The First Step in Locating an ICD Code

The ICD-10-CM code set contains over 70,000 diagnosis codes, creating a vast and complex classification system that can overwhelm even experienced clinicians. Yet the fundamental principle of locating the correct code is remarkably straightforward. It begins with a single, non-negotiable rule: the first step in locating an ICD code is always to consult the Alphabetic Index.

This principle is foundational to ICD-10-CM coding and is consistently emphasised across all official coding guidelines and training materials. The Alphabetic Index, sometimes referred to as the Index to Diseases and Injuries, serves as the entry point into the classification system.

As the ICD-10-CM Official Guidelines for Coding and Reporting make clear, “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.” This is not a suggestion—it is a requirement of the coding system itself.

The Anatomy of the Three‑Step Process

Locating the correct ICD-10-CM code follows a structured three-step process. However, the critical first move is always the same.

Step 1: Begin in the Alphabetic Index. Locate the main term that describes the patient's condition. The Alphabetic Index is organised alphabetically, with main terms in bold type displayed on the far left margin and subterms indented beneath them. This index provides the entry point to the classification system, identifying potential code options based on the documented diagnosis.

Step 2: Verify the code in the Tabular List. Once a potential code is identified in the Alphabetic Index, turn to the Tabular List to confirm its accuracy and identify the highest level of specificity. This step is essential because the Alphabetic Index does not always provide the complete code.

Step 3: Review chapter-specific coding guidelines. The final step involves consulting the Official Coding Guidelines and any chapter-specific instructions that may affect code selection, sequencing, or reporting requirements.

Why the Alphabetic Index Must Be the Starting Point

The Alphabetic Index is designed as the gateway into the ICD-10-CM classification system for a reason: it organises the vast universe of medical terminology into a searchable, alphabetical structure that mirrors how clinicians actually document diagnoses.

When you begin in the Alphabetic Index, you are searching by the language of clinical documentation—the terms your clinicians actually use. This is far more intuitive than attempting to navigate the Tabular List, which is organised by body system and disease category. As one coding expert notes, “The Alphabetic Index does not always provide the full code. Selection of the full code, including laterality and any applicable 7th character can only be done in the Tabular List.”

How the Alphabetic Index Is Structured

The Alphabetic Index contains several distinct sections, each serving a specific purpose in the code‑location process:

• Index to Diseases and Injuries: The primary section where most diagnostic terms are located. Main terms appear in bold type, with subterms indented beneath them. Subterm entries are listed in alphabetical order, including those with numbers written in their spelled‑out form.

• Table of Neoplasms: A specialised table for locating neoplasm codes, organised by anatomical site and behaviour (malignant, benign, in situ, uncertain, unspecified).

• Table of Drugs and Chemicals: A reference for locating codes related to poisoning, adverse effects, and underdosing of substances.

• External Cause of Injuries Index: For locating codes that describe the cause of injury or poisoning.

Understanding the Dash (-) in the Alphabetic Index

One of the most critical aspects of using the Alphabetic Index is recognising the dash (-) that may appear at the end of an entry. This dash indicates that additional characters are required to complete the code.

For example, an Alphabetic Index entry ending with a dash signals that the code is incomplete and must be verified and completed in the Tabular List. Even if no dash appears, it remains essential to refer to the Tabular List to verify that no additional characters (such as a 7th character for episode of care) are required.

Common Pitfalls When Starting in the Alphabetic Index

Even experienced clinicians can make errors when beginning their code search. Here are the most common pitfalls to avoid:

1. Skipping the Alphabetic Index entirely: Some clinicians attempt to locate codes directly in the Tabular List using the search function or by browsing categories. This bypasses the structured entry point of the classification system and increases the risk of selecting an incorrect or incomplete code.

2. Stopping at the first code found: The Alphabetic Index may direct you to a less specific code or to a starting point in the Tabular List. It is essential to verify the code in the Tabular List and scan surrounding codes to ensure the most accurate level of detail has been obtained.

3. Coding from the Alphabetic Index alone: The Alphabetic Index is a finding aid, not a final authority. “You cannot code from the Alphabetic Index,” as coding experts emphasise. The complete code, including laterality and any applicable 7th character, can only be determined in the Tabular List.

4. Missing the dash (-) indicator: Failing to notice a dash at the end of an Alphabetic Index entry leads to incomplete codes and claim denials.

5. Failing to consult the Official Coding Guidelines: The guidelines provide additional instruction beyond what appears in the Alphabetic Index and Tabular List, including sequencing rules and reporting requirements.

Why This Matters for Clinicians

For mental health professionals who document diagnoses and may occasionally assign codes themselves, understanding this foundational step is essential for accurate documentation and defensible coding. The Alphabetic Index serves as the bridge between clinical language and the structured terminology of the ICD-10-CM classification system.

When clinicians document with precision, the Alphabetic Index can effectively guide them to the correct code. Vague or incomplete documentation—such as “anxiety” without specifying the type—makes it difficult to navigate the Alphabetic Index and increases the risk of selecting an incorrect or unspecified code.

Official Guidelines and Resources

The authoritative source for ICD-10-CM coding guidance is the ICD-10-CM Official Guidelines for Coding and Reporting, published annually by the Centers for Medicare & Medicaid Services (CMS) and the National Center for Health Statistics (NCHS). For the 2026 fiscal year (October 1, 2025 – September 30, 2026), these guidelines remain the definitive reference for proper code selection and sequencing.

These guidelines “complement the official conventions and instructions provided within the ICD-10-CM itself” and have been developed to assist both healthcare providers and coders in identifying diagnoses to be reported.

FAQ

Why can‘t I start my code search directly in the Tabular List?

The Tabular List is organised by body system and disease category, not by clinical terminology. Attempting to locate a code directly in the Tabular List without first consulting the Alphabetic Index is like trying to find a word in a dictionary without knowing how to spell it. The Alphabetic Index translates clinical language into the structured terminology of the classification system, providing the entry point to the correct code family. Even when using electronic search functions, experienced coders recommend starting with the Alphabetic Index to ensure accurate code selection.

What if I can‘t find the condition I’m looking for in the Alphabetic Index?

If a condition does not appear under its primary term, try searching under synonyms, alternative terms, or related conditions. The Alphabetic Index may also direct you to a main term that is not immediately obvious—for example, “depression” may be located under “depressive disorder” or a specific type such as “major depressive disorder.” If searching the Alphabetic Index yields no results, consult the Tabular List‘s table of contents to identify the relevant chapter and category, but this should be a last resort, not the first step.

What is the most common error clinicians make when locating ICD codes?

The most common error is attempting to code directly from the Alphabetic Index without verifying the code in the Tabular List. As coding experts note, “you cannot code from the Alphabetic Index” because it does not always provide the complete code. The Alphabetic Index may direct you to a less specific code or to a starting point in the Tabular List, but only the Tabular List can confirm the full code, including laterality and any required 7th character.

What does the dash (-) at the end of an Alphabetic Index entry mean?

A dash at the end of an Alphabetic Index entry indicates that additional characters are required to complete the code. The Alphabetic Index provides an incomplete code that must be verified and completed in the Tabular List. Even when no dash appears, it is still essential to refer to the Tabular List to verify that no additional characters (such as a 7th character for episode of care) are required.

Why are the Official Coding Guidelines necessary if I have the Alphabetic Index and Tabular List?

The Official Coding Guidelines provide additional instruction beyond what appears in the Alphabetic Index and Tabular List. They address complex coding scenarios, sequencing rules, reporting requirements, and chapter-specific guidance that may not be immediately apparent from the code books alone. For example, the guidelines clarify when a diagnosis may be reported as a principal diagnosis versus a secondary diagnosis, and when additional codes are required to fully capture the clinical picture. Failing to consult the guidelines can result in incorrect code assignment and claim denials.

References

1. CDC. (2025). ICD-10-CM Official Guidelines for Coding and Reporting: FY 2026.

2. Woolstenhulme, C. (2015). Locating a code in ICD-10-CM. Find-A-Code.

3. Edwards, B. (2015). Rules of the ICD-10 Road. AAPC.

4. AAP News. (2015). ICD-10-CM: back to the basics. American Academy of Pediatrics.

5. Medical Billers and Coders. (2023). General Coding Guidelines for ICD-10-CM.