ICD code T25.792S – Third-degree corrosion of the left ankle and foot, sequela

Key takeaways

T25.792S is the billable ICD-10-CM code for third-degree chemical corrosion at multiple sites of the left ankle and foot.

The 7th character S means the visit treats a late effect, such as scarring or contracture, rather than the acute burn.

Corrosion is chemical and a burn is thermal, so a heat injury takes a T25.3x code instead of T25.7x.

The encounter note has to say the current condition followed from the original corrosion, or payers deny the claim.

Pair the claim with an external cause code carrying the same S character, such as X46.XXXS.

ICD-10 code T25.792S is billable, and it already names the left side

ICD-10 code T25.792S is a valid, billable ICD-10-CM diagnosis code for fiscal year 2026, effective October 1, 2025. Five facts sit inside those characters.

The cause was chemical rather than thermal, and the depth reached third degree. More than one site on the left ankle and foot was involved. The visit itself treats what that injury left behind, not the injury.

Billable status means T25.792S can go on a claim by itself, with no more specific code available below it. Laterality is already settled, so the side is not what puts this claim at risk. The 7th character and the documentation behind it are.

Reach for T25.799S instead only when the record genuinely does not say which side was burned. Picking the unspecified sibling while the chart names the left side is a specificity error, and auditors look for it.

Every character in T25.792S tells the payer something different

Read the code in order and the usual miscodes in this family stop happening. Five elements do the work.

• Corrosion: a chemical agent caused the injury, such as an acid, an alkali, or another caustic substance. Never swap it with a thermal burn.
• Third degree: full-thickness destruction of skin and subcutaneous tissue, sometimes deeper. The physician has to state third-degree severity, since coders cannot infer it from wound appearance.
• Multiple sites: more than one anatomical site of the ankle or foot was corroded. If only the ankle was involved, the T25.71x codes apply; the T25.72x codes cover the foot alone.
• Left ankle and foot: the 6th character 2 names the left side. Right takes a 1, and an undocumented side takes a 9.
• Sequela, 7th character S: this encounter treats a consequence of the earlier corrosion, not the acute injury.

Per the CMS ICD-10-CM coding guidelines, chemicals cause corrosions while heat, electricity, radiation, and friction cause burns. The two run in separate code families, so a thermal injury never takes a T25.7x code.

Three choices stand between the category and the billable code, and the chart below walks through all three.

The 7th character decides whether the claim survives

The 7th character in T25.792S is S, and it stands for sequela. Choosing the wrong one denies more claims in this family than any other error. Here is how the three options divide up.

A worked example makes the split concrete. A patient spills drain cleaner across the left foot and ankle in March, and the wounds close by June. She returns in November with a contracture that limits dorsiflexion. March is T25.792A, the June dressing changes are T25.792D, and November is T25.792S.

The common mistake is staying on D once the healing phase has ended. Practices that keep billing T25.792D for late-effect visits get caught on post-payment audits, because the note describes a residual condition rather than a healing wound.

Corrosion is chemical, a burn is thermal, and the code families do not overlap

Burn codes and corrosion codes sit in different halves of category T25. Swapping them is a common root cause of rejection on injury claims, because the diagnosis and the external cause code stop agreeing.

The CDC/NCHS ICD-10-CM web tool carries the official tabular list, where the two sub-blocks are set out separately. Whenever the operative note or the ED note uses chemical-agent language, corrosion codes apply, whatever lay term the patient or family used.

Where T25.792S sits in the ICD-10-CM hierarchy

T25.792S is the bottom rung of a five-level ladder. Knowing the ladder is how a coder moves sideways when the site or the side changes.

Climbing the ladder is never the fix on a claim. Payers want the bottom rung, so T25.79 or T25 on its own will not pass. The only legitimate moves are sideways, into the siblings below.

Three siblings look like T25.792S, and one character separates them

Subcategory T25.79 holds three sequela codes. Only the 6th character changes between them, which is why they get mixed up so often.

Coders also cross-reference the other phases of the same code, T25.792A and T25.792D. If the injury turns out to be thermal rather than chemical, the sequela code moves into the T25.3x range instead.

T25.399S covers that injury when no side is documented. Browsing the ICD-10-CM code index beats guessing once the site moves off the ankle and foot.

Four documentation points keep a sequela claim alive

Section I.C.19 of the ICD-10-CM Official Guidelines sets what a sequela claim has to show. Miss any one of these four and the claim comes back, either denied or asking for records.

• The original corrosion is on record. The acute episode, coded with 7th character A, has to appear in the patient’s history, and the current note should point at it.
• The causal link is explicit. The provider has to write that the presenting condition came from the prior corrosion. Wording such as “secondary to prior chemical burn” or “post-corrosion scarring” does it, while implied linkage does not.
• The current condition is named. Scarring, contracture, hypersensitivity, a chronic open wound, and functional limitation all qualify. “Follow-up” or “wound check” on its own does not.
• Third-degree severity is confirmed. The original record has to document third degree. Assigning T25.792S when the acute note says second degree is a severity error.

Practices that keep this work in-house lean on software for billing teams to hold that thread. The original injury encounter and every later sequela visit then stay linked on one record.

Strong denial management starts by naming which of the four points was missing on each returned claim, then repairing the intake step that dropped it.

External cause codes travel with the corrosion code

Section I.C.20 asks for an external cause code alongside every injury code. For T25.792S that means naming the chemical, plus the place and the activity where the original injury happened. Some payers treat these as optional and others reject without them, so the safe policy is to treat them as required.

The right range depends on the agent, and two codes cover most chemical exposures. Place and activity then sit in separate chapters of their own.

• X46.XXXA/D/S: accidental poisoning by and exposure to organic solvents and halogenated hydrocarbons and their vapors, with the 7th character matching the primary code.
• X49.XXXA/D/S: accidental poisoning by and exposure to other and unspecified chemicals and noxious substances.
• Y92.-: the place of occurrence, such as a home garage or a manufacturing plant, when the history records it.
• Y93.-: the activity code, meaning what the patient was doing when the corrosion happened.

On a sequela claim the external cause code carries S as well, so X46.XXXS pairs with T25.792S. Mixing a sequela primary with an initial-encounter external cause is a mismatch that some payers catch automatically and return unprocessed. The remittance advice on that claim usually cites a remark code pointing straight at the 7th character.

T25.792S has not changed, but check it every October

T25.792S has been in the ICD-10-CM tabular list since the United States adopted the code set. Recent fiscal years look like this.

No code in this family moved across the last three updates. Even so, confirm the status in the CDC/NCHS tool before each October 1, since a retired code is a silent denial.

The AAPC ICD-10-CM code lookup carries the same fiscal year history, with effective and termination dates. Build that check into the billing calendar, so no claim goes out under a superseded edition.

How Pabau keeps a sequela claim tied to the original injury

The hard part of a sequela claim is rarely the code. It is proving, months later, that the contracture in front of you grew out of a corrosion recorded last spring. That proof usually sits spread across a chart, a scanned ED note, and a billing system that never saw either.

Practice management software like Pabau keeps the original encounter, the wound photos, and every follow-up note on one patient record. A coder working the sequela visit can open the acute corrosion note without leaving the chart. The causal sentence can then quote the first clinician’s own wording.

Claims leave from that same record and go electronically to Claim.MD, our US clearinghouse partner. Rejections and remittance advice come back to the same place, so whoever repairs the documentation is reading the documentation.

Keep sequela claims tied to the original injury

Pabau keeps the acute corrosion note, the follow-up visits, and the claim on one patient record. Claims go out electronically through Claim.MD, so your team can support the causal link without hunting across separate systems.

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Conclusion

Sequela corrosion claims fail for three reasons. The 7th character is wrong, or the note never links the current problem to the original burn. The third is an external cause code that points at the wrong agent. Each one is repaired at the documentation source, not on the claim form.

T25.792S already names the left side, so laterality is one argument you do not have to win. Spend the effort on the causal sentence in the encounter note instead. That sentence is what a payer reads when it decides whether this late effect is yours to bill.

Book a demo to see how Pabau keeps the original corrosion note and the sequela claim on the same patient record.

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