The conventions, general guidelines and chapter-specific guidelines are applicable to all health care settings unless otherwise indicated. The conventions and instructions of the classification take precedence over guidelines.
A. Conventions for the ICD-10-CM
The conventions for the ICD-10-CM are the general rules for use of the classification independent of the guidelines. These conventions are incorporated within the Alphabetic Index and Tabular List of the ICD-10-CM as instructional notes.
1. The Alphabetic Index and Tabular List
The ICD-10-CM is divided into the Alphabetic Index, an alphabetical list of terms and their corresponding code, and the Tabular List, a structured list of codes divided into chapters based on body system or condition. The Alphabetic Index consists of the following parts: the Index of Diseases and Injury, the Index of External Causes of Injury, the Table of Neoplasms and the Table of Drugs and Chemicals.
2. Format and Structure
The ICD-10-CM Tabular List contains categories, subcategories and codes. Characters for categories, subcategories and codes may be either 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. That is, each level of subdivision after a category is a subcategory. The final level of subdivision is a code. Codes that have applicable 7th characters are still referred to as codes, not subcategories. A code that has an applicable 7th character is considered invalid without the 7th character. The ICD-10-CM uses an indented format for ease in reference.
3. Use of codes for reporting purposes
For reporting purposes only codes are permissible, not categories or subcategories, and any applicable 7th character is required.
4. Placeholder character
The ICD-10-CM utilizes a placeholder character "X". The "X" is used as a placeholder at certain codes to allow for future expansion. An example of this is at the poisoning, adverse effect and underdosing codes, categories T36-T50.
Where a placeholder exists, the X must be used in order for the code to be considered a valid code.
5. 7th Characters
Certain ICD-10-CM categories have applicable 7th characters. The applicable 7th character is required for all codes within the category, or as the notes in the Tabular List instruct. The 7th character must always be the 7th character in the data field. If a code that requires a 7th character is not 6 characters, a placeholder X must be used to fill in the empty characters.
a. Alphabetic Index abbreviations
NEC "Not elsewhere classifiable"
This abbreviation in the Alphabetic Index represents "other specified." When a specific code is not available for a condition, the Alphabetic Index directs the coder to the "other specified" code in the Tabular List.
NOS "Not otherwise specified"
This abbreviation is the equivalent of unspecified.
b. Tabular List abbreviations
NEC "Not elsewhere classifiable"
This abbreviation in the Tabular List represents "other specified". When a specific code is not available for a condition, the Tabular List includes an NEC entry under a code to identify the code as the "other specified" code.
NOS "Not otherwise specified"
This abbreviation is the equivalent of unspecified.
[ ] Brackets are used in the Tabular List to enclose synonyms, alternative wording or explanatory phrases. Brackets are used in the Alphabetic Index to identify manifestation codes.
( ) Parentheses are used in both the Alphabetic Index and Tabular List to enclose supplementary words that may be present or absent in the statement of a disease or procedure without affecting the code number to which it is assigned. The terms within the parentheses are referred to as nonessential modifiers. The nonessential modifiers in the Alphabetic Index to Diseases apply to subterms following a main term except when a nonessential modifier and a subentry are mutually exclusive, the subentry takes precedence. For example, in the ICD-10-CM Alphabetic Index under the main term Enteritis, "acute" is a nonessential modifier and "chronic" is a subentry. In this case, the nonessential modifier "acute" does not apply to the subentry "chronic".
: Colons are used in the Tabular List after an incomplete term which needs one or more of the modifiers following the colon to make it assignable to a given category.
8. Use of "and".
See Section I.A.14. Use of the term "And"
Other and Unspecified codes
a. "Other" codes
Codes titled "other" or "other specified" are for use when the information in the medical record provides detail for which a specific code does not exist. Alphabetic Index entries with NEC in the line designate "other" codes in the Tabular List. These Alphabetic Index entries represent specific disease entities for which no specific code exists so the term is included within an "other" code.
b. "Unspecified" codes
Codes titled "unspecified" are for use when the information in the medical record is insufficient to assign a more specific code. For those categories for which an unspecified code is not provided, the "other specified" code may represent both other and unspecified.
See Section I.B.18 Use of Signs/Symptom/Unspecified Codes
10. Includes Notes
This note appears immediately under a three character code title to further define, or give examples of, the content of the category.
11. Inclusion terms
List of terms is included under some codes. These terms are the conditions for which that code is to be used. The terms may be synonyms of the code title, or, in the case of "other specified" codes, the terms are a list of the various conditions assigned to that code. The inclusion terms are not necessarily exhaustive. Additional terms found only in the Alphabetic Index may also be assigned to a code.
12. Excludes Notes
The ICD-10-CM has two types of excludes notes. Each type of note has a different definition for use but they are all similar in that they indicate that codes excluded from each other are independent of each other.
A type 1 Excludes note is a pure excludes note. It means "NOT CODED HERE!" An Excludes1 note indicates that the code excluded should never be used at the same time as the code above the Excludes1 note. An Excludes1 is used when two conditions cannot occur together, such as a congenital form versus an acquired form of the same condition.
An exception to the Excludes1 definition is the circumstance when the two conditions are unrelated to each other. If it is not clear whether the two conditions involving an Excludes1 note are related or not, query the provider. For example, code F45.8, Other somatoform disorders, has an Excludes1 note for "sleep related teeth grinding (G47.63)," because "teeth grinding" is an inclusion term under F45.8. Only one of these two codes should be assigned for teeth grinding. However psychogenic dysmenorrhea is also an inclusion term under F45.8, and a patient could have both this condition and sleep related teeth grinding. In this case, the two conditions are clearly unrelated to each other, and so it would be appropriate to report F45.8 and G47.63 together.
A type 2 Excludes note represents "Not included here." An excludes2 note indicates that the condition excluded is not part of the condition represented by the code, but a patient may have both conditions at the same time. When an Excludes2 note appears under a code, it is acceptable to use both the code and the excluded code together, when appropriate.
13. Etiology/manifestation convention ("code first", "use additional code" and "in diseases classified elsewhere" notes)
Certain conditions have both an underlying etiology and multiple body system manifestations due to the underlying etiology. For such conditions, the ICD-10-CM has a coding convention that requires the underlying condition be sequenced first, if applicable, followed by the manifestation. Wherever such a combination exists, there is a "use additional code" note at the etiology code, and a "code first" note at the manifestation code. These instructional notes indicate the proper sequencing order of the codes, etiology followed by manifestation.
In most cases the manifestation codes will have in the code title, "in diseases classified elsewhere." Codes with this title are a component of the etiology/ manifestation convention. The code title indicates that it is a manifestation code. "In diseases classified elsewhere" codes are never permitted to be used as first-listed or principal diagnosis codes. They must be used in conjunction with an underlying condition code and they must be listed following the underlying condition. See category F02, Dementia in other diseases classified elsewhere, for an example of this convention.
There are manifestation codes that do not have "in diseases classified elsewhere" in the title. For such codes, there is a "use additional code" note at the etiology code and a "code first" note at the manifestation code, and the rules for sequencing apply.
In addition to the notes in the Tabular List, these conditions also have a specific Alphabetic Index entry structure. In the Alphabetic Index both conditions are listed together with the etiology code first followed by the manifestation codes in brackets. The code in brackets is always to be sequenced second.
An example of the etiology/manifestation convention is dementia in Parkinson's disease. In the Alphabetic Index, code G20 is listed first, followed by code F02.80 or F02.81 in brackets. Code G20 represents the underlying etiology, Parkinson's disease, and must be sequenced first, whereas codes F02.80 and F02.81 represent the manifestation of dementia in diseases classified elsewhere, with or without behavioral disturbance.
"Code first" and "Use additional code" notes are also used as sequencing rules in the classification for certain codes that are not part of an etiology/ manifestation combination. See Section I.B.7. Multiple coding for a single condition.
The word "and" should be interpreted to mean either "and" or "or" when it appears in a title.
For example, cases of "tuberculosis of bones", "tuberculosis of joints" and "tuberculosis of bones and joints" are classified to subcategory A18.0, Tuberculosis of bones and joints.
The word "with" should be interpreted to mean "associated with" or "due to" when it appears in a code title, the Alphabetic Index, or an instructional note in the Tabular List. The classification presumes a causal relationship between the two conditions linked by these terms in the Alphabetic Index or Tabular List. These conditions should be coded as related even in the absence of provider documentation explicitly linking them, unless the documentation clearly states the conditions are unrelated. For conditions not specifically linked by these relational terms in the classification, provider documentation must link the conditions in order to code them as related.
The word "with" in the Alphabetic Index is sequenced immediately following the main term, not in alphabetical order.
16. "See" and "See Also"
The "see" instruction following a main term in the Alphabetic Index indicates that another term should be referenced. It is necessary to go to the main term referenced with the "see" note to locate the correct code.
A "see also" instruction following a main term in the Alphabetic Index instructs that there is another main term that may also be referenced that may provide additional Alphabetic Index entries that may be useful. It is not necessary to follow the "see also" note when the original main term provides the necessary code.
17. "Code also" note
A "code also" note instructs that two codes may be required to fully describe a condition, but this note does not provide sequencing direction.
18. Default codes
A code listed next to a main term in the ICD-10-CM Alphabetic Index is referred to as a default code. The default code represents that condition that is most commonly associated with the main term, or is the unspecified code for the condition. If a condition is documented in a medical record (for example, appendicitis) without any additional information, such as acute or chronic, the default code should be assigned.
19. Code assignment and Clinical Criteria
The assignment of a diagnosis code is based on the provider's diagnostic statement that the condition exists. The provider's statement that the patient has a particular condition is sufficient. Code assignment is not based on clinical criteria used by the provider to establish the diagnosis.