ICD-10 Background

The International Classification of Diseases (ICD) is a diagnosis coding system implemented by the World Health Organization (WHO) to track diseases. ICD-10 (tenth revision) was implemented in 1993 to replace the ICD-9 system developed in the 1970s. The United States is one of the few countries in the world which has not implemented ICD-10, but instead continues to use the ICD-9 coding system.


In August 2008, as part of the HIPAA Administrative Simplification, HHS mandated the implementation of ICD-10-CM (clinical modification) to replace ICD-9-CM for diagnosis coding and the ICD-10-PCS (Procedure Coding System) for inpatient hospital procedure coding with the compliance date originally set for October 1, 2013 and postponed twice; now set for October 1, 2015.


The decision to switch cited many important reasons (see ICD-10-CM/PCS: An Introduction, at www.cms.gov/ICD10):

  • ICD-9 is 30 years old and has outdated and obsolete terminology, uses outdated codes that produce inaccurate and limited data, and is inconsistent with current medical practice by not accurately describing the diagnoses and inpatient procedures of care delivered in the 21st century. ICD-10 has the ability to expand codes in order to capture additional advancements in clinical medicine.
  • There is no more room in the ICD-9 system to add new codes as medical science continues to make new discoveries.
  • Computer science, combined with new, more detailed codes will allow for better analysis of disease patterns and treatment outcomes that can advance medical care.

The ICD-10-CM system consists of more than 69,000 codes, compared to approximately 14,000 ICD-9-CM codes. The guidelines, rules and organization of ICD-10 are very similar to ICD-9, but the coding format and description of codes are different.

  • All ICD-10-CM codes are alpha-numeric and include all letters except, “U”, providing a greater pool of code numbers.
  • ICD-9-CM codes have a maximum of 5 digits, while ICD-10-CM codes have a maximum of seven digits and letters. The first character is alpha, 2nd – 7th is alpha or numeric and the 7th character is used in certain situations (obstetrics, musculoskeletal, injuries, and external causes of injuries).

ICD-9-CM Format

ICD-10-CM Format
xxx.xx xxx.xxx x



  •  Laterality (side of the body affected) has been added to relevant codes
  • Injuries are grouped by anatomical site rather than type of injury
  • Code titles are more complete; no need for coders to refer back to category, subcategory, or sub-classification level to determine the complete meaning of the code
  • The seventh digit will indicate an initial or subsequent encounter or the sequela (abnormal condition resulting from a previous disease)
  • Many more codes have been added to describe post-operative or post-procedural conditions
  • Expanded use of combination codes which are used for both symptom and diagnosis, and etiology and manifestations
  • Excludes Notes: Indicates where 2 conditions cannot occur together and where the condition excluded is not part of the condition represented by the code but the patient may have both conditions at the same time
  • Many new codes have been added, such as codes for blood type and alcohol level
  • Codes reflect modern medicine and updated medical terminology