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Comparing ICD-9 to ICD-10, Code Structure and Organization

The ABC's of ICD-10
1. Comparing ICD-9 to ICD-10, Code Structure and Organization

Last week we provided an overview of ICD-10 and ran through some of the new features of the codeset. This week, let’s take a little bit deeper look at how ICD-10 differs from ICD-9, and how the new codes are organized. 

Expansion of Codes

Last week we discussed how ICD-10-CM allows for a greater level of specificity and clinical detail.  One of ICD-9’s issues is that some chapters are full and, thus were limited in the ability to add new codes. Because ICD-10 codes have increased in character length, the number of codes available for use has been greatly expanded. The biggest difference between the two code structures is that ICD-9 had 14,4000 codes, while ICD-10 contains over 69,823. ICD-10 codes consists of three to seven characters, while ICD-9 contained three to five digits. 

  • 14,400 codes in ICD-9-CM to over 69,823 in ICD-10-CM 
  • 34,250 (50%) of all ICD-10 codes are related to the musculoskeletal  systems
  • 17,045 (25%) of all ICD-10 codes are related to fractures 
  • 10,582 (62%) of these fracture codes distinguish “right” from “left
  • ~25,000 (36%) of all ICD-10 codes distinguish “right” from “left” 

Code Structure: Comparing ICD-9 to ICD-10

Consists of three to five digits Consists of three to seven characters
First character is numeric or alpha ( E or V) First character is alpha
Second, Third, Fourth and Fifth digits are numeric All letters used except U
Always at least three digits Character 2 always numeric; 3 through 7 can be alpha or numeric
Decimal placed after the first three digits Decimal placed after the first three characters
Alpha characters are not case-sensitive Alpha characters are not case sensitive

ICD-10-CM codes may consist of up to seven characters, with seventh character representing visit encounter or sequelae for injuries and external causes.


Diagnosis Code Structure



ICD-10 codes are organized differently than ICD-9 codes. For example:

  • Sense organs are separated from nervous system disorders
  • Injuries are grouped by anatomical site rather than injury category
  • Post-operative complications have been moved to the procedure-specific body system chapter

Focus Areas

  • Disease type 
  • Disease acuity 
  • Disease stage 
  • Site specificity 
  • Laterality 
  • Missing combination code detail 
  • Changes in timeframes associated with familiar codes

Examples of Specificity

  • S72.044G Nondisplaced fracture of base of neck of right femur, subsequent encounter for closed fracture with delayed healing
  • I69.351 Sequelae of cerebral infarction, hemiplegia and hemiparesis following cerebral infarction affecting right dominant side
  • T43.621S Poisoning by amphetamines, accidental (unintentional), sequela
  • M80.011A Age-related osteoporosis with current pathological fracture, right shoulder, initial encounter for fracture

Seventh Character Determination

  • Initial vs. subsequent encounter vs. sequela Injuries
  • Poisoning, adverse effects, and underdosing
  • Most external cause codes (except for place of occurrence, activity or status)

Seventh characters for the initial encounter are used while the patient is receiving active treatment for the condition. For example, surgical treatment, emergency department encounter, and evaluation and continuing treatment by the same or a different provider or physician. While the patient may be seen by a new or a different provider or physician over the course of a treatment of an injury, assignment of the seventh character is based on whether the patient is undergoing active treatment and not whether the provider is seeing the patient for the first time.

The seventh characters for the subsequent encounter are used after the patient has received treatment for the condition and is receiving routine care for the condition during the healing or recovery phase. For example, cast change or removal, an x-ray to check the healing status of a fracture, the removal of an external or internal fixation device, medication adjustment, or other aftercare and followup visits following treatment of the condition.

The seventh character, S, for sequela, is for use for complications or conditions that arise as a result of a condition, such as scar formation after a burn. Scars are sequela of the burn. When using the seventh character, S, it is necessary to use both the injury code that precipitated the sequela and the code for the sequela itself. The S identifies the injury responsible for the sequela and is added only to the injury code, not the sequela code. The specific type of sequela, scar, is sequenced first, followed by the injury code. 

The aftercare Z codes should not be used for aftercare for conditions such as injuries or poisonings where the seventh character is provided to identify subsequent cases. For example, for aftercare of injury, assign the acute care injury code with the appropriate seventh character for the subsequent encounter. 

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