If you’re learning ICD-10-CM coding, one of the first things you’ll come across is the concept of coding conventions. These conventions are the “rules of the road” for how diagnosis codes are structured, written, and reported. Understanding them is essential for accurate medical coding, billing, and compliance.
In this article, we’ll break down the main ICD-10-CM conventions in simple terms, with examples to make them easy to remember.
What Are ICD-10-CM Conventions?
ICD-10-CM conventions are the official instructions built into the Alphabetic Index and Tabular List of the ICD-10-CM coding manual. They guide coders on how to correctly select and apply codes.
Think of them as built-in rules that apply across all chapters of the ICD-10-CM, no matter which diagnosis you’re coding.
Key ICD-10-CM Conventions You Must Know
1. Alphabetic Index and Tabular List
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Alphabetic Index: An A–Z listing of diagnoses and conditions. Includes special tables like the Table of Neoplasms and the Table of Drugs & Chemicals.
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Tabular List: Organized by body systems and conditions, broken down into chapters.
💡 Tip: Start with the Alphabetic Index, but always verify your choice in the Tabular List.
2. Code Structure and Format
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Codes can be 3 to 7 characters long.
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Categories = 3 characters (example: J20).
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Subcategories = 4–5 characters.
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Codes with 7th characters require placeholders (using “X”) if the code isn’t long enough.
3. Use of Codes for Reporting
Only complete codes can be reported—not just categories or subcategories.
4. Placeholder “X”
The letter X acts as a filler when a 7th character is required but the code isn’t long enough.
Example: Certain injury and poisoning codes require placeholders before the 7th character.
5. 7th Characters
These identify details like:
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Initial encounter
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Subsequent encounter
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Sequela (late effects)
6. Abbreviations
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NEC = “Not elsewhere classifiable” (other specified).
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NOS = “Not otherwise specified” (unspecified).
7. Punctuation Rules
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[ ] Brackets = Synonyms or explanatory phrases.
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( ) Parentheses = Words that may or may not appear without changing the code.
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: Colon = An incomplete statement that needs extra words to assign a code.
8. Other vs. Unspecified Codes
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Other (NEC): Used when detail is provided, but no exact code exists.
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Unspecified (NOS): Used when documentation doesn’t provide enough detail.
9. Excludes Notes
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Excludes1 = The two conditions cannot be coded together (unless clearly unrelated).
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Excludes2 = The two conditions can both be coded if appropriate.
10. Etiology and Manifestation
Some conditions require two codes:
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The underlying cause (etiology).
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The resulting condition (manifestation).
Example: Dementia due to Parkinson’s disease.
11. Default Codes
When a diagnosis is listed without extra detail, use the default code (the code shown first in the Alphabetic Index).
12. Code Assignment and Provider Documentation
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Code what the provider documents.
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Coders don’t assign diagnoses—they translate provider statements into the correct code.
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If documentation is unclear, query the provider.
Why These Conventions Matter
Mastering ICD-10-CM conventions helps you:
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Avoid coding errors that delay claims.
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Ensure compliance with CMS and insurance rules.
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Improve coding speed and accuracy.
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Support accurate data for research and public health.
Final Thoughts
Understanding ICD-10-CM conventions is the foundation of medical coding. Whether you’re preparing for the CPC exam, working as a professional coder, or training in healthcare documentation, these rules will guide your coding decisions every day.
By learning how to navigate the Alphabetic Index, use Excludes notes, and apply placeholders and 7th characters, you’ll be well on your way to accurate and compliant coding.
✅ Pro Tip: Keep a personal cheat sheet of the most common conventions, so you can reference them quickly while coding.
Official Guidelines & Resources for ICD-10-CM
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FY 2026 ICD-10-CM Official Guidelines (CMS PDF)
→ Published by CMS and NCHS as a companion to the official ICD-10-CM manual—that’s the definitive resource for coding conventions and rule updates Centers for Medicare & Medicaid Services. -
ICD-10-CM Central Hub (CDC / NCHS Website)
→ Provides direct access to the ICD-10-CM Browser Tool, coding guidelines, tabular/index lists, and downloadable files. This is the official location maintained by NCHS (CDC) CDC+1.
Disclaimer:
This blog post is for educational purposes only and does not replace the ICD-10-CM Official Guidelines for Coding and Reporting (FY 2026) or the official resources available on the CDC / NCHS ICD-10-CM website. Always consult the official sources for the most up-to-date coding rules, and follow official guidance when assigning codes. When documentation is unclear, be sure to query the provider.
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