ICD-10-CM coding conventions are the instructional rules, notes, punctuation, and abbreviations in the ICD-10-CM code book that tell coders how to select and sequence diagnosis codes. They are part of the ICD-10-CM Official Guidelines for Coding and Reporting (Section I.A). In addition, HIPAA requires coders to follow them. Below you will find each convention explained in plain language, followed by guidelines and practical tips.
ICD-10-CM Coding Conventions, Guidelines & Tips: A Comprehensive Overview
Medical coding is a crucial aspect of the healthcare system. It helps ensure accurate documentation and reimbursement for the services that providers render. In the United States, coders use ICD-10-CM (International Classification of Diseases, 10th Revision, Clinical Modification) for diagnosis coding. This system provides standardized codes that represent various diseases, conditions, and injuries for medical billing, statistical purposes, and clinical research. Understanding ICD-10-CM coding conventions, guidelines, and best practices is essential for accurate coding and optimal reimbursement. Below is an in-depth exploration of ICD-10-CM coding conventions, guidelines, and practical tips.
ICD-10-CM Coding Conventions
ICD-10-CM conventions are standard rules that guide coders as they select and assign the correct diagnosis codes. These conventions help ensure the integrity of the coding system and uniformity in diagnosis reporting.
Key ICD-10-CM Coding Conventions
Code Structure and Format
ICD-10-CM codes are alphanumeric, with a maximum of 7 characters. The first character is always a letter, and the second is a number. Characters 3 through 7 can be letters or numbers, and the 7th character adds detail when a code requires it. For example:
Example: E11.9 (Type 2 diabetes mellitus without complications)
Example with 7th character: S06.0X0A (Concussion without loss of consciousness, initial encounter)
ICD-10 Steps to Code Medical Coding Steps For ICD-10-CM
Alphabetical Index
The alphabetical index is the first step in selecting a code. It lists entries alphabetically by disease, condition, or symptom. After locating the disease or condition in the index, the coder must verify the code in the Tabular List.
Tabular List
The Tabular List is a structured list of codes arranged by category. It begins with broad disease groups and narrows down to specific conditions. Always consult this list after finding a code in the alphabetical index, because it provides the full code description and inclusion/exclusion notes.
Excludes Notes
ICD-10-CM includes two types of Excludes notes:
Excludes 1: This means “not coded here.” The two conditions cannot occur together, so do not report the codes together. One exception applies: when the two conditions are unrelated to each other, you may report both.
Excludes 2: Coders may use these codes together if the conditions are not related. In these cases, the coder can assign both codes, but the two conditions must be distinct and separate.
Brackets [ ] and Parentheses ( )
Brackets [ ] enclose synonyms, alternative wording, or explanatory phrases in the Tabular List. For example: J00 Acute nasopharyngitis [common cold]. In the Alphabetic Index, italicized brackets show a manifestation code that follows the underlying condition.
Parentheses ( ) enclose nonessential modifiers. In other words, the condition or description within parentheses does not affect the selection of the code.
Colons (:)
A colon (:) appears in the Tabular List after an incomplete term. The term needs one or more of the modifiers that follow the colon before it fits the category. As a result, coders confirm that the documentation includes one of those modifiers before they assign the code.
Includes/Excludes Notes
Includes: Specifies the conditions that a code category covers.
Excludes: Specifies conditions that a code category does not cover. It also helps coders decide whether to select a different code.
Seventh Character
Some ICD-10-CM codes require a seventh character, which applies primarily to injuries, fractures, and certain conditions that have a defined progression. The seventh character provides additional detail about the encounter, such as:
A: Initial encounter
D: Subsequent encounter (routine care during healing or recovery)
S: Sequela (a condition that results directly from the injury)
ICD-10-CM Coding Guidelines
ICD-10-CM coding guidelines provide instructions and rules that coders must follow to ensure correct diagnosis coding. The guidelines help coders understand how to apply the codes accurately in various clinical contexts.
Key ICD-10-CM Coding Guidelines
Principal Diagnosis
The principal diagnosis is the condition established after study to be chiefly responsible for the patient’s admission to the hospital. Therefore, list it first on the claim form. If two or more conditions equally meet this definition, either one may be sequenced first, unless the circumstances of admission or the guidelines indicate otherwise.
Unspecified Codes
Whenever possible, avoid using unspecified codes (such as codes ending in “.9”) because they are vague and do not provide enough information. Use unspecified codes only when the documentation does not specify a more detailed diagnosis.
Medical Coding Unlisted and Unspecified Codes Medical Coding Unlisted and Unspecified Codes
Coding for Acute and Chronic Conditions
When the same condition is documented as both acute (new, short-term) and chronic (long-lasting), and the Index lists separate subentries at the same level, code both and sequence the acute code first. However, some conditions have a combination code instead. For example, J44.0 reports COPD with an acute lower respiratory infection, and you also code the infection, such as J20.9 for acute bronchitis.
Acute vs Chronic in Medical Coding
Use of Laterality
ICD-10-CM includes codes that specify laterality for conditions affecting the left or right side of the body. Always ensure that the code indicates the correct side (e.g., left hip pain vs. right hip pain).
Multiple Conditions
When a patient has multiple conditions that require treatment or are relevant to the encounter, code all related conditions. However, the coder must determine which conditions the provider treated during the encounter and code those first.
Combination Codes
A combination code applies when a single code describes multiple aspects of a condition (e.g., a condition with associated complications). For example, E11.9 refers to Type 2 diabetes without complications, while E11.65 refers to Type 2 diabetes with hyperglycemia. Using combination codes can reduce the number of codes needed.
Coding for Sequelae
Sequelae are the aftereffects of a disease or condition. When coding sequelae, generally code the residual condition first, followed by the code for the cause of the sequela. However, some categories use combination codes instead. For example, hemiplegia after a stroke uses a code from category I69, such as I69.351, rather than the acute stroke code.
Medical Coding Sequela Medical Coding Sequela
External Causes Codes
Use codes that describe external causes of injury or diseases, such as V00-Y99 codes, together with codes that describe the injury or condition. External causes codes help provide additional details such as how the injury occurred.
ICD-10 External Cause Codes for Medical Coding ICD-10 External Cause Codes for Medical Coding
Z Codes
Z codes describe reasons for encounters other than illness or injury. These include screening, preventive care, or issues related to pregnancy and childbirth. For example, Z12.31 describes a screening mammogram for breast cancer.
Medical Coding Z Codes ICD-10-CM Z Codes 2026: SDOH, Homelessness, Unemployment, Sequencing & Billing Guide
ICD-10-CM Coding Tips
Always Consult the Tabular List
After finding a code in the alphabetical index, always verify it in the Tabular List for the correct coding description, as well as for any important inclusion or exclusion notes.
Ensure Proper Code Sequencing
When coding multiple diagnoses, always sequence the codes in order of importance. Then list the primary diagnosis first. Follow payer-specific guidelines for sequencing when applicable.
Sequencing in Medical Coding Sequencing in Medical Coding 2026
Review the Medical Record Thoroughly
Ensure that the medical record documentation supports all diagnoses and conditions. If the documentation lacks specificity, ask the provider for clarification to avoid coding errors.
Stay Updated with Coding Changes
ICD-10-CM codes and guidelines may change annually, typically on October 1st. Be sure to stay informed about updates and revisions through official sources such as CMS and the National Center for Health Statistics (NCHS). Also, follow professional resources like the American Health Information Management Association (AHIMA) and AAPC.
Avoid Over-Coding or Under-Coding
Over-coding means using a higher-level code than the documentation supports. As a result, it can lead to compliance issues.
Under-coding means failing to capture all diagnoses and conditions. Consequently, it can result in lower reimbursement and failure to meet payer requirements.
Medical Coding Bundling and Upcoding Medical Coding Bundling and Upcoding
Use Specific Codes When Available
Always use the most specific code possible. Avoid using unspecified codes unless no additional information is available.
In summary, ICD-10-CM coding is a critical aspect of medical billing and coding. It ensures accurate documentation of diagnoses for proper reimbursement and quality reporting. By adhering to the ICD-10-CM coding conventions and guidelines, medical coders can help reduce errors and ensure compliance. They also contribute to the smooth functioning of healthcare billing. Finally, regularly reviewing coding guidelines, staying updated with revisions, and carefully consulting the coding resources will enhance the accuracy and efficiency of the coding process
Medical Coding and ICD-10-CM Coding Guidelines Medical Coding and ICD-10-CM Coding Guidelines
ICD-10-CM 2025 Guidelines CMS FY 2027 ICD-10-CM Official Guidelines
ICD-10-CM Instructional Notes: Quick Reference
- Includes and inclusion terms: clarify or give examples of the conditions a code or category covers. The list is not exhaustive.
- Excludes1: means “not coded here.” The two conditions cannot occur together, so do not report the codes together. Read more in Excludes1 explained.
- Excludes2: means “not included here.” The excluded condition is not part of this code. However, the patient can have both, so you may report both codes. See Excludes2 explained.
- Code first and Use additional code: used for etiology and manifestation pairs. Sequence the underlying condition first. Manifestation codes appear in brackets in the Alphabetic Index, and you cannot report them as the first listed diagnosis.
- Code also: you may need two codes to fully describe the condition. Sequencing depends on the circumstances of the encounter.
- “And”: means “and/or” in a code title.
- “With” or “in”: means “associated with” or “due to.” ICD-10-CM presumes a causal relationship between the two conditions unless the provider documents that they are unrelated.
- See and See also: “See” directs you to another main term you must use. “See also” points to another main term that may provide more options.
- Default code: the code listed next to a main term in the Alphabetic Index is the default when documentation gives no further detail.
- Placeholder X: used to fill empty character positions when a code requires a 7th character but has fewer than six characters.
- NEC and NOS: NEC (not elsewhere classifiable) means the documentation is specific but ICD-10-CM has no specific code. NOS (not otherwise specified) means the documentation is not specific, which is the same as “unspecified.”
Conventions work hand in hand with the official guidelines. For a plain language summary of the full guidelines, see our ICD-10-CM Official Guidelines overview and the step by step ICD-10-CM coding process.
ICD-10-CM Conventions FAQs
What is the difference between Excludes1 and Excludes2?
Excludes1 means you cannot report the two conditions together. Excludes2 means the code does not include the condition. However, you can report both codes when the patient has both conditions.
What is the difference between NEC and NOS?
Coders use NEC when the provider is specific but no specific code exists. In contrast, coders use NOS when the documentation itself is not specific.
What does “with” mean in ICD-10-CM?
“With” links two conditions and presumes a causal relationship, such as diabetes with a complication, unless the provider states the conditions are unrelated.
Where are the ICD-10-CM conventions found?
They are in Section I.A of the ICD-10-CM Official Guidelines for Coding and Reporting, printed at the front of every ICD-10-CM code book.