ICD-10-CM (International Classification of Diseases, Tenth Revision, Clinical Modification) is the standard diagnostic coding system used in the United States for medical claims, quality reporting, and public health surveillance. Understanding and correctly applying ICD-10 coding guidelines is essential for healthcare organizations to ensure accurate documentation, proper reimbursement, and regulatory compliance.
This comprehensive guide covers all essential ICD-10 coding rules, conventions, and best practices that every medical coder, clinical documentation improvement (CDI) specialist, and healthcare professional should know. Whether you're preparing for certification, auditing encounters, or implementing coding compliance programs, this resource provides the foundation you need.
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ICD-10 Coding Rules Covered in This Guide
Understanding ICD-10 Coding Guidelines
The ICD-10-CM Official Guidelines for Coding and Reporting are developed by the cooperating parties: the American Hospital Association (AHA), the American Health Information Management Association (AHIMA), CMS, and the National Center for Health Statistics (NCHS). These guidelines are updated annually to reflect changes in medical practice and coding conventions.
The guidelines are organized into four sections:
- Section I: Conventions, general coding guidelines, and chapter-specific guidelines
- Section II: Selection of principal diagnosis (inpatient)
- Section III: Reporting additional diagnoses (inpatient)
- Section IV: Diagnostic coding and reporting guidelines for outpatient services
Code First / Etiology-Manifestation Convention
Certain conditions have both an underlying etiology and multiple body system manifestations due to the underlying etiology. For such conditions, the ICD-10-CM has a coding convention that requires the underlying condition be sequenced first, if applicable, followed by the manifestation. Wherever such a combination exists, there is a "use additional code" note at the etiology code, and a "code first" note at the manifestation code. These instructional notes indicate the proper sequencing order of the codes: etiology followed by manifestation.
Key Principle
Manifestation codes with "in diseases classified elsewhere" in the title are never permitted to be used as first-listed or principal diagnosis codes. They must be used in conjunction with an underlying condition code and must be listed following the underlying condition.
In most cases, manifestation codes will have in the code title "in diseases classified elsewhere." Codes with this title are a component of the etiology/manifestation convention. The code title indicates that it is a manifestation code. There are also manifestation codes that do not have "in diseases classified elsewhere" in the title. For such codes, there is a "use additional code" note at the etiology code and a "code first" note at the manifestation code, and the rules for sequencing apply.
Alphabetic Index Structure
In addition to the notes in the Tabular List, these conditions also have a specific Alphabetic Index entry structure. In the Alphabetic Index, both conditions are listed together with the etiology code first followed by the manifestation codes in brackets. The code in brackets is always to be sequenced second.
Example: Dementia with Parkinson's Disease
An example of the etiology/manifestation convention is dementia with Parkinson's disease. In the Alphabetic Index, a code from category G20 is listed first, followed by code F02.80 or F02.81- in brackets. A code from category G20- represents the underlying etiology (Parkinson's disease) and must be sequenced first, whereas codes F02.80 and F02.81- represent the manifestation of dementia in diseases classified elsewhere, with or without behavioral disturbance.
"Code first" and "Use additional code" notes are also used as sequencing rules in the classification for certain codes that are not part of an etiology/manifestation combination.
Learn more about the Code First Rule →
Use Additional Code Convention
The Use Additional Code note indicates that a secondary code should be added to provide more complete information about a condition. Unlike Code First, this instruction appears on the etiology code, directing the coder to add manifestation or other related codes.
Common scenarios requiring additional codes include:
- Infectious agents causing diseases (use additional code to identify the organism)
- Conditions with associated manifestations
- External causes of morbidity
- Tobacco use, alcohol use, or BMI as applicable
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Excludes1 vs Excludes2
The ICD-10-CM has two types of excludes notes. Each type of note has a different definition for use, but they are all similar in that they indicate that codes excluded from each other are independent of each other.
Excludes1 - "NOT CODED HERE!"
A type 1 Excludes note is a pure excludes note. It means "NOT CODED HERE!" An Excludes1 note indicates that the code excluded should never be used at the same time as the code above the Excludes1 note. An Excludes1 is used when two conditions cannot occur together, such as a congenital form versus an acquired form of the same condition.
Exception to Excludes1
An exception to the Excludes1 definition is the circumstance when the two conditions are unrelated to each other. If it is not clear whether the two conditions involving an Excludes1 note are related or not, query the provider.
Example of Excludes1 Exception
Code F45.8 (Other somatoform disorders) has an Excludes1 note for "sleep related teeth grinding (G47.63)" because "teeth grinding" is an inclusion term under F45.8. Only one of these two codes should be assigned for teeth grinding. However, psychogenic dysmenorrhea is also an inclusion term under F45.8, and a patient could have both this condition and sleep related teeth grinding. In this case, the two conditions are clearly unrelated to each other, and so it would be appropriate to report F45.8 and G47.63 together.
Excludes2 - "NOT INCLUDED HERE"
A type 2 Excludes note represents "Not included here." An Excludes2 note indicates that the condition excluded is not part of the condition represented by the code, but a patient may have both conditions at the same time. When an Excludes2 note appears under a code, it is acceptable to use both the code and the excluded code together, when appropriate.
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Specificity Requirements
ICD-10-CM requires codes to be reported to the highest level of specificity documented in the medical record. This includes:
- Anatomical site: Specific body location
- Severity: Mild, moderate, severe
- Episode of care: Initial, subsequent, sequela
- Trimester: For obstetric codes
Using unspecified codes when more specific information is available in the documentation can lead to claim denials, quality measure issues, and audit findings.
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Laterality
Many ICD-10-CM codes include laterality to specify which side of the body is affected:
- Right
- Left
- Bilateral
- Unspecified (only when documentation doesn't specify)
Laterality is particularly important for conditions affecting paired organs and structures such as eyes, ears, lungs, kidneys, arms, and legs. When laterality is documented, the appropriate laterality code must be assigned.
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Combination Codes
Combination codes are single codes that classify:
- Two diagnoses
- A diagnosis with an associated secondary process (manifestation)
- A diagnosis with an associated complication
When a combination code exists that accurately identifies both the diagnostic conditions or a condition with an associated complication, only the combination code should be assigned. Multiple codes should not be used when a combination code clearly identifies all elements documented in the diagnosis.
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Mutual Conflicts and Code Validation
Beyond the official coding conventions, coders must also be aware of mutual conflicts between codes. These are situations where two codes in the same encounter create logical or clinical inconsistencies:
Gender Conflicts
Certain diagnosis codes are specific to one gender. Coding a male-only condition for a female patient (or vice versa) represents a gender conflict that will likely result in claim rejection.
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Active vs History Conflicts
A patient cannot have both an active condition and a personal history of that same condition in the same encounter. For example, coding both active breast cancer and history of breast cancer creates a conflict.
Learn more about Active vs History Conflicts →
Other Conflict Types
- Acquired vs Congenital: Conditions that cannot be both acquired and congenital
- Symptom vs Diagnosis: When a definitive diagnosis explains the symptom
- Mental Health Conflicts: Mutually exclusive psychiatric diagnoses
Staying Current with Annual Updates
ICD-10-CM codes and guidelines are updated annually, with changes typically effective October 1 each year. Staying current with these updates is essential for:
- Accurate claim submission
- Proper reimbursement
- Quality measure compliance
- Audit readiness
Tools for ICD-10 Validation
Given the complexity of ICD-10 guidelines and the numerous rules that must be applied simultaneously, automated validation tools have become essential for coding compliance. Validate against ICD 10 CM coding guidelines (FY2026) using our free Code Auditor, which checks encounter-level codes against all official rules. Need to find a specific diagnosis? Look up ICD 10 CM codes by keyword or description. The auditor validates against these rules:
- Code First / Use Additional Code conventions
- Excludes1 validations
- Specificity and Laterality requirements
- Combination code opportunities
- Gender, Active/History, and other mutual conflicts
Audit ICD-10 Codes Against Official Guidelines
Check up to 30 codes per encounter against ICD 10 CM official guidelines for 2024-2026.
Look up ICD 10 CM codes to find the right diagnosis codes before validation.