2.1 ICD-10-CM Structure & Official Guidelines
Key Takeaways
ICD-10-CM codes can be 3-7 characters long: the first character is always a letter, characters 2-3 are numeric, and characters 4-7 can be alphanumeric.
The ICD-10-CM code book has two main parts: the Alphabetic Index (used first to look up terms) and the Tabular List (used to verify and assign the final code).
Never code directly from the Alphabetic Index alone — always verify in the Tabular List for complete code selection and instructional notes.
Placeholder character "X" is used to fill empty positions when a 7th character extension is required but the code has fewer than 6 characters.
The 7th character extension is used in certain chapters (e.g., injuries, fractures) to indicate the type of encounter: A (initial), D (subsequent), S (sequela).
Laterality is indicated by specific characters within the code: typically 1 = right, 2 = left, 3 = bilateral, and 9 = unspecified.
The Official Coding Guidelines (OCG) are mandatory rules published by CMS and NCHS that govern how ICD-10-CM codes are selected and sequenced.
Coders must follow the convention hierarchy: instructional notes at the code level override category-level notes, which override chapter-level notes.
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.
ICD-10-CM Code Structure
Code Format
ICD-10-CM codes consist of 3 to 7 characters:
| Position | Character Type | Example (S52.301A) |
|---|---|---|
| 1st character | Always a letter (A-Z, except U) | S |
| 2nd character | Always numeric (0-9) | 5 |
| 3rd character | Always numeric (0-9) | 2 |
| Decimal point | Placed after the 3rd character | . |
| 4th character | Alphanumeric | 3 |
| 5th character | Alphanumeric | 0 |
| 6th character | Alphanumeric | 1 |
| 7th character | Alphanumeric (extension) | A |
Example: S52.301A = Unspecified fracture of shaft of right radius, initial encounter for closed fracture
Code Hierarchy
| Level | Characters | Example | Description |
|---|---|---|---|
| Chapter | Based on 1st character | S00-T88 | Injury, poisoning (Chapter 19) |
| Block/Category | 3 characters | S52 | Fracture of forearm |
| Subcategory | 4-5 characters | S52.3 | Fracture of shaft of radius |
| Code | Up to 7 characters | S52.301A | Full specificity with extension |
Placeholder Character "X"
The placeholder "X" serves two purposes:
- Filling empty character positions when a 7th character is required but the code has fewer than 6 base characters
- 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.
7th Character Extensions
Certain ICD-10-CM chapters require a 7th character to indicate the type of encounter:
| 7th Character | Meaning | When to Use |
|---|---|---|
| A | Initial encounter | First active treatment for the condition |
| D | Subsequent encounter | Follow-up care after initial treatment has been completed |
| S | Sequela | Late effect or complication of a previously treated condition |
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:
| 7th Char | Meaning |
|---|---|
| A | Initial encounter for closed fracture |
| B | Initial encounter for open fracture type I or II |
| C | Initial encounter for open fracture type IIIA, IIIB, or IIIC |
| D | Subsequent encounter for closed fracture with routine healing |
| G | Subsequent encounter for closed fracture with delayed healing |
| K | Subsequent encounter for closed fracture with nonunion |
| P | Subsequent encounter for closed fracture with malunion |
| S | Sequela |
Laterality
ICD-10-CM includes laterality designations to identify which side of the body is affected:
| Digit Value | Meaning |
|---|---|
| 1 | Right |
| 2 | Left |
| 3 | Bilateral |
| 9 | Unspecified |
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.
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
Official Coding Guidelines (OCG) — Key Rules
The OCG are mandatory guidelines that every coder must follow:
General Guidelines
- Code to the highest level of specificity — Use the most specific code available based on documentation
- Code only confirmed diagnoses — Do NOT code suspected, probable, or rule-out diagnoses in outpatient settings (code the signs/symptoms instead)
- Code what is documented — Coders cannot interpret clinical data or assume diagnoses
- Acute vs. chronic conditions — When both exist, code both; sequence the acute condition first if it is the reason for the encounter
- Late effects (sequelae) — Code the sequela condition first, followed by the cause code with 7th character "S"
- Signs and symptoms — Code signs and symptoms when no definitive diagnosis has been established
A patient presents for a follow-up visit for routine healing of a closed femoral shaft fracture that was treated with internal fixation 6 weeks ago. Which 7th character extension should be used?
A — Initial encounter
B — Initial encounter for open fracture
D — Subsequent encounter for closed fracture with routine healing
S — Sequela
In the code T36.0X1A, what does the "X" represent?
An external cause code indicator
A placeholder character to maintain correct code length
The letter X representing a specific drug category
An excludes notation requiring additional research
A patient is seen in the office for a right wrist fracture. The operative report and documentation clearly state "right." The coder should:
Use the unspecified laterality code since wrist laterality is not clinically significant
Use the laterality code for right, as documentation supports it
Query the physician to confirm laterality before coding
Use the bilateral code to cover both possibilities
Which of the following are TRUE about the ICD-10-CM Alphabetic Index and Tabular List? (Select all that apply)
Select all that apply
The Alphabetic Index should be consulted first to locate the main term
Codes from the Alphabetic Index can be reported directly without verification
The Tabular List contains instructional notes such as Includes, Excludes1, and Excludes2
The Neoplasm Table is part of the Alphabetic Index
The Tabular List is organized alphabetically by disease name
In ICD-10-CM, an Excludes___ note means the conditions listed are mutually exclusive and CANNOT be coded together.
Type your answer below
In an outpatient setting, a physician documents "suspected pneumonia." According to the Official Coding Guidelines, the coder should:
Code the confirmed diagnosis of pneumonia (J18.9)
Code the signs and symptoms that prompted the evaluation
Code both the suspected pneumonia and the symptoms
Query the physician to confirm the diagnosis before coding anything
Which ICD-10-CM convention indicates that the listed conditions are NOT included in the code but MAY be coded together if documented?
Includes note
Excludes1
Excludes2
Code first
Sections you finish are checked off in the contents.