ICD-10-CM Structure, Conventions, and Notes

ICD-10-CM is the diagnosis code set used in the United States to report why a patient received care and what conditions, symptoms, findings, injuries, or health factors were documented. For CBCS purposes, ICD-10-CM supports medical necessity, claim processing, reporting, and sequencing. The exam does not allow or require coding manuals as of 2024-09-24, and questions include needed coding information. Still, the candidate must understand how ICD-10-CM works because many questions test application of conventions rather than manual lookup. ICD-10-CM codes are alphanumeric.

Key Concepts

The first character is a letter, the second is a number, and later characters may be letters or numbers. A three-character category is the base. Additional characters add detail such as etiology, anatomic site, severity, laterality, episode of care, manifestation, or complication. Some valid codes have three characters. Others require four, five, six, or seven characters. A code is not complete unless it contains the required number of characters and follows the instructions for that category.

A decimal is placed after the third character when more characters are used, but the decimal is formatting, not a separate character.

The Alphabetic Index is used to locate a starting point by main term, subterm, and modifying term. However, the coder must verify the code in the Tabular List. The Tabular List contains the final instructions, exclusions, inclusion terms, sequencing notes, and specificity requirements. In exam questions, this may be represented by a code excerpt or a note that tells the candidate what is required. A common error is to stop at an index-style lead without checking the tabular instruction.

Another error is ignoring notes that appear above the specific code, such as notes at the chapter, block, category, or subcategory level.

Those notes apply to codes beneath them unless the structure indicates otherwise. Inclusion terms show examples of conditions included in a code. They are not necessarily exhaustive. An Includes note may define the content of a category. An Excludes1 note generally means the two conditions should not be coded together because they are mutually exclusive or one is included in the other. An Excludes2 note means the excluded condition is not part of the code, so both codes may be used together if the patient has both conditions. Code first notes and use additional code notes direct sequencing and complete reporting.

Workflow and Documentation

For example, a condition caused by an underlying disease may require the underlying disease to be sequenced before the manifestation. A use additional code note may instruct the coder to add a code for exposure, organism, tobacco use, insulin use, stage, or another detail when documented. In diseases classified elsewhere categories, the manifestation code often cannot be first-listed because it depends on the underlying condition. NEC and NOS are also important conventions.

NEC means not elsewhere classifiable and is used when the documentation provides detail but ICD-10-CM does not offer a more specific code for that detail.

NOS means not otherwise specified and is used when the documentation lacks detail needed for a more specific code. A CBCS candidate should understand that unspecified is not automatically wrong. It may be correct when the record truly lacks specificity and clarification is not available. However, unsupported unspecified coding can cause denials or reduce data quality when the documentation contains more detail. Placeholder X is used in some codes to hold character positions so a seventh character can be placed correctly.

This is common in injury and external cause codes. The placeholder is not optional when required.

Exam Application

Seventh characters often identify the episode of care, such as initial encounter, subsequent encounter, or sequela, but the meaning varies by chapter and category. The coder must apply the seventh character instructions supplied for that code family. Punctuation and terms matter. Parentheses contain nonessential modifiers, meaning the code may apply whether or not those words appear. Brackets may identify synonyms, alternative wording, or manifestation codes depending on context. The word and in code descriptions can mean and/or under ICD-10-CM conventions.

With can sometimes indicate a presumed causal relationship in specific index contexts or guideline areas, especially certain chronic disease combinations, but the coder must follow current guidelines and documentation. The practical CBCS method is: identify the documented condition or reason, locate the likely code, verify the complete code and required characters, read all notes, apply excludes and sequencing instructions, add required additional codes, and confirm the diagnosis supports the billed service.

High-Yield Checkpoints

• ICD-10-CM is used to report diagnoses, reasons for encounters, symptoms, abnormal findings, injuries, external causes, status, and other health factors.
• Codes are alphanumeric and may require three to seven characters depending on category instructions and specificity.
• The Tabular List, Alphabetic Index, instructional notes, inclusion terms, Excludes notes, code first notes, and use additional code notes guide code assignment.
• A placeholder X may be needed when a code requires a seventh character but has fewer than six characters before that extension.
• The coder should verify index leads in the Tabular List and follow all notes at the category, subcategory, and code levels.