How to read an ICD-10-CM code

Categories, subcategories, codes

All categories are 3 characters. A three-character category that has no further subdivision is equivalent to a code.

Subcategories are either 4 or 5 characters. Codes may be 3, 4, 5, 6 or 7 characters.

In FY2027 the classification has 98,403 lines. The table counts them by length: a billable line is a code you can report, a header groups the lines below it.

Billable or not

For reporting purposes only codes are permissible, not categories or subcategories, and any applicable 7th character is required.

215 of the 1,921 categories have no subdivision and are billable as they stand, like I10. Every other category is a header. On this site each code shows its status next to its title, with the edition and its dates.

One code from top to bottom

E11.9 sits in chapter 4, block E08-E13. Each level is a line of the Tabular List with its own title:

1. E11 Type 2 diabetes mellitus header
2. E11.9 Type 2 diabetes mellitus without complications billable

Notes that travel with a code

Instructional notes sit at the chapter, block, category or code where NCHS placed them and apply to everything below. Code pages here print the notes of every level above the code and say where each came from. Lines of the Tabular List carrying each kind of note in FY2027:

Index first, then the Tabular List

To select a code in the classification that corresponds to a diagnosis or reason for visit documented in a medical record, first locate the term in the Alphabetic Index, and then verify the code in the Tabular List.

The FY2027 Alphabetic Index has 6,307 main terms and 63,405 entries that give a code; 6,996 of those end in a dash, meaning more characters are needed, and the Tabular List decides the final code. The search box on the home page reads the same entries.

Medicare rows on a code page

Next to the NCHS text, each billable code shows what the CMS files of FY2027 list for it. None of this is part of ICD-10-CM, and payers apply their own rules.

• CC / MCC (MS-DRG v44 Appendix C): 15,113 codes are a CC and 3,357 an MCC. Either counts only as a secondary diagnosis, and not when the principal diagnosis is on the code’s PDX exclusion list; the code page gives the list number and its size. 8 codes of Appendix C Part 2 are an MCC only if the patient is discharged alive (marked †).
• Appendix B: the MDC and the MS-DRGs under which Appendix B lists the code.
• Code Editor: Medicare Code Editor v44 edits that name the code, such as an age or sex conflict or a manifestation code.
• POA: “Exempt” when the code is on the FY2027 list of codes exempt from present-on-admission reporting (38,399 codes); otherwise a POA indicator is reported.
• HCC V28: the payment HCC of the CMS-HCC V28 model in the 2027 Initial ICD-10-CM Mappings, which covers codes valid in FY2025–FY2026 (8,004 FY2027 billable codes are mapped). Codes new in FY2027 are pending until CMS maps them.

Quotes: ICD-10-CM Official Guidelines for Coding and Reporting FY 2027 (CMS and NCHS), by section. Counts: computed from the NCHS FY2027 files on this site's build.