ICD-10-CM conventions and instructions

Where you are

You can move from the index to a full code. Keep that workflow. This lesson adds the instructions that decide whether codes belong together and how they should be ordered.

Why this matters

A code title cannot carry every rule. Notes and conventions supply information that may change your final answer. The classification's conventions and instructions take precedence over the general guidelines. Read them before deciding that a familiar code combination must be correct. [icd-guidelines, introduction; I.A]

Other specified and unspecified

NEC means “not elsewhere classifiable.” It points toward other specified coding: the record gives detail, but the classification has no separate code for that particular condition. NOS means “not otherwise specified.” It corresponds to unspecified coding, where the record lacks the detail needed for a more specific selection. [icd-guidelines, I.A.6; I.A.9]

For a course comparison, imagine two folders. The first contains a clear description of an unusual condition for which the index directs an other specified code. The second names a condition but leaves a required distinguishing detail unknown. These are different reasons for selecting a broad code. Neither gives you permission to invent information. When the classification lacks an unspecified option, read its direction carefully; an other specified entry can sometimes serve both purposes. [icd-guidelines, I.A.9]

Punctuation has a job

Brackets in the Tabular List can enclose synonyms or explanatory wording. In the index, brackets identify manifestation codes. Parentheses enclose nonessential modifiers: words that can be present or absent without changing the assignment, subject to the index's subentry rules. A colon follows an incomplete term that needs one or more of the listed modifiers. Commas have several uses that depend on the entry's context. [icd-guidelines, I.A.7]

Do not treat “nonessential modifier” as a service modifier such as one used with a procedure code. Here it describes the effect of a parenthetical word in the diagnosis classification. The same English word can refer to different tools in different books.

Includes and inclusion terms

An Includes note under a category explains or gives examples of what belongs there. Inclusion terms under a code can be synonyms or conditions assigned to it. The list is not necessarily exhaustive. Use the index and tabular instructions together instead of assuming that a condition must appear verbatim beneath the code. [icd-guidelines, I.A.10–11]

Excludes1 and Excludes2

An Excludes1 note generally means the excluded code should not be assigned with the code above the note. The guidelines also describe an exception when the two conditions are unrelated. If their relationship is unclear, query the provider. Do not reduce Excludes1 to either “never together under any circumstances” or “always together when both are documented.” Both shortcuts lose a necessary part of the rule. [icd-guidelines, I.A.12.a]

An Excludes2 note means the excluded condition is not included in the first code. The patient may have both, and both may be coded when appropriate. This is permission to report supported coexisting conditions, not a requirement to add a second condition that the record does not establish. [icd-guidelines, I.A.12.b]

Code first, use additional code, code also

In an etiology/manifestation pair, the etiology is the underlying cause and the manifestation is the condition it produces. The convention puts the underlying condition first and the manifestation after it. Manifestation codes identified as being in diseases classified elsewhere cannot stand alone as first-listed diagnoses. Index brackets can identify the code that follows the underlying condition. [icd-guidelines, I.A.13]

“Code first” and “use additional code” also appear outside classic etiology/manifestation pairs. Read the actual wording and any conditions it places on the instruction. “Code also” tells you that additional coding may be needed but does not, by itself, set the sequence. [icd-guidelines, I.A.13; I.A.17; I.B.7]

And, with, see, and see also

In a code title, and can mean either and or or. The classification uses with and in to indicate a presumed relationship when those words connect conditions in specified index, title, or tabular contexts. Apply the presumption unless the documentation states that the conditions are unrelated or another guideline requires an explicit link. For conditions not linked by the classification, you cannot create that relationship yourself. [icd-guidelines, I.A.14–15]

That last point deserves care. “Never infer any relationship” is too broad because the classification provides certain presumptions. “All coexisting conditions are related” is also wrong. Your task is to find out whether this particular relationship is supplied by the classification and whether an exception applies.

A see direction must be followed. A see also direction offers another useful term, but you need not follow it when the original term supplies the necessary code. Tabular verification remains required in both cases. [icd-guidelines, I.A.16; I.B.1]

Worked example: the note changes the order

In a fictional chart, the provider documents an underlying condition and its related manifestation. Your verified tabular entries carry a code-first direction at the manifestation and a matching additional-code instruction at the underlying condition.

Write both candidates on separate lines. Draw an arrow from the underlying condition to the manifestation. Your sequence follows the convention. It does not follow the order in which the clinician happened to mention the conditions. Do not substitute a manifestation-only answer just because it sounds more like the immediate problem. [icd-guidelines, I.A.13]

This example deliberately leaves the disease and code choices to your book. Find a real etiology/manifestation pair there and explain how its notes support the order.

Book drill

Find one example each of an Includes note, Excludes1, Excludes2, code first, use additional code, and code also. Record the location and write the effect in your own words. For Excludes1, also locate the unrelated-conditions exception in the guidelines. For code also, identify where you would look for sequencing instructions beyond the note itself.

Common traps

Do not confuse NEC with NOS. Do not treat an Excludes2 note as a ban. Do not turn code also into a universal order rule. Do not treat the ordinary wording of a chart as though it overrides a classification convention.

Practice

1. The record gives a specific condition but no unique classification entry exists. Which abbreviation fits?

2. Two conditions fall under an Excludes1 note, and their relationship is unclear. What is the next professional action?

3. A code also note identifies an additional condition. Does that phrase alone tell you which code comes first?

Compare your reasoning

1. NEC. 2. Seek clarification rather than deciding the relationship yourself. 3. No. Use the encounter circumstances and any other applicable sequencing instructions.

Checkpoint

Explain six different notes without reading a memorized slogan. For each, say what it requires, what it does not require, and where you confirmed that interpretation.

Sources

1. CDC/NCHS and CMS — ICD-10-CM April 1 2026 Guidelines. FY2026 April. Accessed 2026-09-10.