Billing & Coding Blog

Creation and structure of ICD 10 CM CODES


International Classification of Diseases-Tenth Revision-Clin­ical Modification (ICD-10-CM) was endorsed by the 43rd World Health Assembly in May 1990 and came into use in World Health Organization (WHO) Member States in 1994.

The United States is a member of WHO. The classification is the latest in a series, which has its origins in the 1850s. The first edition, known as the International List of Causes of Death, was adopted by the International Statistical Institute in 1893. WHO took over the responsibility for the ICD at its creation in 1948 when the sixth Revision, which included causes of morbidity for the first time, was published. The World Health Assembly adopted in 1967 the WHO Nomenclature Regula­tions that stipulate use of ICD in its most current revision for mortality and morbidity statistics by all Member States.

ICD is the foundation for the identification of health trends and statistics globally, and the international standard for reporting diseases and health conditions. It is the diagnostic classification standard for all clinical and research purposes. ICD defines the universe of diseases, disorders, injuries and other related health conditions, listed in a comprehensive, hierarchical fashion.

The National Center for Health Statistics (NCHS) devel­oped ICD-10-CM in consultation with a technical advisory panel, physician groups, and clinical coders to assure clinical accuracy and utility.

Structure of ICD-10-CM

ICD-10-CM is published in two sections:

  1. Alphabetic Index or Index to Diseases and Injuries: Diagnostic terms organized in alphabetic order for the disease descriptions in the Tabular List. In this curric­ulum, the terms Alphabetic Index and Index to Diseases and Injuries are used interchangeably.
  2. Tabular List: Diagnosis codes organized in sequential order and divided into chapters based on body system or condition.

Index to Diseases and Injuries

The Index to Diseases and Injuries (Alphabetic Index) is divided into sections and organized by main terms. The sections include:

  1. Index to Diseases and Injuries (Alphabetic Index)
  2. Table of Neoplasms
  3. Table of Drugs and Chemicals
  4. External Cause of Injuries Index

Main terms in the Alphabetic Index usually reference the disease, condition, or symptom. Sub terms modify the main term to describe differences in site, etiology, or clinical type. Main terms are bold in the Alphabetic Index. Sub terms add specificity to the main term. The code listed next to the main term is considered the default code. The default code represents the condition most commonly associated with the main term. As with all code assignment, always verify the default code in the Tabular List to assure proper reporting.

Tabular List

The Tabular List contains 21 chapters and contains categories, subcategories, and codes. Each character for all categories, subcategories, and codes may be either a letter or a number. Codes can be three, four, five, six, or seven characters. The 1st character of a category is a letter. The 2nd and 3rd characters may be either letters or numbers. Subcategories are either four or five characters and may be either letters or numbers. Codes are three, four, five, or six characters and the final character in a code may be either a letter or number. Certain categories have a 7th character extension (discussed later in this chapter).

There are symbols throughout the Tabular List to identify when a code requires an additional character.

At the beginning of each chapter, there may be excludes and includes notes that apply to the entire chapter.

The 4th character in an ICD-10-CM code further defines the site, etiology, and manifestation or state of the disease or condition. The four-character subcategory includes the three-character category plus a decimal with an additional character to further identify the condition to the highest level of specificity.

The 5th or 6th character sub classifications represent the most accurate level of specificity regarding the patient’s condition or diagnosis.

Certain ICD-10-CM categories have applicable 7th characters. The applicable 7th character is required for all codes within the category, or as the notes in the Tabular List instruct. The 7th character must always be in the 7th position. If a code is three, four, or five characters, but requires a 7th character extension, a placeholder X must be used to fill the empty characters.


Add Comment

For medical coding references please go to
By Coding
Billing & Coding Blog

Get in touch's Blog is a website for medical coding reference.

We are happy to post interesting topics for the medical coders community.