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1.1 ICD-10-CM Structure, Formats, and Core Conventions

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AHIMA Certified Coding Specialist CCS

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1 Chapter 1: ICD-10-CM Diagnosis Coding Fundamentals & UHDDS Guidelines

1.1 ICD-10-CM Structure, Formats, and Core Conventions 1.2 UHDDS Principal Diagnosis Selection Rules 1.3 Secondary Diagnoses Reporting & Present on Admission (POA) Indicators

2 Chapter 2: Circulatory, Cardiac & Vascular Inpatient Coding

2.1 Hypertension, Hypertensive Heart & Chronic Kidney Disease, and CAD 2.2 Acute Myocardial Infarction (AMI) Types & Sequencing 2.3 Heart Failure Types, Acuity & Cardiomyopathy 2.4 Cerebrovascular Disease, Acute Stroke, and Late Effects

3 Chapter 3: Respiratory, Sepsis & Infectious Conditions Coding

3.1 Sepsis, Severe Sepsis, and Septic Shock Guidelines 3.2 Pneumonias, Aspiration Pneumonia, and Influenza 3.3 COPD, Asthma, and Acute Respiratory Failure

4 Chapter 4: Renal, Endocrine, Gastrointestinal & Nutritional Disorders

4.1 Acute Kidney Injury (AKI) & Chronic Kidney Disease (CKD) 4.2 Diabetes Mellitus Types, Manifestations & Complications 4.3 Protein-Calorie Malnutrition & Endocrine Emergencies 4.4 Gastrointestinal Bleeding, Ulcers, and Acute Abdominal Pathologies

5 Chapter 5: Neoplasms, Obstetrics, Perinatal, Injury & Trauma Coding

5.1 Neoplasm Table, Primary, Secondary & Encounters for Therapy 5.2 Obstetrics, Normal/Complicated Deliveries & Puerperium 5.3 Perinatal Conditions & Congenital Malformations 5.4 Traumatic Injuries, Fractures & 7th Character Extensions

6 Chapter 6: ICD-10-PCS Architecture, Conventions & Approaches

6.1 ICD-10-PCS 7-Character System Architecture & Conventions 6.2 ICD-10-PCS The Seven Surgical Approaches 6.3 General PCS Guidelines & Multiple Procedure Rules

7 Chapter 7: ICD-10-PCS Root Operations: Cutting Out, Taking Out & Altering Body Parts

7.1 Excision vs Resection vs Destruction 7.2 Extraction, Drainage, and Extirpation 7.3 Detachment & Amputation Coding 7.4 Release vs Division

8 Chapter 8: ICD-10-PCS Root Operations: Putting In, Putting Back & Moving Body Parts

8.1 Replacement vs Supplement vs Insertion 8.2 Reposition, Transfer, and Reattachment 8.3 Bypass, Dilation, Occlusion, and Restriction

9 Chapter 9: High-Volume Inpatient Surgical & Medical Procedures in ICD-10-PCS

9.1 Coronary Artery Bypass Grafting (CABG) & Percutaneous Coronary Interventions 9.2 Total Joint Arthroplasties & Spinal Fusion 9.3 Mechanical Ventilation & Extracorporeal Therapies

10 Chapter 10: Outpatient CPT & HCPCS Level II Coding & Modifiers

10.1 CPT Evaluation and Management (E/M) Guidelines 10.2 CPT Surgical, Radiology, Pathology & HCPCS Level II Coding 10.3 CPT Modifiers in Hospital Outpatient & Professional Coding

11 Chapter 11: Inpatient Record Review, Clinical Validation & Documentation Integrity

11.1 Inpatient Health Record Analysis & UHDDS Documentation Hierarchy 11.2 Clinical Validation of Diagnoses vs Coding Compliance 11.3 Identifying Reportable Secondary Conditions & Complications

12 Chapter 12: Compliant Provider Queries & AHIMA/ACDIS Standards

12.1 AHIMA/ACDIS Compliant Query Standards & Ethical Principles 12.2 Compliant Query Formats: Open-Ended, Multiple-Choice & Yes/No 12.3 High-Impact Clinical Query Scenarios & Provider Workflow

13 Chapter 13: Inpatient Prospective Payment System (IPPS) & MS-DRG Mechanics

13.1 IPPS Reimbursement Structure & MS-DRG Grouping Logic 13.2 Complications and Comorbidities (CC) & Major CC (MCC) Impact 13.3 All-Patient Refined DRG (APR-DRG) System 13.4 Inpatient Transfer Policies, Cost Outliers & Short-Stay Rules

14 Chapter 14: Outpatient Reimbursement, Coding Edits & Fraud/Abuse Compliance

14.1 Hospital Outpatient Prospective Payment System (OPPS) & APCs 14.2 National Correct Coding Initiative (NCCI) Edits & MUEs 14.3 Hospital-Acquired Conditions (HAC) Reduction & Quality Programs 14.4 Healthcare Fraud, Abuse Laws, Audits & AHIMA Ethical Standards

15 Chapter 15: Health Information Technology, CAC, Encoders & Data Security

15.1 Electronic Health Records (EHR) & Computer-Assisted Coding (CAC) 15.2 Logic-Based Encoders, Groupers & Health Data Systems 15.3 HIPAA Privacy/Security, Cybersecurity & Internal Coding Audits

1.1 ICD-10-CM Structure, Formats, and Core Conventions

Key Takeaways Explain This Topic AI

ICD-10-CM codes are alphanumeric and range from 3 to 7 characters; codes longer than three characters place a decimal after the third character, and designated categories use placeholder 'X' when needed to position a required extension.

Coders must strictly adhere to the two-step lookup workflow, verifying all candidate codes found in the Alphabetic Index within the Tabular List to capture essential instructional notes and tabular conventions.

Excludes1 notes signify pure mutual exclusivity ('NOT CODED HERE!'), whereas Excludes2 notes indicate distinct non-included conditions ('NOT INCLUDED HERE') that may be co-reported if documented.

The relational terms 'with' and 'in' in the Alphabetic Index or Tabular List establish a presumed causal relationship between two conditions, requiring them to be coded as related unless explicitly documented otherwise.

Last updated: August 2026

ICD-10-CM Structure, Formats, and Core Conventions

AHIMA CCS Exam Focus: Mastering the structural rules, punctuation, and instructional conventions of the International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) is the foundation of all inpatient and outpatient medical coding. On the CCS exam, candidates are evaluated on their ability to interpret instructional notes, differentiate between Excludes1 and Excludes2 notes, apply 7th character extensions accurately, and recognize presumed causal relationships established by conventions such as "with."

1. Regulatory Foundation and Architecture of ICD-10-CM

ICD-10-CM is the official HIPAA-mandated code set for reporting diagnoses and inpatient/outpatient morbidity across all United States healthcare settings. It is maintained under the oversight of the Coordination and Maintenance Committee , co-chaired by the National Center for Health Statistics (NCHS) (responsible for diagnosis coding) and the Centers for Medicare & Medicaid Services (CMS) (responsible for procedure coding and hospital payment policy).

Code Construction and Format

Every ICD-10-CM code is composed of 3 to 7 alphanumeric characters , formatted with distinct structural rules:

Character 1: Always an alphabetical letter (A–Z). The letter U is reserved for special purposes and is used in assigned codes such as U07.1 ( COVID-19 ); it is not excluded from the code set.

Character 2: Always numeric (0–9).

Characters 3 through 7: Can be either alphabetical or numeric (alpha characters are not case-sensitive).

Decimal Placement: A decimal point is placed immediately after the third character in all codes containing four or more characters. Three-character category codes do not feature a decimal point (e.g., I10 , A38 ). [Category] . [Subcategory] [Subclassification] [Extension]

  • E11 . 6 21 -
  • (3 Chars) . (4th Char) (5th & 6th) (7th Char)
  • Type 2 Diabetes . With foot ulcer Right foot [If required]
  • Hierarchical Decomposition

Category (3 Characters): Represents a single disease entity, closely related group of conditions, or broad clinical presentation (e.g., K25 Gastric ulcer , E11 Type 2 diabetes mellitus ).

Subcategory (4 to 5 Characters): Specifies etiology, anatomical site, severity, or clinical manifestations (e.g., E11.6 Type 2 diabetes mellitus with other specified complications , E11.62 Type 2 diabetes mellitus with skin complications ).

Subclassification / Complete Code (6 to 7 Characters): Represents the highest level of clinical specificity, incorporating precise laterality, disease subtype, or external causal mechanisms (e.g., E11.621 Type 2 diabetes mellitus with foot ulcer ).

  • 2. Advanced Mechanics: Placeholders and 7th Character Extensions
  • The Dummy Placeholder 'X'

ICD-10-CM utilizes the letter X as a mandatory placeholder in specific categories to achieve two vital functions:

Future Expansion: Providing space for future subclassifications without disrupting the hierarchical numbering system.

Standardizing 7th Character Position: When a code requires a 7th character extension but contains fewer than six characters in its base code, the placeholder X must be inserted into all empty intervening character positions (positions 4, 5, and/or 6) to ensure the extension sits strictly in the seventh position .

Critical Compliance Rule: If a code requires a 7th character extension and the dummy placeholder X is omitted or placed in the wrong slot, the resulting code is structurally invalid, causing immediate electronic claim rejection and MS-DRG grouping errors.

Example 1 (5-character base code): Accidental poisoning by penicillin, initial encounter.

Base code: T36.0X1 (Poisoning by penicillins, accidental (unintentional))

Character 4 is 0 , Character 5 is X (placeholder), Character 6 is 1 (intent: accidental), Character 7 is A (initial encounter).

Final Valid Code: T36.0X1A

Example 2 (3-character base category): Initial encounter for assault by handgun discharge.

  • Base category: X93
  • Positions 4, 5, and 6 require placeholders: X93XXXA
  • 7th Character Extensions in Injury and Musculoskeletal Coding

In Chapter 19 ( Injury, Poisoning, and Certain Other Consequences of External Causes ) and Chapter 13 ( Diseases of the Musculoskeletal System and Connective Tissue ), 7th character extensions indicate the phase of clinical management:

7th Character Designation Clinical Definition & Criteria

A Initial Encounter The patient is receiving active treatment for the condition (e.g., surgical repair, emergency department evaluation, initial casting, debridement by a new physician).

D Subsequent Encounter The patient has completed active treatment and is receiving routine care during the healing or recovery phase (e.g., cast removal, suture removal, medication adjustment, rehabilitation follow-up).

S Sequela Complications or conditions that arise as a direct late effect of an acute injury or disease after the acute phase has completely resolved. Sequela Coding Rule (Two Codes Required):

  • [Code 1: Residual Condition / Current Complaint] (e.g., Scar, Keloid, Arthritis)
  • +
  • [Code 2: Cause of Sequela Code ending in 'S'] (e.g., Old Burn, Previous Fracture)

Note on Pathological and Traumatic Fractures: Certain fracture categories utilize expanded 7th characters beyond A , D , and S to represent open fracture severity (Gustilo-Anderson Classifications I, II, III-A/B/C) and healing status (e.g., K for subsequent encounter with nonunion, P for subsequent encounter with malunion, G for subsequent encounter with delayed healing).

3. The Mandatory Two-Step Coding Workflow

Accurate diagnosis coding requires strict adherence to a standard two-step search procedure. A coder must never assign a code directly from the Alphabetic Index without verifying the code in the Tabular List.

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Why Index-Only Coding Fails

The Alphabetic Index does not display Excludes1 or Excludes2 notes.

The Alphabetic Index does not indicate required 7th character extensions or instructional sequencing notes ( Code First , Use Additional Code ).

The Alphabetic Index may only list a 3- or 4-character base category when a 5-, 6-, or 7-character code is required by the classification.

4. Instructional Notes and Classification Conventions

ICD-10-CM incorporates precise instructional conventions that dictate code selection, co-reporting validity, and sequencing hierarchy.

Excludes1 vs. Excludes2 Notes

Understanding the fundamental distinction between Excludes1 and Excludes2 is one of the most heavily tested domains on the AHIMA CCS exam.

Feature Excludes1 ("NOT CODED HERE!") Excludes2 ("NOT INCLUDED HERE")

Core Meaning The excluded condition is not coded at the code above the note; the conditions generally are not reported together. The excluded condition is not part of the code, but the patient may have both simultaneously.

Co-Reporting Rule Never code both together (unless official exception criteria are met). Permissible to report both codes together if documentation supports both conditions.

Clinical Rationale The classification treats the listed concepts as mutually exclusive at that location, subject to the official unrelated-condition exception. The classification distinguishes conditions that may coexist and be coded together when documented.

Official Exception If two conditions linked by Excludes1 are completely unrelated to each other (e.g., at different anatomical sites or different etiologies), both may be coded with provider documentation. N/A — Co-reporting is standard when both conditions exist.

Applying Excludes1: Read the note at the exact Tabular location. Do not report the excluded code with the code above the note unless the two documented conditions are unrelated and the official exception applies; query when the relationship is unclear.

Applying Excludes2: The patient may have both conditions. Report both only when each is documented, reportable for the encounter, and otherwise permitted by the Tabular List and official guidelines.

Etiology and Manifestation Conventions ("Code First" & "Use Additional Code")

For conditions where an underlying systemic etiology produces localized manifestations, ICD-10-CM establishes an immutable sequencing hierarchy:

Code First (Underlying Disease): Appears at the manifestation code in the Tabular List. The underlying etiology code is sequenced before the manifestation code. This relative order does not by itself make the etiology the principal/first-listed diagnosis; principal diagnosis selection still follows the applicable setting and encounter rules.

Use Additional Code: Appears at the etiology code in the Tabular List, instructing the coder that a secondary code is required to capture the specific manifestation or complication.

Brackets [ ] in Alphabetic Index: Manifestation codes appear in slanted brackets [ ] in the Alphabetic Index (e.g., Dementia, in Alzheimer's disease G30.9 [F02.80] ). Manifestation codes can never be sequenced as the principal or primary diagnosis . Sequencing Requirement:

1. [Etiology Code—sequenced before the manifestation] (e.g., G30.9 Alzheimer's disease)

2. [Manifestation Code—never principal/first-listed] (e.g., F02.80 Dementia in other diseases without behavioral disturbance)

"Code Also"

A Code Also note indicates that two codes may be required to fully describe a clinical condition, but unlike Code First , it does not mandate a rigid sequencing order . Sequencing depends entirely on the reason for the encounter, UHDDS principal diagnosis rules, or clinical severity.

Relational Conventions: "With" vs. "And"

"And": In code titles and category descriptions, the word "and" must be interpreted as "and/or" (e.g., A18.0 Tuberculosis of bones and joints means tuberculosis of bones, tuberculosis of joints, or tuberculosis of both bones and joints).

"With" / "In": The word "with" or "in" in the Alphabetic Index (immediately under a main term) or in an instructional note in the Tabular List indicates a presumed causal relationship between the two conditions.

The Presumption Rule: Coders must code the two conditions as related even in the absence of explicit provider documentation linking them with terms like "due to" or "secondary to."

The Overriding Rule: If the physician explicitly states that the two conditions are unrelated (e.g., "chronic kidney disease due to amyloidosis, not diabetes"), the presumptive link is broken, and the conditions are coded separately. Conditions with Presumed Causal Link ("With"):

  • Diabetes (E08-E13) WITH: Nephropathy, Neuropathy, Retinopathy, CKD, Foot Ulcer, Arthropathy
  • Hypertension (I10) WITH: Heart Failure (I11.-), Chronic Kidney Disease (I12.-), Both (I13.-)
  • Sepsis and acute organ dysfunction are **not** linked by a presumed “with” relationship; severe sepsis requires provider-documented association

Specificity Conventions: NEC vs. NOS

NEC (Not Elsewhere Classifiable): Corresponds to "Other specified." Used when the clinical record contains highly detailed, specific clinical documentation of a condition, but the ICD-10-CM classification system does not contain a unique code designed specifically for that variant.

NOS (Not Otherwise Specified): Corresponds to "Unspecified." Used when the clinical documentation in the medical record is insufficient or non-specific, preventing the coder from assigning a more granular code.