ICD-10 Chapter Reference

ICD-10-CM chapter reference

ICD-10-CM (the United States Clinical Modification of the WHO’s ICD-10) organises every diagnosis code into 22 chapters. Each chapter covers a body system or category of conditions and owns a contiguous block of alphanumeric code ranges, such as A00-B99 for infectious diseases or I00-I99 for the circulatory system. This tool lists all 22 chapters and lets you type a code to see which chapter it falls in.

How it works

An ICD-10-CM code begins with a letter (the first character) followed by two digits, then an optional decimal and further characters. The first three characters — the “category” — fall inside a chapter’s range. To resolve a code, the lookup parses the leading letter and two digits, then walks the chapter list comparing that prefix against each range’s lower and upper bounds. For example I21 (acute myocardial infarction) sorts between I00 and I99, so it belongs to Chapter 9, Diseases of the circulatory system.

Note that a few ranges end on a letter-suffixed boundary like O00-O9A; the comparison treats those coarsely, so always confirm sub-block membership against the published code set.

All 22 ICD-10-CM chapters at a glance

Anomalies and non-obvious patterns

The chapter structure broadly follows alphabetical order, but several features surprise first-time users of the classification:

The letter H is split across two chapters. H00-H59 covers eye and adnexa conditions, while H60-H95 covers ear and mastoid conditions. These are completely different body systems sharing a single letter range because the classification ran low on available letters.

Chapter 22 (U codes) appears at the end despite U coming before V, W, X, Y alphabetically in the table. The U chapter is reserved for special purposes and provisional codes. U07.1 is the code for COVID-19 — added during the pandemic via the provisional mechanism that the U chapter exists to accommodate. U09.9 is the post-COVID condition code.

Chapters 19 and 20 cover injuries and their external causes respectively. A patient who fractures their wrist falling from a ladder needs both an S code (the injury) and a W code (the external cause). The external cause code does not appear in the chapter that contains the diagnosis; it is in a completely separate chapter.

ICD-10-CM versus related systems

• WHO ICD-10: The global version uses 3-digit categories rather than the 7-character codes ICD-10-CM can reach. Chapter ranges differ in some areas.
• ICD-10-PCS: A completely separate system for inpatient procedure coding, not diagnosis. PCS codes begin with a number (0-9) and use a 7-character alphanumeric structure unrelated to ICD-10-CM.
• ICD-11: The WHO’s current revision, adopted internationally but not yet used for US billing. ICD-11 has a different chapter structure and code format.

Tips and notes

• Letters are not all used left-to-right: chapter order broadly follows A-Z but some letters span two chapters (H00-H59 eyes vs H60-H95 ears) and some appear out of sequence (U00-U85 is the last chapter, “special purposes”, and contains U07.1 for COVID-19).
• ICD-10-CM differs from WHO ICD-10 and from ICD-10-PCS (inpatient procedures). This reference is the diagnosis (CM) chapter scheme.
• The code set is updated annually (effective 1 October in the US); verify ranges against the current fiscal-year files before billing.