As a heightened awareness and acceptance of our inevitable transition to ICD-10 develops, many of you may be wondering how to approach the task of preparing your key stakeholders, coders, clinical documentation specialists and providers for the change. Certainly we all realize the need for additional education, but just where do you begin? The HIM profession typically approaches such a change to a new payment methodology by focusing on the differences between the current method and the new method. This would work for the transition to ICD-10-CM too, as there are certainly far more similarities between ICD-9-CM and ICD-10-CM than differences. But another viable option, and a bit of a more holistic approach, focuses on the foundation of ICD-10-CM referred to as the “axis of classification.”


The phrase “axis of classification” simply means a way of classifying and studying diseases. When utilizing an axis of classification for morbid conditions, diseases are assigned to a system of categories based on established criteria. Such criteria may be based on the affected part of the body (anatomy), the nature of a disease process (pathophysiology), etiology or manifestation, etc. These represent some of the many axes of classification available in ICD-10-CM. The use of assorted axes of classification is necessary to meet the needs of a varied group of users in classifying diseases, injuries and other reasons for health services being provided in a wide array of healthcare settings.

Anatomy is the primary axis of classification of ICD-10-CM, as it was in ICD-9-CM. This is evident by the fact that most of the ICD-10-CM chapter titles reflect diseases of a particular body system, such as “Diseases of the Respiratory System” and “Diseases of the Nervous System,” etc. ICD-10-CM employs many other axes as well, such as etiology, as found in Chapter One (“Certain Infectious and Parasitic Diseases”). A combination of multiple and diverse axes are used in classifying some diseases within the same chapter. When designing a disease classification system, the primary axis reflects the most important statistical and clinical aspects of the disease. For example, for a diagnosis of heart failure, the first axis of classification is “type” and the second is “acuity.”


ICD-10-CM utilizes a variety of axes of classification, either alone or in some combination, such as:


  • Etiology
  • Manifestation or complication
  • Specificity of anatomical site
  • Chronicity (i.e. acute, subacute, chronic, unspecified vs. acute/subacute, chronic, unspecified, etc.)
  • Degree (i.e., mild, moderate, severe, unspecified vs total/complete, partial/incomplete, etc.)
  • Type (i.e., primary, secondary, unspecified, etc.)
  • Laterality (i.e., R/L/unspecified or R/L/bilateral/unspecified)
  • Episode of care (3-16 “extension” options, depending on code category)
  • Trimester (i.e., 1, 2, 3,unspecified, etc.)
  • Number of fetus (i.e. 1-5, other.)


It is the variation and combination of these axes of classification that contribute to the tremendous increase in the number of codes available for assignment in ICD-10-CM as compared to ICD-9-CM.

To illustrate, let’s talk numbers!


For Fiscal Year 2012, there are a total of 14,567 valid ICD-9-CM diagnosis codes.

For the same year, there are a total of 79,503 draft ICD-10-CM diagnosis codes.


The axes of classification vary not only by diagnosis/condition, but also to a great extent by chapter. In examining code volume variations by chapter in the spreadsheet below, you can see that 54 percent of the ICD-10-CM codes available for assignment are classified to Chapter 19 (“Injury, Poisoning & Certain Other Consequences of External Causes”).


Chapter Chapter Title Code Volume Code Volume Vol Rank
1 Certain Infectious & Parasitic Diseases 1270 1292 21
2 Neoplasms 988 2026 13
3 Diseases of the Blood & Blood-forming Organs 123 298 9
4 Endocrine, Nutritional & Metabolic Diseases 335 879 8
5 Mental & Behavioral Disorders 477 927 14
6 Diseases of the Nervous System 412 792 15
7 Diseases of the Eye & Adnexa (new chapter) 795 3100 6
8 Diseases of the Ear & Mastoid Process (new chapter) 192 855 5
9 Diseases of the Circulatory System 474 1587 7
10 Diseases of the Respiratory System 255 436 18
11 Diseases of the Digestive System 596 895 19
12 Diseases of Skin and Subcutaneous Tissue 204 950 4
13 Diseases of the Musculoskeletal System & Connective Tissue 892 7939 2
14 Diseases of the Genitourinary System 389 726 16
15 Pregnancy, Childbirth & Puerperium 1104 2539 10
16 Certain Conditions Originating in the Perinatal Period 281 504 17
17 Congenital Malformations, Deformations & Chromosomal Abnormalities 421 944 11
18 Symptoms, Signs & Abnormal Clinical & Laboratory Findings 372 777 12
19 Injury, Poisoning & Certain Other Consequences of External Causes 2587 42970 1
20 External Causes or Morbidity (new chapter) 1291 7518 3
21 Factors Influencing Health Status & Contact with Health Services (new chapter) 1109 1549 20



In ICD-9-CM, code range 800-999 has 2,587 codes, while the corresponding section in the FY 2012 draft version of ICD-10-CM (code range S00-T98) has 42,970 code options. Yes, that is right: a volume increase of 40,383 codes (or, stated another way, a 1,561 percent increase over the number of code options available in ICD-9-CM). That is more than half of the total codes in the entire ICD-10-CM code set. So essentially, by analyzing and learning just one chapter out of the 21 included in ICD-10-CM, you have just mastered more than half of the codes in the new code set. Not a bad plan of attack, I’d say! Let’s keep going…


The codes in Chapter 19 are used to report a variety of injuries, such as burns and traumatic fractures, poisonings, adverse effects of drugs and a new category, “underdosing.” Let’s begin with the “injuries” category.


When coding injuries using ICD-10-CM, the first axis of classification is the body region affected (such as head, neck, thorax, etc.) Unlike in ICD-9-CM, injuries in ICD-10-CM are grouped by body part affected, not by categories of injury. For example, different injuries of the hand are all grouped together. The second axis of classification of injuries is the specific type of injury (such as superficial, open wound, fracture, dislocation and sprain, injuries to nerves, blood vessels, muscle and tendon damage, crushing injuries, traumatic amputation, other or unspecified injuries, etc.) Laterality is used for most codes in Chapter 19, such as injuries, and a seventh character is used to report the episode of care (such as initial, subsequent or sequel).


Example: S31.123A Laceration of abdominal wall with foreign body, right upper quadrant without penetration into peritoneal cavity


Burns in ICD-10-CM are classified by heat source – thermal (T20-32) vs. sunburn (L55.-) vs. radiation (L55- L59) – in addition to anatomical site.


Example: T23.231A, Burn of second degree of multiple right fingers, not including thumb, initial encounter


The coding of traumatic fractures exposes us to a combination of axes of classification, such as:


  • Specific anatomical site (less specified or unspecified code options are still available).
  • Type of fracture: displaced or nondisplaced. Displaced is the default.
  • Seventh digits used to capture whether a fracture is “open” or “closed” (
  • “closed” is the default).


Open fractures also require documentation of Gustilo Type I, II, IIIA, IIIB, IIIC.


  • Episode of care, with anywhere from 3-16 options available to report .
    • Initial, subsequent or sequela.
      • A subsequent episode of care also requires documentation of:
        • Routine healing
        • Delayed healing
        • Nonunion
        • Malunion
  • Fracture of surgical neck of the humerus, which is further specified as to whether the fracture is a two-, three- or four-part fracture.
  • Fracture of humeral shaft by fracture type, such as greenstick, transverse, oblique, spiral, comminuted, segmental, other or unspecified.


Example: S72.044A, Nondisplaced fracture of base of the neck of right femur, initial encounter


Poisonings by and adverse effects of drugs, medical and biological substances are combined under a single category, listed by specific drug(s) involved. Underdosing, defined as taking less of a medication than is prescribed by a provider or manufacturer’s instructions with negative health consequences, is added as new terminology and combined with poisonings by and adverse effects of drugs, medical and biological substances by specific drug.


Example: T40.1x1A, Poisoning by heroin, accidental (unintentional), initial encounter


This provides a quick overview of Chapter 19 in ICD-10-CM. If you approach each chapter in a similar manner – by focusing on the use of axes of classification – learning ICD-10-CM becomes a little less daunting. By comparing your current documentation to the axes of classification available for the same diagnoses in ICD-10-CM, you will be able to identify those areas where current documentation is inadequate to fully take advantage of the granularity available in ICD-10-CM.


About the Author


Angela Carmichael, MBA, RHIA, CCS, CCS-P, is director of HIM compliance for J.A. Thomas & Associates. Angela earned a Bachelor of Science degree, in Health Services Administration from Barry University and a MBA from Nova Southeastern University. She is a Registered Health Information Administrator and also has achieved the designations of Certified Coding Specialist, and Certified Coding Specialist-Physician and AHIMA Approved ICD-10-CM/PCS Instructor.


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