How ICD-10-CM codes work (and how to read one)

ICD-10-CM (International Classification of Diseases, 10th Revision, Clinical Modification) is the diagnosis code set used for billing and reporting in the United States. It's maintained by the CDC's National Center for Health Statistics and CMS, and updated every October. The current release has roughly 74,000 billable codes plus around 23,000 category headers.

The anatomy of a code

Take E11.9 — 'Type 2 diabetes mellitus without complications':

• The first character is always a letter (the body system / chapter area).
• Characters 2–3 form the category — here E11, the family of type 2 diabetes codes.
• After the decimal point, characters 4–7 add etiology, anatomic site, severity and, for injuries, an encounter type (A, D or S).

So E11 is a category header, E11.9 is a specific billable code within it, and E11.65 ('with hyperglycemia') is a sibling. The dot is purely cosmetic — E119 and E11.9 are the same code.

What 'billable' means

Only leaf codes — the most specific codes with no further children — are 'valid for submission'. A category header like E11 is not billable on its own; you must code to the highest level of detail the record supports. Submitting a non-billable header is one of the most common reasons a claim is rejected.

Chapters

The code set is organized into 22 chapters by body system and cause — infectious diseases (A00–B99), neoplasms (C00–D49), endocrine and metabolic (E00–E89), injuries (S00–T88), external causes (V00–Y99), and so on. Chapters make the set navigable and are how coders orient themselves in an unfamiliar area.

Finding the right code

You usually start from a clinical description ('sprained ankle', 'essential hypertension') and work toward the specific billable code. Patientary lets you search by description or by code prefix, shows the chapter and hierarchy, and tells you immediately whether the result is billable.

For developers and agents

If you're building coding or claims software, you need code validation you can trust. Patientary's API returns the official description, chapter, billable flag and parent for any code, and validates codes in batch — the deterministic tool an AI coding agent should call before proposing a code.