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ICD-10-CM Demystified: From Conventions to Code Lookup

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   Navigating ICD-10-CM diagnostic coding can feel overwhelming for those new to the medical coding field. Accuracy is critical, as medical coders are held to high standards that directly impact patient care and provider reimbursement. Assigning the wrong codes can result in claim denials, delayed authorizations, and incorrect payments, all of which cause unnecessary frustration for both patients and providers. To help ease this challenge, this article takes a closer look at Section I.A. and Section I.B. of the ICD-10-CM Official Guidelines. While not an exhaustive review, this breakdown highlights the conventions and general coding guidelines that lay the foundation for proper code assignment and serve as the cornerstone for every coder’s practice.

Section I.A. Conventions

The Alphabetical Index and Tabular List

  • The ICD-10-CM is divided into the alphabetical index a list of terms with their corresponding code, and then the tabular list broken down by chapter based off the body system or condition

The alphabetical index is dissected into the following 4 sections:

  • Index of Diseases and Injury
  • Index of External Causes and Injury (See Section I.C.19. Adverse effects, poisoning, underdosing, and toxic effects)
  • Table of Neoplasms (See Section I.C.2. Neoplasms)
  • Table of Drugs and Chemicals

Format and Structure

  • There are categories, subcategories and codes present within the tabular list, characters for categories, subcategories and codes may be represented by a letter or number
  • All categories are 3 characters and there are 3-character categories requiring no further subdivision and they are equivalent to a code
  • Example: Multiple Sclerosis (G35)
  • Subcategories fall between 4-5 characters in length
  • Codes can fall between 3-7 characters, each level of subdivision after the category is a subcategory, and the final level of subdivision is a code
  • Codes requiring a 7th character are referred to as codes not subcategories
  • A code requiring a 7th character is considered invalid without the use of the appropriate 7th character

Use of Codes for Reporting Purposes

  • For reporting purposes only codes are appropriate, not categories or subcategories, and any applicable 7th character is necessary

Placeholder Character

  • The ICD-10-CM uses the character X to allow for further expansion of codes, when a placeholder is present the X character must be used for the code to be considered accurate

7th Character

  • Certain categories require a 7th character, and the applicable 7th character is required for all the codes within that category, and it must be the 7th character in the data field
  • If a code requires a 7th character and is not at least 6 characters in length the X placeholder must be used to fill the empty characters

Abbreviations:

NEC Not Elsewhere Classifiable

  • The abbreviation NEC represents that the provider documented more specified information pertaining to the patient’s condition, but no code exists to report it accurately

NOS Not Otherwise Specified

  • NOS is synonymous with unspecified and is to be used only when the coder lacks pertinent information to report a more specific code

[ ] Brackets

  • Brackets are utilized within the tabular list to enclose explanatory phrases, alternate wording, or synonyms
  • Brackets are also used within the alphabetical index to indicate that multiple codes are required. For certain conditions more than one code is necessary to accurately document the condition.
  • Let’s look at an example: When indexing Dementia with Hypothyroidism E03.9 [F02.80] the brackets indicate the second code F02.80, Dementia is diseases classified elsewhere to accurately report Dementia with Hypothyroidism

( )Parentheses

  • Parentheses are referred to as nonessential modifiers and are used to enclose supplementary words that may or may not appear in a statement of a disease or procedure and they do not affect the code number to which they are assigned.
  • An example would be Cyst (colloid) (mucus) (simple) (retention)

Boldface

  • Boldface type is used to present all titles and codes in the tabular list and main terms in the alphabetical index

Other and Unspecified Codes

  • “Other” or “Other Specified” codes are selected when the data in the medical record provides detail for which a code matching its specificity doesn’t exist
  • They appear as NEC within the alphabetical index and are designated to “Other” codes in the tabular list
  • Unspecified codes are reported when the data in the medical record is insufficient to assign a more specific code
  • For category blocks lacking unspecified codes “Other Specified” can represent both other and unspecified (See Section I.B.18. Use of Signs/Symptoms/Unspecified Codes)

Inclusion Notes

  • Inclusion notes appear under a 3-category code to further define or provide examples of the content within the category

Inclusion Terms

  • These list of terms appear beneath some codes presenting various conditions assigned to that code and may not be an exhaustive list
  • Note that only additional terms found only in the alphabetic index may also be assigned to a code

EXCLUDES 1

  • An EXCLUDES 1 note indicates that the condition is not coded here, and tells us that the code excluded should not be used at the same time as the code above the excludes note if the conditions are related to one another
  • This note can also indicate that two conditions should not be reported together, such as a congenital form versus an acquired form of the same condition
  • Example: H26 Other cataract has an EXCLUDES 1 note for congenital cataract Q12.0, being the codes are for the same condition you should not report H26 (acquired condition) with code Q12.0 (congenital condition)
  • There can be an exception when conditions listed can be used together when there is no relation to one another such as the EXCLUDES 1 note beneath subcategory S00.47 Other superficial bite of the ear, states that code S01.35 Open bite of ear should note be reported together, however if the documentation states that there is a superficial bite on the left ear and an open bite on the right ear the two codes can be reported together

EXCLUDES 2

  • An EXCLUDES 2 note represents that the condition is not included here
  • This note indicates that the condition excluded is not part of the condition represented by the code, but a patient may simultaneously have both conditions, and they can be reported together
  • When this note appears it is acceptable to report both the code and the excluded code together

Use Additional Code

  • When use an additional code note appears it signals the coder that if the data is available, to provide a complete picture of the diagnosis
  • Example: Let’s look at the notes presented at category block D70 Neutropenia
  • Use additional code for any associated fever (R50.81) Mucositis (J34.1, K12.3-, K92.81, N76.81)
  • In this example we are instructed to use these additional codes if the data is available to fully capture the patient’s diagnosis
  • If the patient has D70.3 Neutropenia due to an infection in combination with a fever the proper code assignment in the correct sequence is D70.3, R50.81

Code First

  • Certain conditions both have an underlying etiology and multiple body system manifestations, due to the underlying etiology
  • The note indicates that the etiology meaning the underlying disease be reported first and the manifestation be recorded second, this note only appears within the tabular G20- Parkinsons disease is to be sequenced first as the underlying etiology followed by code F02.80 or F02.81- in brackets
  • G20 is sequenced to indicate the underlying etiology versus F02.80 or F02.81- represent manifestations of Dementia in diseases classified elsewhere, with or without behavioral disturbances

Use Additional Code, if Applicable

  • This note indicates that the code may be assigned as a diagnosis when the casual condition is unknown or not applicable
  • If the casual condition is documented, then it should be sequenced first as the first listed diagnosis
  • Example D68.32 Hemorrhagic disorder due to extrinsic circulating anticoagulants the note appears stating use additional code for adverse effect, if applicable to identify drug (T45.515, T45.525)

“And”

  • “And” is to be interpreted as either “and” or “or” when appearing in a title
  • Example: “Tuberculosis of bones” or “Tuberculosis of joints” are classified to subcategory A18.0 Tuberculosis of bones and joints

“With”

  • The “With” convention represents a casual relationship between two conditions and is meant to be interpreted as “associated with” or “due to” when appearing in a code title, the alphabetical index, or within the tabular
  • This classification assumes a casual relationship and automatically links the two conditions even in the absence of the provider linking verbiage and unless the provider specifies, they are unrelated they are linked to one another
  • Example Hypertension “with” Heart Failure if the two conditions are listed together in the medical record there is a casual relationship and I10 Essential Hypertension is not used instead I11.0 with the appropriate Heart Failure code from category I50.- is reported
  • The exception to this convention is when another guideline exists that requires linking verbiage from the provider such as the Sepsis guideline for “acute organ dysfunction that is not clearly associated with Sepsis”

“See and See Also”

  • When the term “see” or “see also” appears in the alphabetical index appears it indicates another term should be referenced and it is necessary to see the other term to select the proper code
  • “Code Also”
  • The “code also” note indicates that another code may be required to capture the full diagnosis, but this note doesn’t provide sequencing directions, and the code assignment will be based off circumstance of the encounter

Default Codes

  • A default code represents the condition most associated with the main term or is the unspecified code for the condition of reference
  • The default code is located next to a main term within the alphabetical index
  • Default codes are utilized when the medical record lacks more specified information describing the condition such as acute or chronic
  • Example: Depression F32A is utilized when no further details other than Depression are documented in the medical record

Code Assignment and Clinical Criteria

  • When assigning a diagnosis code, it is based off the providers diagnostic statement that the patient’s condition exists, and the statement is sufficient to meet the criteria to report the code
  • Code assignment is not to be based off clinical criteria the provider uses to establish the diagnosis, and when conflicting documentation within the medical record is present this is a situation to query the provider

I.B. General Coding Guidelines

Locating a Code in ICD-10-CM

  • Always utilize both the alphabetical index and tabular list never abstract a code straight from the index always verify code selection in the tabular to not miss any pertinent instructions
  • Locate the main term in the alphabetical index
  • Refer to any notes beneath the main term
  • Be sure to read any terms enclosed by parentheses following the main term
  • If modifiers of the main term are present refer to them
  • Never skip over sub terms indented beneath the main term
  • Look out for any cross-reference instructions such as “see” or “see also”

Level of Detail in Coding

  • We are instructed by the guidelines to always code to the highest level of specificity using all applicable characters

Code or Codes from A00.0-T88.9, Z00-Z99.8, U00-U85

  • Code assignments from A00.0-T88.9, Z00-Z99.8 and U00-U85 are used to report diagnoses, symptoms, conditions, problems, complaints or other reason(s) for the encounter

Combination Codes

  • Combination codes are a single code used to report two diagnoses; it can be a diagnosis with a manifestation or a diagnosis with an associated complication described within a single diagnosis code
  • You can find a combination code by referring to subterm entries within the alphabetical index and by reading all notes of inclusion and exclusion in the tabular list
  • To utilize a combination code must fully capture the diagnoses reported, or the alphabetical index has provided instructions to do so
  • Multiple code assignments should not be reported when a combination code exists that fully captures the diagnostic conditions documented in the medical record
  • In the situation where the combination code lacks specificity to fully capture all manifestations or complications, an additional code is required and to be used as a secondary code

Sequela (Late effects)

  • Sequela codes are used to indicate that residual effects are present after the acute phase of an illness or injury is terminated
  • There is no time frame recommended for sequela it can range from early on or months to years later
  • Examples: Scar formation occurring from a previous burn, a deviated septum resulting from a nasal fracture, and the unfortunate infertility arising from a tubal occlusion that was the result of old Tuberculosis
  • Our guidelines for sequela state that the condition causing the sequela be sequenced first followed by the sequela code
  • There are exceptions to this guideline in the situation that the sequela is followed by a manifestation code mentioned in the tabular list and title or the sequela code is expanded using 4–6-character levels and fully describes the manifestation(s)

Acute and Chronic Conditions

  • There are times the same condition may be documented as Acute (Subacute) and chronic, and within the alphabetical index at the same indentation level, in this situation code both and sequence the Acute (Subacute) first

Impending or Threatened Condition

  • If the condition occurred at the time of discharge, code as a confirmed diagnosis
  • In the event the condition did not occur refer to the alphabetical index to determine if the condition presents a subentry for “impending” or “threatened” and you can also refer to main term entries for “impending” and for “threatened”

Syndromes

  • When indexing Syndromes follow the guidance of the alphabetical index, when there is no main term entry for that specific syndrome, report codes for documented manifestations of that syndrome

Borderline Diagnosis

  • If the provider documents a “borderline” diagnosis at the time of the patients discharge, it is coded as confirmed, excluding those conditions that have a specific entry within the alphabetical index such as “borderline Diabetes” then it is coded using the specific entry
  • In the event the documentation is unclear, query the provider

Use of Signs, Symptoms and Unspecified Codes

  • When a definitive has not been established by the end of an encounter it is appropriate to report signs and symptoms
  • If documentation is lacking to represent a specified type of a condition, use the unspecified code
  • Example: Mixed Hyperlipidemia is coded as E78.2, but if the provider only documented Hyperlipidemia the code assignment would be E78.5, Hyperlipidemia, Unspecified type

Scenario Time

   Now that the conventions and guidelines have been reviewed, let’s look at a practical example. In this case, a patient is diagnosed with dementia due to hypothyroidism. This scenario demonstrates how to navigate the Alphabetical Index, confirm the code selection in the Tabular List, and apply rules such as code first and the use of brackets to ensure proper sequencing of both conditions.

Scenario – Dementia with Hypothyroidism

A 72-year-old female presents with progressive confusion, memory loss, and difficulty concentrating. The provider documents dementia due to hypothyroidism.

Step 1: Locate in the Alphabetical Index

  • Main term: Hypothyroidism
  • Subterm: acquired → directs to see also Dementia, in, diseases classified elsewhere
  • Under “Dementia” → with → hypothyroidism = E03.9 [F02.80]

➡️ This gives two pieces of information:

  • E03.9 = Hypothyroidism, unspecified
  • Brackets [F02.80] = secondary code required (dementia as a manifestation).

Step 2: Verify in the Tabular List – E03.9

E03.9 – Hypothyroidism, unspecified

  • Includes: Myxedema NOS
  • Excludes1:
    • Iodine-deficiency related hypothyroidism (E00–E02)
    • Postprocedural hypothyroidism (E89.0)
  • Since the documentation specifies acquired hypothyroidism not related to iodine deficiency or a procedure, E03.9 is valid.

Step 3: Verify in the Tabular List – F02.80

F02.80 – Dementia in other diseases classified elsewhere, without behavioral disturbance

  • Instruction: Code first the underlying physiological condition, such as hypothyroidism.
  • Note: See Official Guidelines I.A.13 Etiology/Manifestation → this explains that the etiology (E03.9) is sequenced first, followed by the manifestation (F02.80).
  • ✅ Code assignment confirmed.

Step 4: Apply Conventions

  • Brackets [ ]: showed that F02.80 must be assigned with E03.9.
  • “Code First” note at F02.80: establishes sequencing order (etiology → manifestation).
  • Excludes1 at E03.9: reminds us not to code together with congenital, iodine-deficiency, or postprocedural hypothyroidism.
  • With Convention: Index automatically links dementia “with” hypothyroidism, no need for provider to write “due to.”

Final Code Assignment

  • E03.9 – Hypothyroidism, unspecified
  • F02.80 – Dementia in other diseases classified elsewhere, without behavioral disturbance

Closing Thoughts

    Understanding and applying ICD-10-CM conventions and general guidelines is essential for ensuring accuracy, compliance, and efficiency in medical coding. Although the details can feel complex at first, these rules exist to create consistency across the healthcare system and to safeguard both patients and providers. By mastering the foundational principles in Sections I.A and I.B, coders can build the confidence needed to navigate more advanced scenarios with precision. Ultimately, the more familiar you become with these guidelines, the stronger your coding accuracy and professional growth will be.

Works Cited

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