ICD code T31.90 – Burns involving 90% or more of body surface with 0% to 9% third

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Key takeaways

T31.90 covers burns across 90% or more of total body surface area (TBSA) with 0% to 9% third degree burns

Up to 9% documented full-thickness involvement still codes to T31.90, and it is also the default when third degree extent is not documented

The fifth character, not the fourth, carries the third degree band: T31.91 starts at 10% and each step adds 10 points

Payers routinely deny T31.90 claims when the TBSA estimation method (Rule of Nines or Lund-Browder chart) is absent from the clinical record

Practice management software like Pabau supports accurate burn diagnosis code documentation and electronic claim submission via Claim.MD

ICD-10 code T31.90: Quick reference details

The table below summarizes the code as published in the CDC/NCHS ICD-10-CM web tool.

Approximate synonyms for T31.90

These clinical synonyms and lay descriptions all map to T31.90 and may appear in physician documentation or query workflows.

• Burns of 90% or more total body surface area, depth unspecified
• Burns involving 90% or more of body surface with up to 9% third degree involvement
• Massive burns, 90%+ TBSA, mainly partial thickness with a minor full-thickness component
• Extensive burn injury, 90-100% body surface area, third degree extent under 10%
• Major thermal burn, 90%+ TBSA, third degree extent not stated

Understanding the T31 category: Burns classified by body surface area

T31 is the parent category for burns classified by the extent of body surface involved. That sets it apart from the T20-T25 series, which classifies burns by anatomical site. Coders use T31 codes when the clinical record documents TBSA as a percentage.

Coding a burn at this extent requires the full T31 hierarchy. The table below lists every active T31 subcategory with its TBSA range.

Each subcategory from T31.1 upward takes a fifth character for third degree extent. T31.0 does not, because a burn under 10% of body surface cannot carry a 10% third degree band.

How TBSA is measured: Rule of Nines and Lund-Browder chart

Payers expect the clinical record to name the estimation method used to arrive at the TBSA percentage. Two tools are standard.

Rule of Nines

The Rule of Nines divides adult body surface into regions, each representing 9% (or a multiple of 9%) of total surface area. It is the most widely used method for initial triage and ICD-10 burn coding in adult patients.

Lund-Browder chart

For pediatric patients, the Lund-Browder chart is the preferred alternative because it adjusts TBSA percentages for age-related differences in body proportion. Children’s heads represent a proportionally larger surface area than adults’, making the Rule of Nines less accurate. Clinicians treating burn patients under 15 years old should document which tool was used.

T31.90 vs. other T31.9X codes: The fifth-character distinction

All T31.9X codes describe burns covering 90% or more of body surface. The fifth character records how much of that area is third degree, in 10-point bands. T31.90 is the lowest band, covering 0% to 9%, and it absorbs cases where depth extent is never stated.

The ladder below shows where each band starts. Every band comes from the CDC/NCHS FY 2026 code descriptions.

Coding implication: A documented third degree component does not by itself rule out T31.90. Query the attending only when the full-thickness estimate could cross 10%, because that is the line between T31.90 and T31.91.

Documentation requirements for T31.90

The medical record must support each element of the T31.90 code. Missing documentation is the primary driver of denial on high-acuity burn claims. Capture all of the following for every burn admission.

• TBSA percentage: The exact percentage (90% or higher) must appear in the clinical note, operative report, or attending physician’s assessment. “Extensive burns” without a numeric TBSA estimate is insufficient.
• Estimation method: State whether the Rule of Nines or the Lund-Browder chart was used. Payers that audit high-acuity burn claims increasingly flag cases where no estimation method is identified.
• Burn depth per region: Document the depth classification for each involved body region. What supports T31.90 over T31.91-T31.99 is a third degree estimate below 10%, or no third degree estimate at all.
• Third degree extent as a percentage: A numeric full-thickness estimate is what picks the fifth character. Without it, the coder defaults to T31.90 and any higher-severity band is lost.
• Body regions affected: List each anatomical region involved. This enables accurate pairing with site-specific T20-T25 codes when applicable.
• Mechanism of injury: Thermal, chemical, electrical, or radiation. This feeds the external cause code selection.
• Present on Admission (POA) indicator: Required for all inpatient claims. T31.90 is a condition-specific POA requirement under CMS inpatient rules.

A templated burn admission note that captures depth, TBSA, and body region cuts the number of coder queries raised after discharge.

Coding guidelines for T31.90

The ICD-10-CM Official Guidelines for Coding and Reporting (Section I.C.19) govern burn code sequencing and use. Key rules for T31.90 are summarized below.

Sequencing: principal vs. secondary diagnosis

When the burn injury is the reason for admission, a site-specific T20-T25 code is typically sequenced as the principal diagnosis. T31.90 is then assigned as an additional code to quantify the extent of burn involvement.

However, TBSA sometimes drives care intensity on its own, as in major burn center admissions. T31.90 may then be sequenced as the principal diagnosis. Follow facility policy and consult the attending when sequencing is ambiguous.

Burns vs. corrosions

T31 codes cover thermal burns. Chemical burns (corrosions) use the T32 series, which mirrors the T31 structure exactly. T32.90 is “Corrosions involving 90% or more of body surface with 0% to 9% third degree corrosion”. The mechanism of injury in the clinical record decides which category applies.

External cause coding

Assign an external cause code from X00-X08 (fire and flames) or X10-X19 (contact with heat sources) alongside T31.90. Place, activity, and status codes may also apply for injuries occurring at work or in specific settings. Missing external cause codes are a frequent trigger for denials on inpatient burn records.

Catching them before submission is the cheapest part of denial management.

Burns at multiple sites

When burns involve multiple body sites, assign the most specific site code available from T20-T25 for each site, then T31.90 as the TBSA extent code. ICD-10-CM guidelines treat T31 codes as supplementary. They do not replace site-specific coding when the site is documented.

MS-DRG mapping and reimbursement for T31.90

Hospital inpatient claims with T31.90 as the principal or significant secondary diagnosis route to burn-specific MS-DRGs under the CMS IPPS. Burns sit in MDC 22, and the burn MS-DRG set is not contiguous. The six codes below come from the CMS FY 2026 MS-DRG definitions manual, version 43.0.

Note that 930 to 932 are not burn DRGs, so the range is easy to misquote. Verify assignments and relative weights against the current CMS IPPS Final Rule before using them in billing.

Billing workflows for burn cases have to capture every complicating condition (CC) and major complicating condition (MCC). Those secondary diagnoses move the MS-DRG, and the payment with it. Electronic claim submission can flag a missing secondary diagnosis before the claim leaves the practice.

Common coding errors to avoid with T31.90

Four errors account for most miscoded T31.9X claims. The first is the most common, and the most expensive.

• Treating T31.90 as a “no third degree burns” code. It is not. Up to 9% full-thickness involvement stays in T31.90, so moving to T31.91 on a small documented patch overcodes the claim.
• Reading the band off the fourth character. The fourth character fixes total TBSA at 90% or more. The fifth character carries the third degree band, and mixing them up shifts every code by one step.
• Coding a chemical injury as a burn. Corrosions belong in T32, not T31. T32.90 is the parallel code, and it carries the same 0% to 9% third degree band.
• Submitting T31.90 with no external cause code. Inpatient burn claims need a code from X00-X08 or X10-X19. Omitting it is a routine rejection reason, and an easy one to prevent.

Related ICD-10 codes to know alongside T31.90

T31.90 is rarely coded in isolation. Burn admissions at this severity almost always generate a cluster of supporting codes. The table below covers the codes most often reported alongside T31.90. Each one has its own entry in our ICD-10-CM codes reference.

How Pabau supports burn diagnosis code documentation

Burn cases at this severity generate a long documentation list before a claim can be built. TBSA calculations, depth by region, external cause codes, and POA indicators all have to be captured.

Practice management software like Pabau captures each of those fields in the clinical note, then carries them into its claims management software. Burn care teams go from note to submission without rekeying the record.

For practices submitting institutional burn claims in the US, Pabau integrates with Claim.MD for electronic claim submission and remittance. Claim.MD supports real-time eligibility checks for over 400 payers. Electronic remittance advice (835) returns denial reason codes automatically, which speeds up correcting fifth-character errors on T31.9X claims.

Pabau’s reporting tools also track denial patterns by code cluster. A burn unit can see which fifth character keeps coming back rejected. Structured intake templates cut down on post-discharge coder queries.

Streamline burn injury documentation and billing

Pabau helps burn care teams capture TBSA percentages, burn depth per region, and mechanism of injury in structured clinical records. That cuts coder queries and speeds up claim submission.

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Conclusion

T31.90 is the correct code when burns cover 90% or more of body surface and third degree extent runs from 0% to 9%. It is also the default when depth extent is never documented. The decision point sits on the fifth character, not the fourth.

Documenting TBSA percentage, estimation method, burn depth by region, and external cause code before discharge eliminates the most common denial triggers on high-acuity burn claims.

To see how Pabau can support structured burn injury documentation and streamlined claim submission for your practice, book a demo with our team.

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