter, to support the T51-T65 code.
• The documented degree if one exists, which moves the claim to T23.521S, T23.621S, or T23.721S.

Common coding errors and how to avoid them

Almost every T23.421S problem traces back to one of the six errors below. The first two are the same mistake seen from different angles, and together they account for most of the misuse of this code.

ICD-9-CM crosswalk for T23.421S

Legacy systems and retrospective research databases still reference ICD-9-CM. Per the General Equivalence Mappings published by the Centers for Medicare and Medicaid Services, T23.421S maps to the code below. The mapping is approximate in one direction only, because ICD-9-CM had no separate late-effect code for corrosions and no finger-level detail.

Effective dates and annual code updates

T23.421S has no reported revisions for FY2025 or FY2026. It has been valid and billable since ICD-10-CM was adopted, and it remains active with no change to its descriptor.

Practices that handle chemical injury aftercare should still confirm the descriptor each October. The fiscal year update can change wording without retiring a code, so an annual check belongs in the coding calendar.

Dermatology, plastics, and hand therapy caseloads carry most of these encounters. Keeping a current code list in the system that produces the claim reduces the chance of submitting a descriptor that no longer matches the chart.

How Pabau supports corrosion sequela claims

A sequela claim depends on a record written months or years earlier. Practices that keep the acute encounter in one system, the operative note in another, and the billing detail in a spreadsheet lose that trail. The coder then guesses at the site, picks the safer unspecified code, and the claim comes back.

Pabau keeps the clinical note, the diagnosis codes your coder assigns, the invoice, and the payer response on the same patient record. When the patient returns for scar release two years later, the original chemical agent and the finger involved are one click away.

Claims go out and come back through our Claim.MD integration, so your team follows each one to remittance without leaving the record. Coding decisions stay with your coder throughout.

The outcome is fewer unspecified codes, fewer rework hours, and less waiting between the visit and the payment. Every Pabau subscription includes the full feature set, so claims tooling is never an upgrade you buy later.

Keep ICD-10 sequela claims on one record with Pabau

Pabau holds the clinical note, the diagnosis codes your coder assigns, the invoice, and the payer response together. Claims go out and come back through our Claim.MD integration, so your team follows each one to remittance.

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Conclusion

The site character is what makes or breaks a T23.4 sequela claim. When the note describes the whole right hand, T23.401S is the correct choice. When it names one right finger other than the thumb, T23.421S is defensible and specific.

Around that code, the claim needs the residual condition sequenced first. It also needs a T51-T65 chemical code with a matching 7th character, and no leftover X46 or X49 entries. Get those three right and corrosion sequela claims stop bouncing.

The harder part is the chart rather than the code. A sequela visit two years out is only codeable if the original encounter’s detail survived, which makes this a records problem first. Book a demo to see how Pabau keeps that detail on one patient record.

Continue your research

Need another corrosion sequela reference? ICD-10 code T22.462S walks through the same 7th character rules for the scapular region.

Coding a burn rather than a corrosion? ICD-10 code T31.61 covers the body surface extent codes that pair with burn claims.

Working through repeat rejections? Denial management in healthcare explains how missing companion codes turn into denials and how to work them back.

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