ICD-10-CM: The Beginner's Guide
Everything a new medical coder needs to understand the ICD-10-CM system — explained plainly, no textbook required.
1. What is ICD-10-CM?
ICD-10-CM stands for the International Classification of Diseases, 10th Revision, Clinical Modification. It is a system of codes published by the US Centers for Medicare & Medicaid Services (CMS) that assigns a unique alphanumeric code to every known disease, condition, injury, and health problem.
Medical coders use these codes to translate a doctor's documentation into standardised codes that insurance companies use to process and pay claims. If the wrong code is used, the claim gets denied — and the provider doesn't get paid.
2. How to read a code
Every ICD-10-CM code follows the same structure:
Chapter letter
Category
Subcategory / etiology
Codes are grouped into 22 chapters. The first character tells you which body system or cause of disease is involved. For example, codes starting with E are endocrine diseases, I are circulatory diseases, S are injuries.
3. Billable vs non-billable codes
Not every code can be submitted on a claim. Header codes (shown in grey on this site) are parent categories — they are too broad to submit. You must always code to the highest level of specificity.
4. Includes, Excludes1, and Excludes2
These instructional notes appear throughout the Tabular List. They are not optional — ignoring them leads to rejected claims.
Lists additional conditions or alternate names that ARE covered by this code. Reading these saves time — you might already have the right code.
These two conditions can never be coded together. They are mutually exclusive (e.g., an acquired form vs. a congenital form of the same condition). If you see Excludes1, pick one code or the other — never both.
The excluded condition is a different condition from the one coded here, but a patient can have both at the same time. When an Excludes2 note applies, it is acceptable to use both codes together.
5. Code First / Use Additional Code
Some conditions have both an underlying cause (etiology) and a body system manifestation. ICD-10-CM requires you to code both — in the right order.
Example: Diabetic neuropathy
Step 1: Code the underlying disease first — E11.40 (Type 2 diabetes with diabetic neuropathy)
Step 2: Use additional code for the specific neuropathy — G63 (polyneuropathy in diseases classified elsewhere)
Sequencing errors are one of the top reasons claims are denied.
6. 7th characters and placeholder X
Many injury codes (especially in Chapter 19) require a 7th character to indicate the type of encounter:
(active treatment)
(routine healing)
(late effect)
Placeholder X rule:
If a code is shorter than 6 characters but requires a 7th character, fill the gap with the letter X. Example: S32.49 (5 characters) becomes S32.49XA for initial encounter. The X is a placeholder — it has no meaning, it just holds the position.
7. How to use the Tabular List
The Tabular List is the official reference organised by code number. When looking up a code, always read up as well as down — instructional notes at the chapter level and category level apply to every code beneath them.
On this site, we replicate the Tabular List experience — every code page shows includes, excludes, use-additional, and code-first notes exactly as they appear in the official CMS file.
Pro tip: Use the Browse by Chapter feature to navigate the full code hierarchy just like the physical book — or use the Alphabetic Index Encoder to start with a condition name and drill down to the code.
8. 5 most common beginner mistakes
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