Understanding ICD-10-CM Codes: A Complete Guide for Medical Coders
- Jan 15, 2025
ICD-10-CM (International Classification of Diseases, 10th Revision, Clinical Modification) is the official diagnostic coding system used in the United States for medical billing, insurance claims, and healthcare reporting. Whether you're a medical coder, biller, or healthcare administrator, this comprehensive guide covers everything you need to know about ICD-10-CM codes.
What is ICD-10-CM?
ICD-10-CM codes are alphanumeric codes used to document patient diagnoses on medical claims submitted to Medicare, Medicaid, and private insurance payers. The coding system replaced ICD-9-CM on October 1, 2015, expanding from approximately 14,000 codes to over 70,000 codes for greater clinical specificity.
Key uses of ICD-10-CM codes:
- Medical billing and insurance reimbursement
- Healthcare statistics and epidemiology
- Clinical decision support
- Quality reporting and value-based care programs
- Public health surveillance
ICD-10-CM Code Structure
ICD-10-CM codes are 3 to 7 characters long and follow a consistent format:
- First character: Always a letter (A-Z, except U which is reserved)
- Second character: Always a number (0-9)
- Third character: A number or letter identifying the category
- Characters 4-7: Provide etiology, anatomic site, severity, and other clinical detail
Code Example: Type 2 Diabetes with Hyperglycemia
The code E11.65 breaks down as:
- E = Chapter 4: Endocrine, nutritional and metabolic diseases
- 11 = Type 2 diabetes mellitus category
- .65 = With hyperglycemia
ICD-10-CM Chapter Overview
ICD-10-CM organizes diagnosis codes into 21 chapters by body system or condition type:
| Chapter | Code Range | Description |
|---|---|---|
| 1 | A00-B99 | Certain infectious and parasitic diseases |
| 2 | C00-D49 | Neoplasms |
| 3 | D50-D89 | Diseases of the blood and immune mechanism |
| 4 | E00-E89 | Endocrine, nutritional and metabolic diseases |
| 5 | F01-F99 | Mental, behavioral and neurodevelopmental disorders |
| 6 | G00-G99 | Diseases of the nervous system |
| 7 | H00-H59 | Diseases of the eye and adnexa |
| 8 | H60-H95 | Diseases of the ear and mastoid process |
| 9 | I00-I99 | Diseases of the circulatory system |
| 10 | J00-J99 | Diseases of the respiratory system |
| 11 | K00-K95 | Diseases of the digestive system |
| 12 | L00-L99 | Diseases of the skin and subcutaneous tissue |
| 13 | M00-M99 | Diseases of the musculoskeletal system |
| 14 | N00-N99 | Diseases of the genitourinary system |
| 15 | O00-O9A | Pregnancy, childbirth and the puerperium |
| 16 | P00-P96 | Certain conditions originating in the perinatal period |
| 17 | Q00-Q99 | Congenital malformations and chromosomal abnormalities |
| 18 | R00-R99 | Symptoms, signs and abnormal clinical findings |
| 19 | S00-T88 | Injury, poisoning and certain other consequences of external causes |
| 20 | V00-Y99 | External causes of morbidity |
| 21 | Z00-Z99 | Factors influencing health status and contact with health services |
Browse all chapters from our ICD-10-CM index.
Billable vs Non-Billable ICD-10-CM Codes
Understanding the difference between billable and non-billable codes is critical for proper claims submission:
Billable codes (also called "valid for submission"):
- Are the most specific codes available
- Can be submitted on insurance claims
- Have no further subdivisions
- Example: J06.9 - Acute upper respiratory infection, unspecified
Non-billable codes (header or category codes):
- Require additional characters for specificity
- Cannot be submitted on claims
- Serve as groupings for related codes
- Example: J06 - Acute upper respiratory infections of multiple and unspecified sites
Rule: Always code to the highest level of specificity supported by the medical documentation.
7th Character Extensions for Injury Codes
Many codes in Chapter 19 (Injury codes S00-T88) require a 7th character extension to indicate the episode of care:
| Extension | Meaning | When to Use |
|---|---|---|
| A | Initial encounter | First visit for active treatment of the condition |
| D | Subsequent encounter | Follow-up care during healing or recovery phase |
| S | Sequela | Late effects or complications from the original injury |
Placeholder X Rule: If a code requires 7 characters but the base code has fewer than 6 characters, use the letter "X" as a placeholder.
Example: S52.001A - Unspecified fracture of upper end of right ulna, initial encounter
ICD-10-CM Coding Guidelines
Follow these official coding guidelines from CMS and NCHS:
- Code to the highest specificity - Select the code that most accurately describes the documented diagnosis
- Use combination codes - When a single code describes both the condition and a common manifestation or complication
- Understand Excludes notes:
- Follow sequencing instructions - "Code first" and "Use additional code" notes indicate proper ordering
- Apply laterality - Specify left, right, or bilateral when the code provides the option
- Document medical necessity - Ensure diagnosis codes support the services billed
How to Look Up ICD-10-CM Codes
Finding the correct ICD-10-CM code requires systematic searching:
- Use our ICD-10 code search to find codes by keyword
- Browse by chapter starting from the ICD-10-CM index
- Try our AI clinical note analyzer to automatically extract codes from physician documentation
- Check common codes by specialty for frequently used diagnoses
Frequently Asked Questions
How many ICD-10-CM codes are there?
The current ICD-10-CM code set contains approximately 72,000+ diagnosis codes, updated annually by CMS.
When was ICD-10-CM implemented?
ICD-10-CM was mandated for use in the United States on October 1, 2015, replacing the ICD-9-CM system.
What is the difference between ICD-10-CM and ICD-10-PCS?
ICD-10-CM is used for diagnosis coding in all healthcare settings. ICD-10-PCS (Procedure Coding System) is used only for inpatient hospital procedure coding.
How often is ICD-10-CM updated?
CMS releases annual updates effective October 1st each year, with new codes, revised codes, and deleted codes. Check our 2026 new codes page for the latest changes.
Who maintains ICD-10-CM?
ICD-10-CM is maintained by the National Center for Health Statistics (NCHS) and the Centers for Medicare & Medicaid Services (CMS).
Start Searching ICD-10-CM Codes
Ready to find the codes you need? Use FindICD10's powerful search tools:
- Search ICD-10-CM codes - Fast keyword search with instant results
- AI Clinical Note Analyzer - Extract codes automatically from clinical documentation
- Browse all chapters - Navigate the complete code hierarchy
- Common codes by specialty - Quick reference for your practice area