ICD-10-CM Primer

by Ann Barta, MSA, RHIA; Gale McNeill, RHIA, CCS; Peggy Meli, PhD, MS, RHIA, LHRM; Kathleen Wall, MS, RHIA; and Ann Zeisset, RHIT, CCS, CCS-P

Although the US is still waiting on legislative or regulatory action establishing an ICD-10-CM implemention date, HIM professionals should start familiarizing themselves with the classification system to prepare for its future use.

What Is ICD-10-CM?

ICD-10-CM is a clinical modification of the World Health Organization’s ICD-10, which consists of a diagnostic system. ICD-10-CM includes the level of detail needed for morbidity classification and diagnostic specificity. It also provides code titles and language that complement accepted clinical practice. As with ICD-9-CM, ICD-10-CM is maintained by the National Center for Health Statistics.

The system consists of more than 68,000 codes, compared to approximately 13,000 ICD-9-CM codes. ICD-10-CM codes have the potential to reveal more about quality of care, so that data can be used in a more meaningful way to better understand complications, better design clinically robust algorithms, and better track the outcomes of care. ICD-10-CM incorporates greater specificity and clinical detail to provide information for clinical decision making and outcomes research.

Comparing ICD-9-CM and ICD-10-CM
ICD-10-CM differs from ICD-9-CM in its organization and structure, code composition, and level of detail.
  • Consists of three to five characters
  • First digit is numeric or alpha (E or V)
  • Second, third, fourth, and fifth digits are numeric
  • Always at least three digits
  • Decimal placed after the first three characters
  • Consists of three to seven characters
  • First digit is alpha
  • All letters used except U
  • Second and third digits are numeric
  • [note: Due to updates to ICD-10-CM since the time of original publication, the third digit, like digits 4-7 in the following bullet, can be alpha OR numeric]
  • Fourth, fifth, sixth, and seventh digits can be alpha or numeric
  • Decimal placed after the first three characters

Code Structure of ICD-10-CM versus ICD-9-CM

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

ICD-9-CM Code Format ICD-10-CM Code Format

ICD-10-CM Structure

ICD-10-CM has an index and tabular list similar to those of ICD-9-CM. However, the ICD-10-CM index is much longer. As with ICD-9-CM, ICD-10-CM uses an indented format for both the index and tabular list. Categories, subcategories, and codes are contained in the tabular list.

The ICD-10-CM index and tabular list were released in July 2007; the “Draft ICD-10-CM Official Guidelines” were released in June 2003 (available at www.cdc.gov/nchs/data/icd9/draft_i10guideln.pdf [no longer available at this URL]). Coding professionals will need to monitor the National Center for Health Statistics Web site for any new versions of the guidelines, index, and tabular list before implementation.

As with ICD-9-CM, proper coding relies on use of the guidelines, which house all information about the coding conventions for ICD-10-CM, general use guidelines, and chapter-specific guidelines for the tabular list. Coding guidelines are also in the index.

The two parts of the ICD-10-CM index are the index to diseases and injury and index to external causes of injury. The table of drugs and chemicals and the neoplasm table are housed in the index to diseases and injury.

The former V codes are now Z codes contained in chapter 21, “Factors Influencing Health Status and Contact with Health Services.”

Differences between ICD-10-CM and ICD-9-CM

ICD-10-CM differs from ICD-9-CM in its organization and structure, code composition, and level of detail. The table above provides a comparison of the two classification systems.

ICD-10-CM codes may consist of up to seven digits, with the seventh digit extensions representing visit encounter or sequelae for injuries and external causes. The difference in code structure is shown in the figure above.

Organizational Changes

While ICD-10-CM has the same type of hierarchical structure as ICD-9-CM, some differences are seen in the organization, including:

  • ICD-10-CM consists of 21 chapters.
  • Some chapters include the addition of a sixth character.
  • ICD-10-CM includes full code titles for all codes (no references back to common fourth and fifth digits).
  • V and E codes are no longer supplemental classifications.
  • Sense organs have been separated from nervous system disorders.
  • Injuries are grouped by anatomical site rather than injury category.
  • Postoperative complications have been moved to procedure-specific body system chapter.

New Features

ICD-10-CM has numerous new features allowing for a greater level of specificity and clinical detail. These include:

  • Combination codes for conditions and common symptoms or manifestations
  • Combination codes for poisonings and external causes
  • Added laterality
  • Added extensions for episode of care
  • Expanded codes (injury, diabetes, alcohol/substance abuse, postoperative complications)
  • Inclusion of trimester in obstetrics codes and elimination of fifth digits for episode of care
  • Expanded detail relevant to ambulatory and managed care encounters
  • Changes in timeframes specified in certain codes
  • External cause codes no longer a supplementary classification

ICD-10-CM also includes added standard definitions for two types of excludes notes. Excludes1 indicates not coded here. The code being excluded is never used with the code. The two conditions cannot occur together. For example, B06 Rubella [German measles] has an Excludes1 of congenital rubella (P35.0).

Excludes2 indicates not included here. The excluded condition is not part of the condition represented by the code. It is acceptable to use both codes together if the patient has both conditions. For example, J04.0, Acute laryngitis has an Excludes2 of chronic laryngitis (J37.0).

The sidebar “ICD-10-CM Code Examples” above highlights select new features and shows the level of specificity available.

An additional feature is the expansion of codes for certain conditions. Two examples are diabetes mellitus and postoperative complication codes.

Diabetes mellitus codes are expanded to include the classification of the diabetes and the manifestation. The category for diabetes mellitus has been updated to reflect the current clinical classification of diabetes and is no longer classified as controlled/uncontrolled:

  • E08.22, Diabetes mellitus due to an underlying condition with diabetic chronic kidney disease
  • E09.52, Drug or chemical induced diabetes mellitus with diabetic peripheral angiopathy with gangrene
  • E10.11, Type 1 diabetes mellitus with ketoacidosis with coma
  • E11.41, Type 2 diabetes mellitus with diabetic mononeuropathy

ICD-10-CM provides 50 different codes for “complications of foreign body accidently left in body following a procedure,” compared to only one code in ICD-9-CM. Examples include:

  • T81.535, Perforation due to foreign body accidently left in body following heart catheterization
  • T81.530, Perforation due to foreign body accidently left in body following surgical operation
  • T81.524, Obstruction due to foreign body accidently left in body following endoscopic examination
  • T81.516, Adhesions due to foreign body accidently left in body following aspiration, puncture or other catheterization

Code extensions (seventh character) have been added for injuries and external causes to identify the encounter: initial, subsequent, or sequela. The extensions are:

  • A Initial encounter
  • D Subsequent encounter
  • S Sequelae

For example, ICD-10-CM code S31.623A, Laceration with foreign body of abdominal wall, right lower quadrant with penetration into peritoneal cavity, initial encounter, shows an extension used with a laceration code. Note that in ICD-10-CM, the entire code description is written out.

Fracture codes require a seventh character that identifies if the fracture is open or closed for an initial encounter or if a subsequent encounter is for routine healing, delayed healing, nonunion, malunion, or sequelae. The fracture extensions are:

  • A Initial encounter for closed fracture
  • B Initial encounter for open fracture
  • D Subsequent encounter for fracture with routine healing
  • G Subsequent encounter for fracture with delayed healing
  • K Subsequent encounter for fracture with nonunion
  • P Subsequent encounter for fracture with malunion
  • S Sequelae

An example is code S42.321A, Displaced transverse fracture of shaft of humerus, right arm, initial encounter for closed fracture.

ICD-10-CM Code Examples

ICD-10-CM consists of new features and greater specificity. Sample ICD-10-CM codes are outlined below to illustrate this increased detail.

Combination Codes for Conditions and Common Symptoms

  • I25.110, Arteriosclerotic heart disease of native coronary artery with unstable angina pectoris
  • K50.013, Crohn’s disease of small intestine with fistula
  • K71.51, Toxic liver disease with chronic active hepatitis with ascites

Combination Codes for Poisonings and the External Cause

  • T39.011, Poisoning by aspirin, accidental (unintentional)
  • T39.012, Poisoning by aspirin, intentional self harm
  • T39.013, Poisoning by aspirin, assault
  • T39.014, Poisoning by aspirin, undetermined


  • C50.212, Malignant neoplasm of upper-inner quadrant of left female breast
  • H02.835, Dermatochalasis of left lower eyelid
  • I80.01, Phlebitis and thrombophlebitis of superficial vessels of right lower extremity
  • L89.213, Pressure ulcer of right hip, stage III

Learning the ICD-10-CM System

Regardless of one’s current level of coding expertise, it is important to become aware of the various associated concepts related to ICD-10-CM and to recognize the differences from ICD-9-CM. Coding professionals should download the ICD-10-CM index, tabular, guidelines, and general equivalence mapping files on the National Center for Health Statistics Web site, www.cdc.gov/nchs/about/otheract/icd9/icd10cm.htm.

Other countries that have implemented ICD-10 have found that coding professionals are more prepared for ICD-10 with increased clinical pathophysiological and human anatomy knowledge. Coding professionals need to learn about the most common disease processes and treatments to help better recognize and identify weaknesses in documentation required with the specificity of the new system.

There are many resources available to help nonclinical healthcare professionals learn the physiological mechanisms of diseases, their diagnosis, and their treatment. A series of books titled Clinical Pathophysiology Made Ridiculously Simple by Aaron Berkowitz might be helpful.

AHIMA’s Web site also contains valuable resources, with the latest ICD-10 information, articles in the FORE Library: HIM Body of Knowledge, and news releases. The Coding Community of Practice provides the opportunity to discuss ICD-10 with peers and read the “ICD-10 Checkpoint” column in the Codewrite electronic newsletter.


Johnson, Kerry. “Implementation of ICD-10: Experiences and Lessons Learned from a Canadian Hospital.” IFHRO Congress, AHIMA Convention, October 2004. Available online in the FORE Library: HIM Body of Knowledge at www.ahima.org.

National Center for Health Statistics. “ICD-10-CM Index and Tabular.” 2007. Available online at www.cdc.gov/nchs/about/otheract/icd9/icd10cm.htm.

National Center for Health Statistics. “Draft ICD-10-CM Official Guidelines.” 2003. Available online at www.cdc.gov/nchs/data/icd9/draft_i10guideln.pdf.

The authors are coding specialists with AHIMA’s Foundation of Research and Education.

Article citation:
Barta, Ann; McNeill, Gale C.; Meli, Peggy L.; Wall, Kathleen E.; Zeisset, Ann M.. "ICD-10-CM Primer" Journal of AHIMA 79, no.5 (May 2008): 64-66.