10.2 Injury, Poisoning, and External Causes

Chapter 19 (S00–T88) and Chapter 20 (V00–Y99) are where COC facility cases lose 7th characters, mix up poisoning with an adverse effect, or park an external-cause code in the first-listed slot of a Medicare outpatient UB-04. The injury code describes the wound. The 7th character describes the phase of care. The V/W/X/Y code, when reported at all, only explains how it happened.

7th characters A, D, and S

Most Chapter 19 categories require a 7th character. For injuries that are not in the extra-fracture list, the values are:

Assignment follows the phase of care, not whether this facility has seen the patient before. A patient who had ORIF yesterday at Hospital A and lands in your ED tonight for wound care that is still part of acute postoperative management may still be A if active treatment of the injury is ongoing. A hospital outpatient clinic visit six weeks later for a well-healing Colles fracture is D.

Aftercare Z codes are not used for injuries that have 7th characters. Do not report Z47.89, Encounter for other orthopedic aftercare, in place of S52.531D. Complication codes such as T84.50XA, Infection and inflammatory reaction due to unspecified internal joint prosthesis, initial encounter, take A when this encounter is active treatment of the infection, even though the implant was placed months ago. That example is in the FY 2026 Chapter 19 7th-character guideline: active treatment refers to treatment for the condition described by the code.

Sequela has no time limit. Scar after a healed second-degree burn of the right hand is first-listed L90.5, Scar conditions and fibrosis of skin, then T23.201S, Burn of second degree of right hand, unspecified site, sequela. The S goes only on the injury code.

Fracture 7th characters are more specific than A/D/S

Traumatic fracture categories add extra 7th characters. A fracture not indicated as open or closed codes to closed. A fracture not indicated as displaced or nondisplaced codes to displaced. For forearm (S52), femur (S72), and lower leg including ankle (S82), open-fracture 7th characters follow the Gustilo open-fracture classification. If the record says open but does not give a Gustilo type, assign the 7th character for open type I or II (B, E, H, M, or Q, depending on healing). Other fracture families (for example many S42 shoulder-girdle codes) use a shorter open/closed set (A closed initial, B open initial, D/G/K/P subsequent, S sequela) without the full Gustilo C/F/J/N/R list.

ED example: Gustilo type IIIB open shaft fracture of the right tibia. First-listed S82.201C, Unspecified fracture of shaft of right tibia, initial encounter for open fracture type IIIA, IIIB, or IIIC. S82.201A would treat it as closed. S82.201B is the default only when the fracture is open and no Gustilo type is recorded.

SDS example: closed right Colles fracture taken for ORIF. First-listed S52.531A, Colles' fracture of right radius, initial encounter for closed fracture. The same patient in your hospital clinic six weeks later with routine healing is S52.531D. Nonunion at a later visit is S52.531K (subsequent encounter for closed fracture with nonunion), not a Z aftercare code. A patient who delayed seeking treatment for the fracture or the nonunion still receives an initial-encounter 7th character (A, B, or C as applicable) per the traumatic-fracture guideline. Pathologic fracture from osteoporosis is M80.- , not a traumatic S code, even after a minor fall that would not break a healthy bone.

Sequence multiple injuries with the most serious injury first. Superficial abrasions at the same site as a deeper wound are not coded. Do not use traumatic injury codes for normal healing surgical wounds or for injuries that occur during a medical intervention; those belong in complication codes. Physeal fractures are reported with the physeal code only; do not add a separate code for the bone that is fractured.

Poisoning, adverse effect, and underdosing (T36–T50)

Categories T36–T50 are combination codes that already include the substance and the intent. No extra Chapter 20 code is required for these events. Do not code from the Table of Drugs and Chemicals without verifying the Tabular List. A 5th or 6th character carries intent:

Placeholder X fills empty characters so the 7th character sits in position 7 (for example T36.0X5A and T39.1X1A). Section 10.3 drills that mechanic.

Poisoning sequencing: poisoning code first, then manifestations. ED acetaminophen overdose taken in error with nausea and vomiting: T39.1X1A, Poisoning by 4-aminophenol derivatives, accidental (unintentional), initial encounter, then R11.2, Nausea with vomiting, unspecified. An extra oxycodone tablet not taken as prescribed is poisoning (T40.2X1A), not an adverse effect. If two drugs were taken, code each unless the Table of Drugs lists a combination.

Adverse-effect sequencing: nature of the reaction first, then the T code with character 5. Clinic rash after penicillin taken exactly as prescribed: L27.0, Generalized skin eruption due to drugs and taken internally, then T36.0X5A, Adverse effect of penicillins, initial encounter. Do not use character 1. The same drug can be an adverse effect or a poisoning; the difference is whether use was correct.

Underdosing sequencing: underdosing codes are never first-listed. Observation for hypertensive urgency after the patient ran out of amlodipine: I16.0, Hypertensive urgency, first, then T46.5X6A, Underdosing of other antihypertensive drugs, initial encounter, plus a noncompliance code such as Z91.138, Patient's unintentional underdosing of medication regimen for other reason, when intent is known. Documentation that the patient took less than prescribed is enough to assign underdosing; a documented clinical relapse is not required. The medical condition remains first-listed when it is the reason for the encounter. Provider-directed dose changes are not underdosing.

Toxic effects of non-medicinal substances use T51–T65 and are sequenced like poisoning: toxic-effect code first, then manifestations.

Chapter 20: External causes (V00–Y99) are additional, never primary

External-cause codes capture how the event happened, intent, place (Y92.-), activity (Y93.-), and status (Y99.-). They must never be sequenced as the first-listed or principal diagnosis. The FY 2026 guidelines state that rule in the Chapter 20 introduction and again as a numbered general external-cause guideline. On a typical Medicare hospital outpatient UB-04 (type of bill 13X), form locator 67 is the first-listed diagnosis. A fall code such as W19.XXXA, Unspecified fall, initial encounter, cannot occupy FL 67. The injury (S72.001A, Fracture of unspecified part of neck of right femur, initial encounter for closed fracture) is first-listed; W19.XXXA and, if the facility reports them, Y92.010 (kitchen of a single-family house) plus activity/status codes are additional. Place, activity, and status categories do not take A/D/S 7th characters. Mechanism codes do, and that 7th character should match the injury’s 7th character for the encounter.

There is no national mandate to report Chapter 20 codes. A state mandate or a particular payer may require them. Medicare outpatient processing still will not accept a V/W/X/Y code as the first-listed diagnosis when they are reported. If the reporting format limits how many external-cause codes fit, keep the code most related to the first-listed injury. Assign external-cause codes for each encounter at which the injury is treated, including subsequent and sequela visits, with a matching 7th character.

Transport example: ED driver in a car-to-car traffic collision with a closed right distal radius fracture. First-listed S52.501A, Unspecified fracture of the lower end of right radius, initial encounter for closed fracture. Additional V43.52XA, Car driver injured in collision with other type car in traffic accident, initial encounter. Do not reverse them. T36–T50 already include intent, so do not add a Chapter 20 poisoning mechanism code for those drug events.