About ICD-10-CM Coding Guide
ICD-10-CM Coding Guide 2021
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The International Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM) is a morbidity classification published by the Centers for Medicare and Medicaid Services (CMS) and the National Center for Health Statistics (NCHS), two departments within the U.S. Department of Health and Human Services (DHHS).
The ICD-10-CM is based on the ICD-10, the classification of disease published by the World Health Organization (WHO).
ICD-10-CM is used for classifying diagnoses and reason for visits in all health care settings in the United States. It contains more than 69,000 codes compared to approximately 14,000 codes in its predecessor, ICD-9-CM.
This product contains 2021 ICD-10-CM codes, which are to be used for services provided in the United States from October 1, 2020 through September 30, 2021.
Structure and Format
ICD-10-CM codes have between 3 and 7 characters and have the following characteristics:
- Character types:
- 1st character: always alpha (all letters except "U")
- 2nd character: numeric (0-9)
- 3rd–7th character: alpha or numeric
- Decimal: placed after the 3rd character
- Chapter: ICD-10-CM is organized into 21 chapters based on anatomic or etiologic groupings.
Example: "Chapter 1 - Certain infectious and parasitic diseases (A00-B99)"
- Section: chapters are divided into Sections corresponding to logical grouping of diseases.
Example: "Intestinal infectious diseases (A00-A09)"
- Category: the first 3 characters of an ICD-10-CM code designate the diagnosis category.
Example: "A00 - Cholera"
- Etiology, Anatomic Site, Severity: the 4th–6th character correspond to etiology, anatomic site, severity, or other clinical details.
Example: "A00.0 - Cholera due to Vibrio cholerae 01, biovar cholerae"
- Extension: a 7th character is required in certain sections to encode characteristics of the patient encounter. If there are fewer than 6 characters in the code the placeholder character "X" is used to ensure that the seventh character is always in the seventh position.
Example: "S03.4xxA - Sprain of jaw, initial encounter"
- Chapter: ICD-10-CM is organized into 21 chapters based on anatomic or etiologic groupings.
- NEC: "Not elsewhere classified" - used when there is no specific code available to represent the condition.
Example: "A41.89 - Other specified sepsis" is used when sepsis is caused by an organism not specifically listed in ICD-10-CM as a cause of sepsis
- NOS: "Not otherwise specified" - equivalent to the word "unspecified" and indicates that the condition or documentation does not provide enough information to assign a more specific code.
Example: "597.81 - Urethral syndrome NOS"
How to Use This Product
Finding the Best-Fit Code
The ICD-10-CM Alphabetical Index contains terms and their corresponding code, organized in these areas:
- Diseases and Injuries
- External Causes of Injuries
- Drugs and Chemicals
To find the best-fit code:
- Select one of the indexes above
- Enter your term or phrase in the "Jump To" box (mobile) or browse the A-Z list (web)
- Select the most appropriate index terms provided, until reaching a code
Confirming the Code
To confirm the code:
- Review and investigate any Instructional Notations associated with the code
- Review the Code Tree (see below) to confirm that the most specific code is being assigned
This product provides a complete tree (hierarchy) for each ICD-10-CM code.
Each tree may contain the following levels:
Categories and codes are denoted in color as either Non-Billable or Billable:
This product provides mapping between ICD-9-CM and ICD-10-CM codes.
To Map from ICD-9-CM --> ICD-10-CM
- Browse the "ICD-9 Codes" index and select the desired ICD-9-CM code
Enter the ICD-9-CM code in the search box
- Once at the desired ICD-9-CM code page, go to the "Mapping" section
To Map from ICD-10-CM --> ICD-9-CM
- Browse/search any of the Alphabetical Indexes or the "ICD-10 Codes" index and select the desired ICD-10-CM code
- Once at the desired ICD-10-CM code page, go to the "Mapping" section
- Some codes map directly while others map "approximately"
- Some codes map to one of several options; others map to one of several scenarios
- A scenario describes two or more codes and how those codes are chosen and ordered
- ICD Version
- ⑨ = ICD-9-CM
- ⑩ = ICD-10-CM
- [Billable] = an ICD-10-CM code that can be used to indicate a diagnosis for reimbursement purposes
- [Non-Billable] = an ICD-10-CM diagnosis code that is not billable
- Present on Admission (POA)
- [POA Exempt] = an ICD-10-CM code that is exempt from POA reporting
- A condition is POA if it is present at the time of inpatient admission. Conditions that develop during an outpatient encounter, including in the emergency department or outpatient surgery, are considered POA.
- ICD Structure
- [Chapter] = one of 21 ICD-10-CM chapters that organizes codes based on anatomic or etiologic groups
- [Section] = a logical grouping of disease codes within a chapter
Accompanying many ICD-10-CM codes are instructional notations. Note types are described below.
- "Includes" notes define, or give examples of, the code or category.
"Excludes" notes describe codes that are independent of the main code in two different ways:
Not Coded Here (Excludes1)
- Codes found in this section should never be used at the same time as the main code, because the two conditions cannot occur together. Example: a congenital form versus an acquired form of the same condition.
Not Included Here (Excludes2)
- Codes found in this section are not part of the condition represented by the main code. However, a patient may have both conditions at the same time so it is acceptable to use both codes together.
Code First/Use Additional Code
- Certain conditions have both an underlying etiology and multiple body system manifestations.
- The ICD-10-CM coding convention requires the underlying condition be sequenced first 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 manifestation codes will have in the code title, "in diseases classified elsewhere."
- These 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.
- A "Code Also" note instructs that two codes may be required to fully describe a condition but the sequencing of the two codes is discretionary, depending on the severity of the conditions and the reason for the encounter.
The Centers for Medicare and Medicaid Services (CMS) has published the ICD-10-CM Official Guidelines for Coding and Reporting 2021 [PDF - 126 pages].
Reproduced below are the General Coding Guidelines (Section I. B.).
It is recommended that these guidelines be used as a companion to this product to assure accurate coding.
Locating a code in the ICD-10-CM
To select a code in the classification that corresponds to a diagnosis or reason for visit documented in a medical record, first locate the term in the Alphabetic Index, and then verify the code in the Tabular List. Read and be guided by instructional notations that appear in both the Alphabetic Index and the Tabular List.
It is essential to use both the Alphabetic Index and Tabular List when locating and assigning a code. The Alphabetic Index does not always provide the full code. Selection of the full code, including laterality and any applicable 7th character can only be done in the Tabular List. A dash (-) at the end of an Alphabetic Index entry indicates that additional characters are required. Even if a dash is not included at the Alphabetic Index entry, it is necessary to refer to the Tabular List to verify that no 7th character is required.
Level of Detail in Coding
Diagnosis codes are to be used and reported at their highest number of characters available.
ICD-10-CM diagnosis codes are composed of codes with 3, 4, 5, 6 or 7 characters. Codes with three characters are included in ICD-10-CM as the heading of a category of codes that may be further subdivided by the use of fourth and/or fifth characters and/or sixth characters, which provide greater detail.
A three-character code is to be 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 7th character, if applicable.
Code or codes from A00.0 through T88.9, Z00-Z99.8
The appropriate code or codes from A00.0 through T88.9, Z00-Z99.8 must be used to identify diagnoses, symptoms, conditions, problems, complaints or other reason(s) for the encounter/visit.
Signs and symptoms
Codes that describe symptoms and signs, as opposed to diagnoses, are acceptable for reporting purposes when a related definitive diagnosis has not been established (confirmed) by the provider. Chapter 18 of ICD-10-CM, Symptoms, Signs, and Abnormal Clinical and Laboratory Findings, Not Elsewhere Classified (codes R00.0 - R99) contains many, but not all codes for symptoms.
Conditions that are an integral part of a disease process
Signs and symptoms that are associated routinely with a disease process should not be assigned as additional codes, unless otherwise instructed by the classification.
Conditions that are not an integral part of a disease process
Additional signs and symptoms that may not be associated routinely with a disease process should be coded when present.
Multiple coding for a single condition
In addition to the etiology/manifestation convention that requires two codes to fully describe a single condition that affects multiple body systems, there are other single conditions that also require more than one code. “Use additional code” notes are found in the Tabular List at codes that are not part of an etiology/manifestation pair where a secondary code is useful to fully describe a condition. The sequencing rule is the same as the etiology/manifestation pair, “use additional code” indicates that a secondary code should be added.
For example, for bacterial infections that are not included in chapter 1, a secondary code from category B95, Streptococcus, Staphylococcus, and Enterococcus, as the cause of diseases classified elsewhere, or B96, Other bacterial agents as the cause of diseases classified elsewhere, may be required to identify the bacterial organism causing the infection. A “use additional code” note will normally be found at the infectious disease code, indicating a need for the organism code to be added as a secondary code.
“Code first” notes are also under certain codes that are not specifically manifestation codes but may be due to an underlying cause. When there is a “code first” note and an underlying condition is present, the underlying condition should be sequenced first.
“Code, if applicable, any causal condition first”, notes indicate that this code may be assigned as a principal diagnosis when the causal condition is unknown or not applicable. If a causal condition is known, then the code for that condition should be sequenced as the principal or first-listed diagnosis.
Multiple codes may be needed for sequela, complication codes and obstetric codes to more fully describe a condition. See the specific guidelines for these conditions for further instruction.
Acute and Chronic Conditions
If the same condition is described as both acute (subacute) and chronic, and separate subentries exist in the Alphabetic Index at the same indentation level, code both and sequence the acute (subacute) code first.
A combination code is a single code used to classify:
- Two diagnoses, or
- A diagnosis with an associated secondary process (manifestation)
- A diagnosis with an associated complication
Combination codes are identified by referring to subterm entries in the Alphabetic Index and by reading the inclusion and exclusion notes in the Tabular List.
Assign only the combination code when that code fully identifies the diagnostic conditions involved or when the Alphabetic Index so directs. Multiple coding should not be used when the classification provides a combination code that clearly identifies all of the elements documented in the diagnosis. When the combination code lacks necessary specificity in describing the manifestation or complication, an additional code should be used as a secondary code.
Sequela (Late Effects)
A sequela is the residual effect (condition produced) after the acute phase of an illness or injury has terminated. There is no time limit on when a sequela code can be used. The residual may be apparent early, such as in cerebral infarction, or it may occur months or years later, such as that due to a previous injury. Examples of sequela include: scar formation resulting from a burn, deviated septum due to a nasal fracture, and infertility due to tubal occlusion from old tuberculosis. Coding of sequela generally requires two codes sequenced in the following order: the condition or nature of the sequela is sequenced first. The sequela code is sequenced second.
An exception to the above guidelines are those instances where the code for the sequela is followed by a manifestation code identified in the Tabular List and title, or the sequela code has been expanded (at the fourth, fifth or sixth character levels) to include the manifestation(s). The code for the acute phase of an illness or injury that led to the sequela is never used with a code for the late effect.
Impending or Threatened Condition
Code any condition described at the time of discharge as “impending” or “threatened” as follows:
- If it did occur, code as confirmed diagnosis.
- If it did not occur, reference the Alphabetic Index to determine if the condition has a subentry term for “impending” or “threatened” and also reference main term entries for “Impending” and for “Threatened.”
- If the subterms are listed, assign the given code.
- If the subterms are not listed, code the existing underlying condition(s) and not the condition described as impending or threatened.
Reporting Same Diagnosis Code More than Once
Each unique ICD-10-CM diagnosis code may be reported only once for an encounter. This applies to bilateral conditions when there are no distinct codes identifying laterality or two different conditions classified to the same ICD-10-CM diagnosis code.
Some ICD-10-CM codes indicate laterality, specifying whether the condition occurs on the left, right or is bilateral. If no bilateral code is provided and the condition is bilateral, assign separate codes for both the left and right side. If the side is not identified in the medical record, assign the code for the unspecified side.
Documentation for BMI, Non-pressure ulcers and Pressure Ulcer Stages
For the Body Mass Index (BMI), depth of non-pressure chronic ulcers and pressure ulcer stage codes, code assignment may be based on medical record documentation from clinicians who are not the patient’s provider (i.e., physician or other qualified healthcare practitioner legally accountable for establishing the patient’s diagnosis), since this information is typically documented by other clinicians involved in the care of the patient (e.g., a dietitian often documents the BMI and nurses often documents the pressure ulcer stages). However, the associated diagnosis (such as overweight, obesity, or pressure ulcer) must be documented by the patient’s provider. If there is conflicting medical record documentation, either from the same clinician or different clinicians, the patient’s attending provider should be queried for clarification.
The BMI codes should only be reported as secondary diagnoses. As with all other secondary diagnosis codes, the BMI codes should only be assigned when they meet the definition of a reportable additional diagnosis (see Section III, Reporting Additional Diagnoses).
Follow the Alphabetic Index guidance when coding syndromes. In the absence of Alphabetic Index guidance, assign codes for the documented manifestations of the syndrome. Additional codes for manifestations that are not an integral part of the disease process may also be assigned when the condition does not have a unique code.
Documentation of Complications of Care
Code assignment is based on the provider’s documentation of the relationship between the condition and the care or procedure. The guideline extends to any complications of care, regardless of the chapter the code is located in. It is important to note that not all conditions that occur during or following medical care or surgery are classified as complications. There must be a cause-and-effect relationship between the care provided and the condition, and an indication in the documentation that it is a complication. Query the provider for clarification, if the complication is not clearly documented.
If the provider documents a "borderline" diagnosis at the time of discharge, the diagnosis is coded as confirmed, unless the classification provides a specific entry (e.g., borderline diabetes). If a borderline condition has a specific index entry in ICD-10-CM, it should be coded as such. Since borderline conditions are not uncertain diagnoses, no distinction is made between the care setting (inpatient versus outpatient). Whenever the documentation is unclear regarding a borderline condition, coders are encouraged to query for clarification.
Use of Sign/Symptom/Unspecified Codes
Sign/symptom and “unspecified” codes have acceptable, even necessary, uses. While specific diagnosis codes should be reported when they are supported by the available medical record documentation and clinical knowledge of the patient’s health condition, there are instances when signs/symptoms or unspecified codes are the best choices for accurately reflecting the healthcare encounter. Each healthcare encounter should be coded to the level of certainty known for that encounter.
If a definitive diagnosis has not been established by the end of the encounter, it is appropriate to report codes for sign(s) and/or symptom(s) in lieu of a definitive diagnosis. When sufficient clinical information isn’t known or available about a particular health condition to assign a more specific code, it is acceptable to report the appropriate “unspecified” code (e.g., a diagnosis of pneumonia has been determined, but not the specific type). Unspecified codes should be reported when they are the codes that most accurately reflect what is known about the patient’s condition at the time of that particular encounter. It would be inappropriate to select a specific code that is not supported by the medical record documentation or conduct medically unnecessary diagnostic testing in order to determine a more specific code.
Chapter-Specific Coding Guidelines
In addition to general coding guidelines, there are guidelines for specific diagnoses and/or conditions in the classification (see ICD-10-CM Official Guidelines for Coding and Reporting 2021 [PDF]). Examples include (bolded are new sections or significant updates):
- Methicillin Resistant Staphylococcus aureus (MRSA) Conditions
- Zika virus infections
- Coronavirus infections (COVID-19)
- Diabetes mellitus
- Chronic pain
- Cardiovascular disease and acute myocardial infarction
- Chronic obstructive pulmonary disease (COPD)
- Influenza and ventilator associated pneumonia
- Vaping-related disorders
- Pressure and non-pressure ulcers
- Chronic kidney disease
- General rules for obstetric cases and perinatal care (including COVID-19 infection)
- Adverse effects, poisoning, underdosing and toxic effects
- External cause coding guidelines
- Terrorism guidelines
- Categories of Z Code
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