The ICD-10-CM Alphabetic Index is your gateway to finding diagnosis codes — but it is more than just a simple alphabetical list. It contains multiple specialized tables and cross-references that experienced coders know how to navigate quickly and accurately. This guide walks through every section of the ICD-10-CM index so you can use it with confidence on the CPC exam and in daily coding practice.
The Golden Rule — Never Code From the Index Alone
Just like the CPT index, the ICD-10-CM Alphabetic Index is a navigation tool — not the final authority on code assignment. The index gives you a suggested code or code range. You must always verify the final code in the Tabular List before assigning it. The Tabular List contains instructional notes, inclusion and exclusion notes, and required additional codes that are not visible in the index. Coding directly from the index without tabular verification is one of the most common and consequential errors in medical coding.
The Three Parts of the ICD-10-CM Index
The ICD-10-CM Alphabetic Index is actually made up of three distinct sections, each serving a different purpose:
Part 1 — Index to Diseases and Injuries
The main alphabetic index listing conditions, diseases, symptoms, and injuries. This is where you start for the vast majority of diagnosis code lookups. Organized alphabetically by condition name.
Part 2 — Table of Neoplasms
A special table for finding neoplasm codes organized by anatomical site and behavior (malignant primary, malignant secondary, in situ, benign, uncertain, or unspecified). Used whenever you are coding any type of tumor or growth.
Part 3 — Table of Drugs and Chemicals
A specialized table for coding poisonings, adverse effects, underdosing, and toxic effects of drugs, chemicals, and other substances. Each substance is listed with separate columns for different types of exposure.
How to Navigate the Main Index
The Index to Diseases and Injuries works through a system of main terms and subterms. Main terms are printed in bold and appear at the left margin. They typically represent the condition, disease, or symptom being coded. Subterms are indented beneath the main term and narrow down the diagnosis by site, etiology, type, or other qualifiers.
The critical rule is: always look up the condition — not the anatomical site. The ICD-10-CM index is organized by condition name, not body part. For example, to find a code for a knee fracture you look up “Fracture” as the main term, then find “femur” or “tibia” or the specific bone as a subterm — not “knee” as the main term.
Important Index Conventions
- See: A mandatory cross-reference — you must look up the indicated term instead
- See also: An optional cross-reference — additional codes may be found under the indicated term
- Brackets [ ]: Indicate a manifestation code that must be sequenced second — never first
- NEC (Not Elsewhere Classified): The condition cannot be classified more specifically with available codes
- NOS (Not Otherwise Specified): The condition is unspecified — used when documentation does not provide enough detail for a more specific code
The Table of Neoplasms — How to Use It
Whenever you code a neoplasm — any tumor, growth, cancer, or mass — you use the Table of Neoplasms. The table is organized alphabetically by anatomical site. For each site, six columns provide different codes based on the behavior of the neoplasm:
| Anatomical Site | Malignant Primary | Malignant Secondary | Ca In Situ | Benign | Uncertain | Unspecified |
|---|---|---|---|---|---|---|
| Breast | C50.– | C79.81 | D05.– | D24.– | D48.6– | D49.3 |
| Colon | C18.– | C78.5 | D01.0 | D12.– | D37.4 | D49.0 |
| Lung | C34.– | C78.0– | D02.2– | D14.3– | D38.1 | D49.1 |
To use the table: (1) identify the anatomical site of the neoplasm, (2) identify the behavior from the pathology report or physician documentation, (3) locate the intersection of site and behavior in the table, (4) verify the suggested code in the Tabular List.
The Table of Drugs and Chemicals
The Table of Drugs and Chemicals is used for poisoning, adverse effect, and underdosing codes. Each drug or chemical substance is listed alphabetically. Six columns describe the circumstances of the exposure:
- Poisoning — Accidental: Unintentional overdose, wrong substance taken by mistake
- Poisoning — Intentional Self-Harm: Deliberate overdose or self-poisoning
- Poisoning — Assault: Substance given to harm another person
- Poisoning — Undetermined: Intent unknown
- Adverse Effect: Correct drug, correct dose, taken as prescribed — but a harmful reaction occurs
- Underdosing: Taking less than prescribed — less than the therapeutic dose