ICD-10-CM Structure & Official Guidelines

The International Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM) is the diagnostic coding system mandated for use in the United States. Accurate ICD-10-CM coding is essential for reimbursement, clinical documentation, and public health tracking. The CPC exam devotes approximately 20-25% of its questions to ICD-10-CM coding.

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ICD-10-CM Code Structure

Code Format

ICD-10-CM codes consist of 3 to 7 characters:

Example: S52.301A = Unspecified fracture of shaft of right radius, initial encounter for closed fracture

Code Hierarchy

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Placeholder Character "X"

The placeholder "X" serves two purposes:

1. Filling empty character positions when a 7th character is required but the code has fewer than 6 base characters
2. Future expansion — reserving character positions for potential code additions

Example: T36.0X1A (Poisoning by penicillins, accidental, initial encounter)

• T36.0 = Poisoning by penicillins
• X = Placeholder for the 5th position
• 1 = Accidental (unintentional)
• A = Initial encounter

Rule: The placeholder X must be used; you cannot skip character positions to reach the 7th character.

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7th Character Extensions

Certain ICD-10-CM chapters require a 7th character to indicate the type of encounter:

Common misconception: "Initial encounter" does NOT mean the first visit — it means the patient is receiving active treatment. A patient can have multiple visits with 7th character A if active treatment is still being provided.

Fracture-Specific 7th Characters

Fractures have additional 7th character options:

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Laterality

ICD-10-CM includes laterality designations to identify which side of the body is affected:

Example:

• H40.1111 = Primary open-angle glaucoma, right eye, mild stage
• H40.1121 = Primary open-angle glaucoma, left eye, mild stage
• H40.1131 = Primary open-angle glaucoma, bilateral, mild stage

Coding Rule: Always code to the highest level of specificity. If the documentation states "right knee," use the code for right — do not use the unspecified laterality code.

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ICD-10-CM Book Organization

Alphabetic Index (Volume 2)

• Index to Diseases and Injuries — Main lookup tool, organized alphabetically by main term
• Neoplasm Table — Organizes neoplasm codes by anatomical site and behavior (malignant primary, secondary, in situ, benign, uncertain, unspecified)
• Table of Drugs and Chemicals — Lists substances with columns for poisoning (accidental, intentional self-harm, assault, undetermined), adverse effects, and underdosing
• External Cause Index — Index of external causes of morbidity

Tabular List (Volume 1)

• Contains all codes organized by chapter (21 chapters + supplementary)
• Includes instructional notes: Includes, Excludes1, Excludes2, Code first, Use additional code, Code also
• Excludes1: The conditions listed are mutually exclusive — they cannot be coded together
• Excludes2: The conditions listed are not included in the code but CAN be coded together if both are documented

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Official Coding Guidelines (OCG) — Key Rules

The OCG are mandatory guidelines that every coder must follow:

General Guidelines

1. Code to the highest level of specificity — Use the most specific code available based on documentation
2. Code only confirmed diagnoses — Do NOT code suspected, probable, or rule-out diagnoses in outpatient settings (code the signs/symptoms instead)
3. Code what is documented — Coders cannot interpret clinical data or assume diagnoses
4. Acute vs. chronic conditions — When both exist, code both; sequence the acute condition first if it is the reason for the encounter
5. Late effects (sequelae) — Code the sequela condition first, followed by the cause code with 7th character "S"
6. Signs and symptoms — Code signs and symptoms when no definitive diagnosis has been established