ICD-10 Code T31.42: burns involving 40-49% body surface

Key takeaways

ICD-10 Code T31.42 describes burns involving 40-49% of total body surface area (TBSA) with 20-29% classified as third degree, valid for FY2026 reimbursement.

T31.42 is a billable, specific ICD-10-CM code in the S00-T88 injury chapter. Coders must document the TBSA percentage and the third-degree burn percentage as two separate figures.

Selecting the correct sibling code (T31.40, T31.41, T31.42, T31.43, or T31.44) depends entirely on the documented third-degree burn percentage within the 40-49% TBSA range.

Practice management software like Pabau helps burn care teams capture TBSA percentages and third-degree burn documentation accurately at the point of care.

ICD-10 Code T31.42 is a billable ICD-10-CM diagnosis code. It describes burns involving 40-49% of total body surface area (TBSA), with 20-29% of that area classified as third degree. Coders must document both figures separately, as specific percentages rather than a general estimate.

Without both numbers in the record, a coder cannot assign this code accurately. This guide covers the billable status, code hierarchy, Rule of Nines methodology, sibling code selection, and documentation requirements for T31.42. It also explains when T31 extent-based codes apply alongside T20-T25 site-specific codes, a distinction many code lookups leave unclear.

ICD-10-CM coding for major burns is one of the more nuanced areas of injury coding. The T31 category adds a second dimension to standard burn documentation. It quantifies how much of the body is affected and how severe the deepest injuries are, rather than where the burn is located. Coders working in emergency departments, burn units, and inpatient coding teams frequently use T31 codes alongside site-specific codes from T20-T25. Getting the distinction right matters for DRG assignment, severity scoring, and reimbursement accuracy.

ICD-10 Code T31.42: code description and billable status

ICD-10 Code T31.42 is a billable, specific ICD-10-CM code valid for FY2026 diagnoses, reimbursement claims, and medical records. It cannot be subdivided further. There are no child codes beneath T31.42.

The CDC/NCHS ICD-10-CM web tool and the CMS ICD-10 codes page both confirm T31.42 as valid for FY2026. Facilities using structured documentation software should ensure their code tables reflect the current fiscal year’s tabular list. This matters most for burn-related diagnoses, where TBSA thresholds map directly to specific codes.

Code hierarchy and parent codes

T31.42 sits within a four-level hierarchy. Understanding the hierarchy helps coders navigate to the correct level of specificity and avoid non-billable parent code submissions. The same four-level structure applies to every T31 sibling, including T31.52 for higher TBSA ranges.

Only T31.42 (the fifth-character code) is billable. Submitting T31.4 or T31 as a standalone diagnosis code will result in a claim error. Coders should also note that T31 codes belong to the broader WHO ICD classification system. The WHO ICD-10 browser provides the international reference framework underpinning these classifications.

Rule of Nines and total body surface area in burn coding

The Rule of Nines is the standard clinical method for estimating total body surface area affected by burns. Accurate TBSA calculation is the foundation of T31 coding. Without a documented TBSA estimate, a coder cannot assign any code within the T31 category.

For T31.42, the treating clinician must document that burns cover 40-49% TBSA. A patient with bilateral full-arm burns (18%) plus anterior trunk burns (18%) plus one full leg (18%) would total 54% – exceeding the T31.42 threshold. A patient with anterior trunk (18%) plus one leg (18%) plus head and neck (9%) would total 45%. That places them squarely within the T31.4 subcategory range.

Precise TBSA documentation in the attending physician’s notes is the coder’s primary source. When documentation uses a range (“approximately 40-45%”), the coder should query the provider rather than assume. Digital intake forms built into a practice’s workflow can standardize burn assessment capture and reduce ambiguous documentation at the point of care.

Third-degree burns: clinical definition for ICD-10 Code T31.42

The second numeric qualifier in ICD-10 Code T31.42 is the third-degree burn percentage. This is not the percentage of the body that is burned overall. It is the percentage of the body covered specifically by full-thickness (third-degree) burns, expressed as a share of total body surface.

Third-degree burns destroy all layers of the dermis and may extend into subcutaneous tissue. They do not typically cause pain at the burn site because nerve endings are destroyed. Clinically, they appear white, brown, or leathery, and they require skin grafting. For coding purposes, what matters is the documented surface percentage, not the treatment approach.

• Third-degree (full thickness): destroys epidermis and full dermis; extends into fat, muscle, or bone in severe cases
• Second-degree (partial thickness): destroys epidermis, partial dermis; painful, blistering
• First-degree (superficial): epidermis only; redness, no blistering

For ICD-10 Code T31.42, the attending physician must document that 20-29% of the body surface is third degree. This figure is separate from and in addition to the 40-49% TBSA figure. A patient with 45% TBSA burns, of which 22% are third degree and 23% are second degree, would receive T31.42. A patient with 45% TBSA burns and 35% third-degree involvement would receive T31.43 instead.

Accurate burn depth documentation feeds directly into diagnostic coding accuracy across every TBSA range, including smaller burns such as T31.11. In burn care specifically, the depth percentage drives DRG grouping and reimbursement weight. Clinical documentation platforms that capture structured burn depth fields help coders avoid the most common T31.4x assignment errors.

Related T31.4 subcategory codes

The T31.4 subcategory contains five specific codes, each differentiated only by the third-degree burn percentage. Selecting the wrong sibling code is a common audit trigger. The table below covers the full T31.4x family to support accurate code selection.

T31.40 is used when the physician documents 40-49% TBSA burns but does not specify any third-degree involvement. Coders should not default to T31.40 when third-degree burns are mentioned but not quantified. In that case, query the provider. The AAPC Codify ICD-10-CM lookup and ICD10Data.com both allow coders to browse the full T31.4x family and confirm descriptor details before assigning.

Documentation requirements for T31.42

Burn coding under the T31 category places the burden of specificity on clinical documentation. The ICD-10-CM Official Guidelines for Coding and Reporting require that the assigned code reflects what the physician actually documented. For T31.42 specifically, the record must contain the following elements before a coder can assign the code.

• Total TBSA documented: a specific percentage or range within 40-49%, recorded by the attending physician or burn specialist
• Third-degree burn percentage documented: a specific percentage within 20-29%, separate from the overall TBSA figure
• Assessment method referenced: Rule of Nines, Lund-Browder chart, or equivalent clinical tool noted in the record
• Burn depth classification per region: the record should indicate which body regions have third-degree involvement versus partial or superficial burns
• Cause of burn documented: required for assigning companion external cause codes (X codes) alongside T31.42
• Encounter type specified: initial encounter (A), subsequent encounter (D), or sequela (S) for the 7th character extension

Note that T31.42 does not require a 7th character by itself in the tabular list, but companion T20-T25 site-specific codes do require 7th character extensions. Ensuring all codes in a burn episode use consistent encounter qualifiers is a common area for compliance review. Compliance management workflows that flag incomplete burn documentation before claim submission can prevent the most common T31.42 coding errors.

Facilities using standardized medical forms for burn admission tend to capture TBSA and depth data more consistently. A structured burn assessment form has dedicated fields for total TBSA, third-degree percentage per region, and assessment method. That structure eliminates the ambiguity that leads to provider queries and delayed claim submission.

When to use T31 codes vs T20-T25 site-specific burn codes

This is the most frequently misunderstood aspect of burn coding. T31 and T20-T25 codes serve different purposes. Coders assign them together on the same claim rather than choosing one over the other.

T20-T25 codes describe burns by body site. T20 covers burns of the head and face, T21 the trunk, T22 the shoulder and upper arm, and so on. Each T20-T25 code also specifies burn depth (first, second, or third degree) and the laterality or site within the region. These are site-specific codes.

T31 codes describe the extent of burns by total body surface area and third-degree involvement. They add a severity dimension that site-specific codes cannot convey on their own.

Applying T31 and T20-T25 codes together

The ICD-10-CM Official Guidelines call for a T31 code as an additional code when site-specific codes alone don’t fully convey a major burn’s severity. For T31.42, consider a coder managing a patient with burns to the trunk, both arms, and one leg. That coder would assign T21.xx, T22.xx (bilateral), and T24.xx alongside T31.42. The T31.42 code tells the payer how large and how severe the overall burn injury is. The T20-T25 codes tell the payer where each burn is and how deep it goes.

Facilities managing complex burn admissions often benefit from skin clinic software that supports multi-diagnosis coding per encounter. Burn units working alongside reconstructive teams see the same benefit from plastic surgery EMR tools built around grafting and revision follow-up.

Tracking multiple simultaneous T20-T25 and T31 codes for a single patient requires structured documentation that generic intake systems rarely support well. HIPAA-compliant documentation platforms with configurable diagnosis fields reduce the risk of omitting a T31 code when it should accompany site-specific codes.

Approximate synonyms and clinical terminology

ICD-10-CM recognizes a range of alternate clinical terms that map to T31.42. These synonyms appear in the index to diseases and injuries and may be used as documentation triggers by coding software. Knowing these terms helps coders identify T31.42 from physician notes that use clinical shorthand rather than code-facing language.

• Burns involving 40-49% of body surface area with 20-29% third degree burns
• 40-49% TBSA burn with 20-29% full thickness
• Major burn, 40-49% TBSA, 20-29% third degree
• Burn, total body surface, 40-49 percent, 20-29 percent third degree
• Full thickness burns (20-29%) with total body involvement (40-49%)

Clinical notes may use “full thickness” rather than “third degree.” Both terms refer to the same burn depth and are acceptable documentation for coding purposes. Notes using “deep partial thickness” without specifying full thickness do not support T31.42’s third-degree qualifier. When in doubt, a provider query is the correct step before assigning T31.42 over T31.41.

Teams managing burn documentation alongside prescription management for post-burn wound care can use structured terminology fields to align clinical notes with coding expectations. Consistent coding across an episode of care depends on using the same clinical terminology in every progress note, not just in the admission note.

How Pabau supports accurate burn documentation

Most burn units still capture TBSA and burn depth in free-text physician notes. A coder is then left to interpret phrases like “extensive burns” well after the encounter ends, often with a payer query attached. Practice management software like Pabau replaces that free-text approach with structured digital forms built for burn assessments. Each form captures the TBSA percentage, the third-degree percentage, the assessment method, and the burn cause as separate fields.

Every field a patient or provider completes flows straight into their record. A coder reviewing the chart sees the TBSA and third-degree percentages as distinct, structured data rather than a single free-text description. That structure cuts down on the provider queries that slow down documentation review for major burn cases.

Streamline burn documentation with Pabau

Pabau helps burn care teams capture structured TBSA assessments, third-degree burn percentages, and treatment records in one place. Your coders always have what they need to assign the right ICD-10 code.

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Conclusion

T31.42 only works when the clinical record carries two precise numbers: 40-49% TBSA and 20-29% third-degree burns. When a chart is vague on either figure, query the provider before defaulting to a sibling code or leaving severity understated on the claim. Getting this pair right, alongside the matching T20-T25 site codes, keeps DRG assignment and reimbursement accurate for major burn cases.

Pabau’s structured digital documentation tools help burn care teams build the precise, consistent records that coders need at the point of care. To see how Pabau supports clinical documentation and coding workflows, book a demo.

Continue your research

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Coding a sequela of a traumatic injury? S37.502S shows how the 7th-character extension works once the original injury has resolved.

Confirming burn depth needs a tissue sample? CPT 11105 covers the additional punch biopsy code used to verify depth when documentation is unclear.

Need to code wound-care supplies alongside a burn diagnosis? HCPCS A4674 covers the chemicals and antiseptics billed separately from the burn treatment itself.

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