Chapter 20 — External Causes of Morbidity
External cause codes are always secondary, are not nationally mandated, and follow a strict priority order: abuse, terrorism, cataclysmic events, transport accidents.
External cause codes are never principal or first-listed, and there is no national mandate to report them. Report them when a state mandate or a payer requires it; otherwise voluntary reporting is encouraged, because the data drives injury research and prevention.
General external cause rules#
| # | Rule | Detail |
|---|---|---|
| 1 | Range of use | An external cause code may be used with any code in the range A00.0–T88.9, Z00–Z99 that represents a health condition due to an external cause — most applicable to injuries, but also valid for infections or diseases due to an external source, and for conditions such as a heart attack occurring during strenuous physical activity |
| 2 | Length of treatment | Assign the external cause code with the appropriate 7th character for each encounter for which the injury or condition is being treated. The 7th character for the external cause should match the 7th character of the code for the associated injury or condition for that encounter |
| 3 | Full range | Use the full range of external cause codes to describe the cause, the intent, the place of occurrence, and where applicable the activity and the patient's status |
| 4 | How many | Assign as many as necessary to fully explain each cause. If only one can be recorded, assign the code most related to the principal diagnosis |
| 5 | Selection | Guided by the Alphabetic Index of External Causes and by the Inclusion and Exclusion notes in the Tabular List |
| 6 | Never principal | NEVER An external cause code can never be a principal (first-listed) diagnosis |
| 7 | Combination external cause codes | Some identify sequential events resulting in an injury, such as a fall which results in striking against an object. The code used should correspond to the sequence of events, regardless of which caused the most serious injury |
| 8 | When none is needed | NEVER No Chapter 20 code is needed if the external cause and intent are included in a code from another chapter — for example T36.0X1- poisoning by penicillins, accidental |
Place, activity and status#
| Category | Rule |
|---|---|
Y92 Place of occurrence | A secondary code used after other external cause codes to identify the location of the patient at the time of injury. Generally assigned only once, at the initial encounter. In the rare instance that a new injury occurs during hospitalisation, an additional place code may be assigned. NEVER No 7th characters are used for Y92. NEVER Do not use Y92.9 if the place is not stated or is not applicable |
Y93 Activity | Describes the activity of the patient at the time the injury occurred. Used only once, at the initial encounter, and only one Y93 code is recorded on a medical record. NEVER Not applicable to poisonings, adverse effects, misadventures or sequela. NEVER Do not assign Y93.9 if the activity is not stated |
Y99 External cause status | Assigned whenever any other external cause code is assigned, including an activity code, with the exceptions noted. Indicates the work status: military activity, non-military person at work, or an individual including a student or volunteer in a non-work activity. NEVER Not applicable to poisonings, adverse effects, misadventures or late effects. NEVER Do not assign a Y99 code if no other external cause codes apply. Used only once, at the initial encounter, and only one is recorded. NEVER Do not assign Y99.9 if the status is not stated |
| Sequencing of all three | Place of occurrence, activity and external cause status codes are sequenced after the main external cause code(s). Generally there should be only one place code, one activity code and one status code per encounter |
| If the reporting format limits the number of codes | Report |
|---|---|
| Only one external cause code can be recorded | The code for the cause / intent most related to the principal diagnosis |
| Additional external cause codes can be captured | The cause and intent, including medical misadventures, of the additional events — rather than the codes for place, activity or external status |
The priority order when two or more events caused separate injuries#
| Priority | Category |
|---|---|
| 1 | Child and adult abuse — takes priority over all other external cause codes |
| 2 | Terrorism — takes priority over all others except child and adult abuse |
| 3 | Cataclysmic events — take priority over all others except abuse and terrorism |
| 4 | Transport accidents — take priority over all others except cataclysmic events, abuse and terrorism |
| 5 | Activity and external cause status codes are assigned following all causal (intent) external cause codes |
Within that hierarchy, the first-listed external cause code corresponds to the cause of the most serious diagnosis due to an assault, accident or self-harm.
Abuse, intent, sequelae and terrorism#
| Topic | Rule |
|---|---|
| Child and adult abuse | Adult and child abuse, neglect and maltreatment are classified as assault. Any assault code may indicate the external cause of an injury from confirmed abuse. For confirmed cases where the perpetrator is known, a code from Y07 accompanies the assault codes |
| Unknown or undetermined intent | If the intent — accident, self-harm, assault — is unknown or unspecified, code the intent as accidental. ALWAYS All transport accident categories assume accidental intent |
| Undetermined intent | NEVER External cause codes for events of undetermined intent are only for use if the documentation in the record specifies that the intent cannot be determined |
| Sequelae external cause codes | Reported using the external cause code with 7th character S for sequela, with any report of a late effect or sequela resulting from a previous injury |
| Sequela external cause code with a current injury | NEVER A sequela external cause code should never be used with a related current nature of injury code |
| Subsequent visits | Use a late effect external cause code for subsequent visits when a late effect of the initial injury is being treated. NEVER Do not use one for subsequent visits for follow-up care — assessing healing, receiving rehabilitative therapy — where no late effect has been documented |
| Terrorism identified by the FBI | The first-listed external cause code is a code from category Y38. Use an additional code for place of occurrence Y92.-. More than one Y38 code may be assigned if the injury results from more than one mechanism of terrorism |
| Terrorism suspected | NEVER A code from Y38 should not be assigned. Suspected cases are classified as assault |
Y38.9 Terrorism, secondary effects | For conditions occurring subsequent to the terrorist event. NEVER Not assigned for conditions due to the initial terrorist act. ALWAYS It is acceptable to assign Y38.9 with another Y38 code where there is an injury due to the initial event and an injury that is a subsequent result |
Practice questions#
Q1A patient is injured in a building collapse during a hurricane. Which external cause code is sequenced first?
- AThe building collapse code
- B
X37.0-hurricane - CThe place of occurrence code
- DEither, at the coder's discretion
Show answer & rationale
Correct answer: B. X37.0- hurricane
Rationale. Section I.B.19.b states that codes for cataclysmic events such as a hurricane take priority over all other external cause codes except child and adult abuse and terrorism, and should be sequenced before other external cause of injury codes, using this exact example: external cause codes for both the hurricane and the building collapse are assigned, with the hurricane code sequenced first. Section I.C.20.f sets the same hierarchy in general terms.
Q2An intent of injury cannot be established from the record, and the record does not state that the intent cannot be determined. How is the intent coded?
- AUndetermined
- BAccidental
- CAssault
- DNo intent code is assigned
Show answer & rationale
Correct answer: B. Accidental
Rationale. Section I.C.20.h states that if the intent — accident, self-harm, assault — of the cause of an injury or other condition is unknown or unspecified, the intent is coded as accidental, and that all transport accident categories assume accidental intent. Section I.C.20.h.1 restricts undetermined intent codes to cases where the documentation in the record specifies that the intent cannot be determined.
Q3How many activity codes from category Y93 may be recorded on a medical record?
- AAs many as apply
- BOne
- COne per encounter
- DTwo, for multi-stage events
Show answer & rationale
Correct answer: B. One
Rationale. Section I.C.20.c states that an activity code is used only once, at the initial encounter for treatment, and that only one code from Y93 should be recorded on a medical record. The activity codes are not applicable to poisonings, adverse effects, misadventures or sequela, and Y93.9 unspecified activity is not assigned if the activity is not stated.
Q4A poisoning by penicillins, accidental, is coded to T36.0X1-. Is a Chapter 20 external cause code also required?
- AYes, for the intent
- BYes, for the place of occurrence
- CNo — the external cause and intent are included in the code
- DOnly if a state mandate applies
Show answer & rationale
Correct answer: C. No — the external cause and intent are included in the code
Rationale. Section I.C.20.a.8 states that no external cause code from Chapter 20 is needed if the external cause and intent are included in a code from another chapter, and gives T36.0X1- as the example. Section I.C.19.e states the same rule from the poisoning chapter's side: codes in categories T36-T65 are combination codes that include the substance and the intent, so no additional external cause code is required.
Q5When is a code from category Y99 external cause status assigned?
- AOn every encounter for an injury
- BWhenever any other external cause code is assigned, including an activity code, with the stated exceptions
- COnly for work-related injuries
- DOnly when a
Y92place code is also assigned
Show answer & rationale
Correct answer: B. Whenever any other external cause code is assigned, including an activity code, with the stated exceptions
Rationale. Section I.C.20.k states that a code from category Y99 should be assigned whenever any other external cause code is assigned for an encounter, including an activity code, except for the noted events, and that a Y99 code should not be assigned if no other external cause codes are applicable. External cause status codes are not applicable to poisonings, adverse effects, misadventures or late effects, are used only once at the initial encounter, and only one is recorded.
Scenarios#
A warehouse worker falls from a ladder while stacking shelves during a shift and sustains a fracture of the left wrist. This is the initial encounter and active treatment is provided.
Show coded answer & rationale
| Seq | Code | Description |
|---|---|---|
| 1 | S62.- | the applicable left wrist fracture code with the initial-encounter 7th character — principal diagnosis |
| 2 | W11.XXXA | Fall on and from ladder, initial encounter |
| 3 | Y92.59 | Other trade areas as the place of occurrence — verify the specific code in the Tabular List |
| 4 | Y93.H2 | Activity, other involving climbing, or the applicable activity code — verify in the Tabular List |
| 5 | Y99.0 | Civilian activity done for income or pay |
Rationale. Section I.C.20.a.3 directs use of the full range of external cause codes to describe the cause, the intent, the place of occurrence and, where applicable, the activity and the patient's status. Section I.C.20.d requires that the place, activity and status codes be sequenced after the main external cause code, with generally one of each per encounter. Section I.C.20.a.2 requires the 7th character on the external cause code to match that on the injury code. Section I.C.20.b and I.C.20.c limit the place and activity codes to the initial encounter.
Guideline: Section I.C.20.a.2, I.C.20.a.3, I.C.20.b, I.C.20.c, I.C.20.d, I.C.20.k
A patient returns two years after a motor vehicle collision for treatment of a post-traumatic joint contracture documented as a late effect of the original injury.
Show coded answer & rationale
| Seq | Code | Description |
|---|---|---|
| 1 | 1 | the code for the contracture — the nature of the sequela, sequenced first |
| 2 | 2 | the injury code with 7th character S |
| 3 | 3 | the transport accident external cause code with 7th character S |
Rationale. Section I.C.20.i.1 states that sequelae are reported using the external cause code with 7th character S, to be used with any report of a late effect or sequela resulting from a previous injury. Section I.C.20.i.2 prohibits using a sequela external cause code with a related current nature of injury code, and I.C.20.i.3 restricts late-effect external cause codes to visits where a late effect is being treated — not to follow-up visits to assess healing or provide rehabilitative therapy where no late effect is documented. Section I.B.10 and I.C.19.a supply the sequela sequencing.
Guideline: Section I.C.20.i, I.C.19.a, I.B.10
Primary sources#
- ICD-10-CM Official Guidelines for Coding and Reporting, FY 2027CMS — the source document for every page on this site (effective 1 Oct 2026)
- NCHS ICD-10-CM files — Tabular List, Index, POA exempt listOfficial code files, addenda and the list of codes exempt from POA reporting
- ICD-10 Coordination and Maintenance CommitteeWhere code proposals are debated; agendas, summaries and meeting materials
- CMS ICD-10 homeCode sets, transition guidance and Medicare coding policy
- AHA Coding ClinicOfficial coding advice from the AHA Central Office — subscription required
- HHS OIG Work PlanActive audit topics — useful for prioritising internal coding audits
- MEDESUN Medical Coding AcademyTraining, credential preparation and audit services by the author