Every medical claim submitted to an insurance payer requires two types of codes — procedure codes that describe what was done, and diagnosis codes that describe why it was done. ICD-10-CM is the system used for diagnosis coding in the United States. Understanding how it works is just as important as understanding CPT, and it is tested heavily on the CPC exam. This guide introduces ICD-10-CM from the very beginning.
What Does ICD-10-CM Stand For?
ICD-10-CM stands for the International Classification of Diseases, 10th Revision, Clinical Modification. Let’s break that down:
- International Classification of Diseases: The ICD system was originally developed by the World Health Organization (WHO) to standardize the recording and reporting of diseases and health conditions worldwide
- 10th Revision: This is the tenth major update to the ICD system, implemented in the United States on October 1, 2015
- Clinical Modification: The CM designation means this is the U.S.-specific version, modified by the Centers for Disease Control and Prevention (CDC) and CMS for use in clinical settings and billing
Why Was ICD-10-CM Created?
The previous system, ICD-9-CM, had been in use since 1979 and had run out of room. With only about 14,000 codes, ICD-9 could not keep up with advances in medicine, new diseases, and the need for more specific documentation. ICD-10-CM solved these problems dramatically — it contains over 70,000 codes, allowing for a level of clinical specificity that was impossible under ICD-9.
More specificity means better data. Better data means more accurate tracking of disease patterns, more precise research, improved public health reporting, and more accurate payment for services. Specificity is the core purpose of ICD-10-CM.
ICD-10-CM vs ICD-9-CM — Key Differences
| Feature | ICD-9-CM | ICD-10-CM |
|---|---|---|
| Code structure | 3–5 characters, numeric with some letters | 3–7 alphanumeric characters |
| First character | Always a number (except E and V codes) | Always a letter |
| Total codes | ~14,000 | ~70,000+ |
| Laterality | Not specified in most codes | Specified — left, right, bilateral |
| Implementation date | 1979–2015 | October 1, 2015 to present |
| Specificity | Limited | Highly detailed |
How ICD-10-CM Codes Are Organized
The ICD-10-CM codebook is divided into two main parts — the Alphabetic Index and the Tabular List. Just like the CPT codebook, you always start in the index and verify in the tabular list — never code from the index alone.
The Alphabetic Index
The Alphabetic Index lists conditions, diseases, symptoms, injuries, and other health problems in alphabetical order. Main terms are listed alphabetically with indented subterms beneath them that narrow down the diagnosis. The index contains three sections: the Index to Diseases and Injuries, the Index to External Causes, and the Table of Neoplasms.
The Tabular List
The Tabular List organizes all ICD-10-CM codes numerically into 21 chapters based on body system or type of condition. Each chapter contains categories, subcategories, and codes with full descriptions, includes and excludes notes, and coding instructions.
The 21 Chapters of ICD-10-CM
| Chapter | Code Range | Content |
|---|---|---|
| 1 | A00–B99 | Infectious and parasitic diseases |
| 2 | C00–D49 | Neoplasms |
| 3 | D50–D89 | Blood and blood-forming organ diseases |
| 4 | E00–E89 | Endocrine, nutritional, metabolic diseases |
| 5 | F01–F99 | Mental, behavioral, neurodevelopmental disorders |
| 6 | G00–G99 | Nervous system diseases |
| 7 | H00–H59 | Eye and adnexa diseases |
| 8 | H60–H95 | Ear and mastoid process diseases |
| 9 | I00–I99 | Circulatory system diseases |
| 10 | J00–J99 | Respiratory system diseases |
| 11 | K00–K95 | Digestive system diseases |
| 12 | L00–L99 | Skin and subcutaneous tissue diseases |
| 13 | M00–M99 | Musculoskeletal and connective tissue diseases |
| 14 | N00–N99 | Genitourinary system diseases |
| 15 | O00–O9A | Pregnancy, childbirth, and puerperium |
| 16 | P00–P96 | Perinatal conditions |
| 17 | Q00–Q99 | Congenital malformations and chromosomal abnormalities |
| 18 | R00–R99 | Symptoms, signs, abnormal findings |
| 19 | S00–T88 | Injury, poisoning, external causes |
| 20 | V00–Y99 | External causes of morbidity |
| 21 | Z00–Z99 | Factors influencing health status (Z-codes) |