FY 2027 MEDESUN®
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Section I.A · FY 2027

ICD-10-CM conventions — Section I.A

Nineteen conventions that govern the whole classification, and that outrank every guideline in this document.

Dr. Santosh Kumar Guptha, CCS, CPC, COC, CIC, CRC, CPMA, CDIP, CPCO — AHIMA-Approved ICD-10-CM/PCS Trainer, MEDESUN® Medical Coding Academy Guidelines effective October 1, 2026 – September 30, 2027

The conventions are the general rules for using ICD-10-CM, and they outrank the guidelines. Section I opens by saying so directly: the conventions and instructions of the classification take precedence over guidelines. There are nineteen of them, and they live inside the Alphabetic Index and the Tabular List as instructional notes — not in a separate rule book.

Diagnostic statementin the medical recordAlphabetic Indexmain term, then subtermsTabular Listverify the full codeReport the codehighest specificityWatch fordash (-) = more charactersnonessential modifiersNEC / NOS entries"with" sequenced firstRead and obeyIncludes / Inclusion termsExcludes1 vs Excludes2Code first / Use additional7th character notesNever begin the search in the Tabular List — Section IV.A
Index first, Tabular always — and read the instructional notes in both

What are the nineteen ICD-10-CM conventions?#

#ConventionWhat it requires
1Alphabetic Index and Tabular ListICD-10-CM has two parts. The Index has four components: the Index of Diseases and Injury, the Index of External Causes of Injury, the Table of Neoplasms and the Table of Drugs and Chemicals.
2Format and structureAll categories are 3 characters. Subcategories are 4 or 5 characters. Codes may be 3, 4, 5, 6 or 7 characters. A 3-character category with no further subdivision is a code. A code with an applicable 7th character is invalid without it.
3Use of codes for reportingOnly codes are reportable — never categories or subcategories — and any applicable 7th character is required.
4Placeholder character XUsed where the classification allows for future expansion, for example at T36–T50. Where a placeholder exists, the X must be used or the code is invalid.
57th charactersRequired for all codes in the category, or as the Tabular notes instruct. The 7th character must sit in the 7th position; if the code is shorter than 6 characters, fill the gap with placeholder X.
6AbbreviationsNEC = not elsewhere classifiable, which means other specified. NOS = not otherwise specified, which is the equivalent of unspecified. Both appear in the Index and the Tabular List.
7Punctuation[ ] brackets — synonyms and alternative wording in the Tabular; manifestation codes in the Index.
( ) parentheses — nonessential modifiers.
: colon — an incomplete term needing a modifier that follows.
, comma — meaning varies by context.
8Use of “and”Cross-reference to convention 14.
9“Other” and “unspecified” codesOther / other specified — the record has detail but no specific code exists (Index NEC entries point here).
Unspecified — the record does not contain enough information to be more specific.
10Includes notesAppear immediately under a 3-character code title to further define or give examples of the category content.
11Inclusion termsThe conditions for which that code is to be used — synonyms, or the list of conditions assigned to an “other specified” code. Not exhaustive; the Index may hold additional terms.
12Excludes notesTwo types with different meanings. Both mean the excluded codes are independent of each other. See the table below.
13Etiology / manifestation convention“Use additional code” at the etiology, “code first” at the manifestation. The underlying condition is sequenced first. In the Index the two are listed together, etiology first and the manifestation in brackets.
14“And”In a code title, “and” means either and or or. A18.0 covers tuberculosis of bones, of joints, and of bones and joints.
15“With”“With” or “in” means associated with or due to. A causal relationship is presumed between the two conditions linked by the term. See the decision chart below.
16“See” and “See also”See must be followed — another term has to be referenced. See also is optional if the original main term already gives the needed code.
17“Code also” noteTwo codes may be required to describe the condition, but this note gives no sequencing direction — the circumstances of the encounter decide.
18Default codesThe code listed next to a main term in the Index. It is either the condition most commonly associated with the term or the unspecified code. Assign it when the record gives no further detail.
19Code assignment and clinical criteriaCode assignment rests on the provider's diagnostic statement that the condition exists. The statement is sufficient; code assignment is not based on the clinical criteria the provider used. If documentation conflicts, QUERY the provider.

Excludes1 or Excludes2 — what is the difference?#

Excludes1 means never together; Excludes2 means the conditions are separate and both may be coded. Excludes1 is a pure excludes note — NOT CODED HERE — used when two conditions cannot occur together, such as a congenital form versus an acquired form of the same condition.

Excludes note under the code you selectedExcludes1 — "NOT CODED HERE"the two codes may never be reported togetherExceptionif the two conditions are clearly unrelated, both maybe reported — if unclear, query the providerExcludes note under the code you selectedExcludes2 — "Not included here"the excluded condition is a separate conditionResultreport both codes together when bothconditions are documented
Convention I.A.12 — the two excludes notes
Excludes1Excludes2
Meaning“NOT CODED HERE!”“Not included here”
Why it existsThe two conditions cannot occur together — for example a congenital versus an acquired form of the same condition.The excluded condition is not part of the condition represented by the code, but a patient may have both at the same time.
Can both codes be reported?NEVER Not at the same time.ALWAYS Yes, when both conditions are documented.
The exceptionWhen the two conditions are genuinely unrelated to each other, both may be reported. If it is not clear whether they are related, QUERY the provider.None — the note itself permits both codes.
Worked example from the guidelinesF45.8 Other somatoform disorders has an Excludes1 for sleep related teeth grinding G47.63, because teeth grinding is an inclusion term at F45.8 — so only one code is assigned for teeth grinding. But psychogenic dysmenorrhea is also an inclusion term at F45.8, and a patient can have both that and sleep related teeth grinding. Those two are clearly unrelated, so F45.8 and G47.63 may be reported together.Report the code and the excluded code together whenever both conditions are present and documented.

How does the “with” convention work?#

When “with” or “in” links two conditions in a code title, the Alphabetic Index or a Tabular instructional note, the classification presumes a causal relationship — and you code them as related even if the provider never wrote a linking statement. Two things defeat that presumption: documentation that clearly states the conditions are unrelated, or another guideline that specifically requires documented linkage.

Two conditions documentedAre they linked by"with", "in" or "due to"?in the Index, a code title or a Tabular noteYES — causal relationship presumedcode as related even with no explicitprovider statement linking themNO — linkage must be documentedthe provider must state the link beforeyou may code the conditions as relatedTwo exceptionsdocumentation saysthey are unrelated, ora guideline demandsexplicit linkageSection I.A.15 — the "with" convention
Convention I.A.15 — the "with" decision
SituationCode as related?Why
Diabetes with chronic kidney diseaseALWAYS YesLinked by “with” in the Index under Diabetes. No provider statement is needed.
Hypertension with heart failureALWAYS YesLinked by “with” — assign a code from I11 plus a code from I50 for the type of heart failure. See Section I.C.9.a.
Documentation states the heart failure is unrelated to the hypertensionNEVER NoThe documentation clearly states the conditions are unrelated, which defeats the presumption. Code I10 and the heart condition separately.
Sepsis with acute organ dysfunctionQUERY Only if linkedThe sepsis guideline specifically requires that the acute organ dysfunction be associated with the sepsis, so this is the ‘another guideline’ exception. Query if the record is unclear.
Two conditions not linked by a relational term anywhere in the classificationNEVER No, unless documentedProvider documentation must link them before they may be coded as related.

Nonessential modifiers — and the one time they do not apply#

Terms in parentheses in the Index are nonessential modifiers: they may be present or absent in the diagnostic statement without changing the code. They apply to the subterms that follow a main term — except when a nonessential modifier and a subentry are mutually exclusive, in which case the subentry wins.

Index structureStatementResult
Main term Enteritis (acute), with a subentry for chronic“Acute enteritis”The nonessential modifier applies — code the main term entry.
Same structure“Chronic enteritis”The nonessential modifier acute does not apply to the subentry chronic. The subentry takes precedence.

Etiology and manifestation — the bracket rule#

In the Alphabetic Index, the code in brackets is always sequenced second. Codes titled “in diseases classified elsewhere” are manifestation codes and may never be reported as a principal or first-listed diagnosis.

ElementWhere the note sitsSequence
Underlying etiologycarries a use additional code noteFirst
Manifestationcarries a code first note; the title often reads “in diseases classified elsewhere”Second
Index presentationboth listed together, manifestation code in bracketsThe bracketed code is always second

The guideline's own example is dementia with Parkinson's disease: in the Index a code from category G20 is listed first, followed by F02.80 or F02.81- in brackets. G20- is the underlying etiology and must come first; the F02.8- code is the manifestation.

Practice questions#

Five questions on Section I.A. Work each one before opening the answer.

Q1A code that requires a 7th character is reported without it. What is the status of that code?

  1. AValid, because the first six characters identify the condition
  2. BInvalid
  3. CValid only in the outpatient setting
  4. DValid if the 7th character is not documented in the record
Show answer & rationale

Correct answer: B. Invalid

Rationale. Convention I.A.2 states that a code with an applicable 7th character is considered invalid without the 7th character. Convention I.A.3 adds that any applicable 7th character is required for reporting. Whether the record documents the information needed is a documentation problem to be resolved, not a reason to report an invalid code.

Q2Code A has an Excludes1 note for code B. The two conditions documented for this patient are clearly unrelated to each other. What may be reported?

  1. ACode A only, because Excludes1 is absolute
  2. BCode B only
  3. CBoth code A and code B
  4. DNeither code; query the provider first
Show answer & rationale

Correct answer: C. Both code A and code B

Rationale. Convention I.A.12.a provides an exception to the Excludes1 definition for the circumstance in which the two conditions are unrelated to each other — both codes may then be reported. The guideline's own example is F45.8 with G47.63. A query is required only when it is not clear whether the two conditions are related.

Q3The record documents “enteritis, chronic.” In the Alphabetic Index the main term is Enteritis (acute), with a separate subentry for chronic. Which entry governs?

  1. AThe main term entry, because “acute” is a nonessential modifier and applies to all subterms
  2. BThe subentry for chronic
  3. CEither entry may be used
  4. DCode both the acute and the chronic form
Show answer & rationale

Correct answer: B. The subentry for chronic

Rationale. Convention I.A.7 states that nonessential modifiers apply to subterms following a main term except where a nonessential modifier and a subentry are mutually exclusive — in which case the subentry takes precedence. Acute and chronic are mutually exclusive here, so the chronic subentry governs.

Q4A provider documents a diagnosis that the coder does not believe is supported by the clinical indicators in the record. What does Section I.A.19 direct?

  1. AAssign the code anyway without further action
  2. BDo not assign the code, because the clinical criteria are not met
  3. CThe provider's statement that the condition exists is sufficient; if the record contains conflicting documentation, query the provider
  4. DAssign an unspecified code instead
Show answer & rationale

Correct answer: C. The provider's statement that the condition exists is sufficient; if the record contains conflicting documentation, query the provider

Rationale. Convention I.A.19 states that code assignment is based on the provider's diagnostic statement that the condition exists, that the statement is sufficient, and that code assignment is not based on the clinical criteria the provider used. The guideline's own remedy for a problem in the record is a query where there is conflicting documentation — not unilateral refusal or downgrading of the code.

Q5In the Alphabetic Index, two codes are listed together for a single condition with the second code in brackets. What does that tell you?

  1. AThe bracketed code is optional
  2. BThe bracketed code is always sequenced second
  3. CThe bracketed code is a synonym and only one code is reported
  4. DEither code may be sequenced first depending on the encounter
Show answer & rationale

Correct answer: B. The bracketed code is always sequenced second

Rationale. Convention I.A.13 states that in the Alphabetic Index both conditions are listed together with the etiology code first, followed by the manifestation code in brackets, and that the code in brackets is always to be sequenced second. In the Tabular List, brackets have a different job — they enclose synonyms, alternative wording or explanatory phrases.

Scenarios#

Scenario 1An excludes note that looks absolute

A patient is seen for two conditions. The code selected for the first condition carries an Excludes2 note naming the code for the second condition. Both conditions are documented and both were evaluated at this visit. The coder's encoder fires a warning on the pair.

Show coded answer & rationale
SeqCodeDescription
1Code 1the first condition — the code carrying the Excludes2 note
2Code 2the excluded condition, reported alongside it

Rationale. An Excludes2 note means “not included here”: the excluded condition is not part of the condition represented by the code, but the patient may have both at the same time. The guideline states that when an Excludes2 note appears under a code it is acceptable to use both the code and the excluded code together, when appropriate. The encoder warning is informational, not an error.

Guideline: Section I.A.12.b

Scenario 2A diagnosis documented with no further detail

The only diagnostic statement in an office record reads “appendicitis.” There is no statement of acute or chronic, and no further detail anywhere in the note.

Show coded answer & rationale
SeqCodeDescription
1Defaultthe code listed next to the main term Appendicitis in the Alphabetic Index, verified in the Tabular List

Rationale. Convention I.A.18 covers this exactly: the code listed next to a main term in the Index is the default code, representing either the condition most commonly associated with the term or the unspecified code, and where a condition is documented without additional information the default code should be assigned. The guideline names appendicitis as its own example. A query for greater specificity is reasonable practice but is not required by the convention.

Guideline: Section I.A.18

Scenario 3A Tabular instruction that contradicts a habit

A coder has always sequenced a particular pair of codes in one order. On verifying the code in the Tabular List, a “code first” note appears at the second code, directing the opposite sequence.

Show coded answer & rationale
SeqCodeDescription
11the underlying condition named in the “code first” note
22the code that carried the note

Rationale. Section I opens by stating that the conventions and instructions of the classification take precedence over the guidelines, and Section II repeats that in determining the principal diagnosis the coding conventions in the Tabular List and Alphabetic Index take precedence over the official coding guidelines. A Tabular instructional note is not advisory. It is also the reason Section I.B.1 insists that the Tabular List must always be consulted even when the Index appears to give a complete code.

Guideline: Section I, opening paragraph; Section I.A.13; Section II introduction

Primary sources#