8.1 ICD-10-CM Structure, Index, and Tabular Conventions
Key Takeaways
- ICD-10-CM codes have 3 to 7 characters; a three-character category is reported only if it is not further subdivided, and any required seventh character must occupy the seventh position.
- Locate the term in the Alphabetic Index first, then verify and complete the code in the Tabular List; laterality and seventh characters are finished only in the Tabular List.
- Placeholder X is required where it exists (T36–T50 is the printed example) so empty positions are filled and the seventh character stays seventh.
- Report valid codes from A00.0–T88.9, Z00–Z99 (Section I.B.3 also prints Z00–Z99.8), and U00–U85; do not report a category heading as if it were a code.
- Excludes1 means not coded here except the unrelated-conditions exception; Excludes2 can allow both codes; code-first and use-additional notes sequence etiology before manifestation.
ICD-10-CM Structure, Index, and Tabular Conventions
Quick Answer: International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) diagnosis codes have 3 to 7 characters. Start in the Alphabetic Index, then verify and complete the code in the Tabular List. Laterality and any required seventh character are finished only in the Tabular List. Placeholder X fills empty positions so a seventh character stays in the seventh slot. Report valid codes from A00.0–T88.9, Z00–Z99, and U00–U85. Excludes1 means not coded here; Excludes2 can allow both codes when both conditions belong on the claim.
This independent OpenExamPrep chapter helps learners study ICD-10-CM structure and hospital outpatient diagnosis rules for the American Academy of Professional Coders (AAPC) Certified Outpatient Coder (COC) exam. It is not a Centers for Medicare & Medicaid Services (CMS), National Center for Health Statistics (NCHS), American Hospital Association (AHA), American Health Information Management Association (AHIMA), or AAPC product, and it does not claim partnership or official review by those organizations.
Why structure belongs in the 15-question ICD-10-CM domain
AAPC's Taking the COC exam page assigns 15 of 100 questions to ICD-10-CM across chapters plus the Official Guidelines. An earlier chapter in this guide introduced Section I conventions for the three-question coding-guidelines domain. This chapter is the ICD-10-CM domain's structure and outpatient Section IV lesson. Do not try to memorize all 21 ICD-10-CM chapters here. Later chapters cover infectious disease, neoplasms, circulatory and respiratory conditions, injury, external causes, and seventh-character application. If you cannot build a valid code from Index plus Tabular, chapter-specific knowledge will not rescue the claim.
Hospital outpatient and ambulatory surgery center (ASC) facility claims still need a complete diagnosis on the UB-04. An invalid three-character heading, a missing placeholder X, or a seventh character typed into the fifth position is not a documentation style choice. It is an invalid code. Professional claims you will see in the same record pointer to the same diagnosis set; the structure rules do not change because the form is a CMS-1500 instead of a UB-04.
The Cooperating Parties (CMS, NCHS, AHA, and AHIMA) publish the FY 2026 ICD-10-CM Official Guidelines for Coding and Reporting, in force October 1, 2025 through September 30, 2026. The guidelines say adherence is required under the Health Insurance Portability and Accountability Act (HIPAA). Conventions printed in the Index and Tabular List take precedence over the guidelines. Recheck the CMS PDF if a newer fiscal year has started before you sit.
Two books, one code: Index then Tabular
ICD-10-CM is divided into the Alphabetic Index and the Tabular List. The Index is an alphabetical list of terms and corresponding codes. The Tabular List is a structured list of codes divided into chapters based on body system or condition.
The Alphabetic Index includes four parts you must know exist:
- Index of Diseases and Injury
- Index of External Causes of Injury
- Table of Neoplasms
- Table of Drugs and Chemicals
Section IV repeats the lookup rule in outpatient language: begin the search for the correct code through the Alphabetic Index. Never begin searching initially in the Tabular List, because that path produces coding errors. Opening Chapter 13 because the case is “a knee problem” is how people assign a look-alike subcategory the Index never supported.
Locating a code (Section I.B.1): locate the term in the Index, then verify the code in the Tabular List. Read instructional notations that appear in both. It is essential to use both the Index and the Tabular List. The Index does not always provide the full code. Selection of the full code, including laterality and any applicable seventh character, can only be done in the Tabular List. A dash (-) at the end of an Index entry means additional characters are required. Even if a dash is not printed, still open the Tabular List to confirm that no seventh character is required.
See instructions in the Index redirect you to another term; follow them. See Also points to additional Index entries when the listed subterms are not enough. Those notes are lookup instructions, not optional commentary.
Categories, subcategories, and codes
The Tabular List contains categories, subcategories, and codes. Characters may be a letter or a number. 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. Each level of subdivision after a category is a subcategory. The final level of subdivision is a code. Codes that have applicable seventh characters are still referred to as codes, not subcategories. A code that has an applicable seventh character is invalid without the seventh character.
For reporting purposes, only codes are permissible, not categories or subcategories, and any applicable seventh character is required.
Level of detail (Section I.B.2 and Section IV.F): diagnosis codes are used and reported at their highest number of characters available and at the highest specificity the medical record supports. A three-character code is used only if it is not further subdivided. A code is invalid if it has not been coded to the full number of characters required for that code, including the seventh character if applicable. Section IV.F restates the same three outpatient points: codes have 3, 4, 5, 6, or 7 characters; use the full number required; code to the highest specificity the documentation supports.
Placeholder X and seventh-character placement
ICD-10-CM uses placeholder X at certain codes to allow future expansion. The guidelines' printed example is the poisoning, adverse effect, and underdosing categories T36–T50. Where a placeholder exists, X must be used for the code to be considered valid.
Certain categories require a seventh character. The applicable seventh character is required for all codes within the category, or as Tabular notes instruct. The seventh character must always be the seventh character in the data field. If a code that requires a seventh character is not six characters long, a placeholder X must fill the empty characters. Sliding the seventh-character value into the next open slot (fifth or sixth) does not produce a valid code.
Injury seventh-character meanings (A initial encounter, D subsequent encounter, S sequela) are taught with the injury chapters. The placement rule belongs here because outpatient fracture clinic visits, poisoning codes, and aftercare traps all fail if X is dropped or the seventh character is left in the wrong position.
Reportable diagnosis range
Section I.B.3 states that the appropriate code or codes from A00.0 through T88.9, Z00–Z99.8, and U00–U85 must be used to identify diagnoses, symptoms, conditions, problems, complaints, or other reasons for the encounter or visit.
Section IV.B prints the outpatient counterpart: A00.0 through T88.9, Z00–Z99, and U00–U85. The practical instruction is the same: assign a complete, valid ICD-10-CM diagnosis (or Z or U) code for the reason for the visit. Do not invent a local fourth character because the Index looked close enough. Do not report an external-cause code as a substitute for a diagnosis code when a diagnosis code exists. U codes live in Chapter 22 (U00–U85). Outpatient facility coders most often meet U07.1 (COVID-19) and U09.9 (post COVID-19 condition, unspecified). Do not invent a U code that is not in the Tabular List.
Laterality is part of completing the code
Some ICD-10-CM codes specify left, right, or bilateral (Section I.B.13):
- If no bilateral code is provided and the condition is bilateral, assign separate codes for the left and right sides.
- If the side is not identified in the record, assign the code for the unspecified side. Unspecified laterality should be rare when the record or a query can identify the side.
- Each unique ICD-10-CM diagnosis code may be reported only once for an encounter. That includes bilateral conditions when there are no distinct laterality codes.
- When a patient has a bilateral condition and each side is treated during separate encounters, assign the bilateral code for the encounter that treats the first side, because the condition still exists on both sides. For the later encounter, after one side has been treated and the condition no longer exists on that side, assign the unilateral code for the remaining side. The guidelines' example is cataract surgery on each eye at separate encounters. If treatment on the first side did not completely resolve the condition, the bilateral code remains appropriate.
Laterality characters are selected in the Tabular List. The Index may point to a family; it does not finish left versus right for you.
Sequencing notes you still read while the code is being built
A prior chapter taught punctuation, NEC versus NOS, and etiology/manifestation in more depth. Use this recap only as a sequencing check while you complete a code — not as a second copy of that lesson, and not as a dump of every ICD-10-CM chapter.
| Instruction | What it does when you sequence |
|---|---|
| Excludes1 | NOT CODED HERE. Do not report the excluded code with the code above the note. Exception: both codes when the two conditions are clearly unrelated; query the provider if relatedness is unclear. |
| Excludes2 | Not included here. The excluded condition is not part of this code, but a patient may have both; report both when documented and appropriate. |
| Code first / use additional / in diseases classified elsewhere | Etiology then manifestation. Manifestation codes with that title are never first-listed. |
| Code also | Two codes may be required; the note does not tell you which is first. Sequence from the reason for the encounter and other applicable guidelines. |
| NEC | Other specified: the record is specific; the classification has no dedicated code. |
| NOS | Unspecified: the record lacks detail the classification could have used. |
Code first and use additional also appear as sequencing rules for some pairs that are not classic etiology/manifestation combinations. Follow the Tabular note at the code you actually assign. Code also never silently converts a secondary code into the first-listed diagnosis.
Facility scenario
Same-day surgery: left shoulder arthroscopy for a documented labral tear. The Index entry ends with a dash. A coder who stops in the Index reports an incomplete stem. The Tabular List supplies laterality (left) and confirms whether additional characters apply. If the same patient later returns for a traumatic injury still in the healing phase, any required seventh character must sit in position seven, with X filling empty slots. Reporting unspecified shoulder when the operative note says left is a level-of-detail miss. Reporting a three-character category heading because it “looks like Chapter 13” is an invalid code.
Source
FY 2026 ICD-10-CM Official Guidelines for Coding and Reporting (CMS PDF)
A hospital outpatient coder needs a complete ICD-10-CM diagnosis that requires laterality and may require a seventh character. Where do the FY 2026 Official Guidelines say those characters are completed?
Which statement correctly describes ICD-10-CM placeholder X in the FY 2026 Official Guidelines?
How should an Excludes1 note be applied when you sequence diagnoses on a hospital outpatient or ASC claim?