What Is ICD-10?

The 10th edition of the International Classification of Diseases (ICD-10) is a medical coding system designed by the World Health Organization (WHO) to catalog health conditions by similar disease categories under which more specific conditions are listed, thus mapping complex diseases to broader morbidities.

Many countries now use national variations of ICD-10, each modified to align with their unique healthcare infrastructure.

The U.S. version of ICD-10, created by the Centers for Medicare & Medicaid Services (CMS) and the National Center for Health Statistics (NCHS), consists of two medical code sets — the International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) and International Classification of Diseases, Tenth Revision, Procedure Coding System (ICD-10-PCS).

ICD-10-PCS is a classification system of medical codes used in hospital settings to report inpatient procedures.

ICD-10-CM is used for medical claim reporting in all healthcare settings and is a standardized classification system of diagnosis codes that represent conditions and diseases, related health problems, abnormal findings, signs and symptoms, injuries, external causes of injuries and diseases, and social circumstances.

For a medical provider to receive reimbursement for medical services, ICD-10-CM codes are required to be submitted to the payer. While CPT^® codes depict the services provided to the patient, ICD-10-CM codes depict the patient’s diagnoses that justify the services as medically necessary.

Origins of ICD-10 Coding

The roots of ICD-10 coding go back to the 1850s. The first edition, known as the International List of Causes of Death, was adopted by the International Statistical Institute in 1893.

WHO assumed oversight of the ICD in 1948 with the main intention of tracking — and helping to eliminate — diseases within various populations. At the time, the Sixth Revision, which introduced causes of morbidity to the system, had just been published.

In 1957 and 1968, WHO released ICD-7 and ICD-8, respectively. Shortly after the release of ICD-9 in 1979, the United States created its own version, known as ICD-9-CM.

The development of ICD-9-CM was a tremendous boon. Not only did the new system expand the ability to capture enhanced morbidity data, but it also incorporated surgical procedures and other items necessary to categorize the needs of hospitals.

But ICD-9-CM, updated annually by CMS and the NCHS, was a limited system with a limited capacity for the addition of codes to keep pace with modern healthcare. And it was already a three-volume set, with the first two volumes dedicated to diagnosis codes and the third volume containing inpatient procedural codes.

So, after decades in the making, CMS and NCHS adopted ICD-10 and adapted the classification to create a new version, ICD-10-CM, which no longer included the third volume of inpatient procedure codes.

What happened to the inpatient procedure codes? CMS determined the need for better organization and funded a project with 3M Health Information Systems in 1995 to develop ICD-10-PCS.

Differences Between ICD-10-CM and ICD-10-PCS

Both ICD-10-CM and ICD-10-PCS came into effect for medical claims reporting on Oct. 1, 2015. But the two code sets differ vastly. The primary distinctions are:

• ICD-10-CM is a diagnosis code set used for all healthcare settings.

• ICD-10-PCS is a procedure code set used only in hospital inpatient settings.

The terms ICD-10-CM and ICD-10 are often used interchangeably in the United States, in part because ICD-10-CM is used across all healthcare settings to report diagnoses. It's also important to remember that ICD-10 may refer to the WHO medical coding system, which differs from the ICD-10-CM and ICD-10-PCS versions used in the United States.

ICD-10 Provides Greater Specificity

Although some circumstance still require ICD-9 to ICD-10 code conversion, those circumstances are becoming uncommon, and ICD-9 is gradually fading into coding history. The ICD-10 codes we use today are more specific than ICD-9-CM codes and allow for detailed classifications of patients’ conditions, injuries, and diseases. Medical coders are now equipped to capture anatomic sites, etiologies, and comorbidities and complications, as well as severity of illnesses. The magnitude of ICD-10 codes currently in effect — there are roughly 19 times more procedure codes and five times more diagnosis codes than in ICD-9-CM — illustrates the increased granularity available to represent real-world clinical practice and medical technology advances. ICD-10 also provides greater capacity to track essential data of disease patterns and outbreaks of disease, and to help identify characteristics and circumstances of affected individuals. With greater specificity, providers and payers can use ICD-10 diagnosis codes to track information about patients’ conditions and the types and number of treatments patients receive. They can gather and analyze code use to:

• Measure the safety and efficacy of patient care

• Determine the health status and risk factors of defined populations

• Improve and monitor providers’ performances

• Assess healthcare costs

• Investigate and prevent coding and billing abuses

Structure of ICD-10-CM Codes

ICD-10-CM codes consist of three to seven characters. Every code begins with an alphabetical character that corresponds to the chapter the code is classified in. The second and third characters are numbers. The fourth, fifth, sixth, and seventh characters can be numbers or letters. Here are some examples of ICD-10-CM codes and the conditions they represent.

• G10 Huntington's disease

• K26.1 Acute duodenal ulcer with perforation

• A37.81 Whooping cough due to other Bordetella species with pneumonia

• I25.111 Atherosclerotic heart disease of native coronary artery with angina pectoris with documented spasm

• M80.021G Age-related osteoporosis with current pathological fracture, right humerus, subsequent encounter for fracture with delayed healing

With each additional character, the ICD-10-CM code depicts more specific diagnostic information. Diagnoses must be coded to the highest level of specificity available in the ICD-10-CM code set. For instance, N04.- Nephrotic syndrome should not be assigned for a patient diagnosed with nephrotic syndrome with minor glomerular abnormality. The finding of minor glomerular abnormality calls for an additional digit and would be coded as N04.0 Nephrotic syndrome with minor glomerular abnormality. Payers will not accept incomplete (also called truncated) codes. This degree of coding detail does not need to be memorized. The code set is organized to lead coders to the most specific diagnosis code selection.

Navigating ICD-10-CM Codes

In addition to the Tabular List of codes, ICD-10-CM includes an Alphabetic Index. There are also ICD-10-CM Official Guidelines. These are designed to help medical coders find and assign the correct diagnosis codes for their patients’ conditions.

ICD-10-CM Official Guidelines

CMS and NCHS provide the ICD-10-CM Official Guidelines. Section I includes conventions and general coding guidelines applicable to the overall classification, and chapter-specific guidelines. Section II outlines rules and principles for the selection of a principal diagnosis. Section III explains the rules for reporting additional diagnoses, and Section IV is specific to diagnosis coding and reporting of outpatient services.

Alphabetic Index

This four-part index encompasses the Index of Diseases and Injury, the Index of External Causes of Injury, the Table of Neoplasms, and the Table of Drugs and Chemicals, which are designed to streamline the process of locating the necessary diagnosis codes and ICD-10-CM coding instructions.  

Index to Diseases and Injuries

Arranged alphabetically across hundreds of pages, this index list diseases, injuries, and diagnostic terms, along with their associated codes (or code categories). Sub-lists of related or more-detailed terms with their corresponding codes appear beneath many of the main terms.

Table of Neoplasms

Neoplasms are found in the table by anatomical site, and then the selection can be quickly narrowed down by cross-referencing the anatomical location with six traits, depending on whether the diagnosis is malignant primary, malignant secondary, carcinoma (ca) in situ, benign, of uncertain behavior, or of unspecified behavior. (The neoplasm diagnosis codes are laid out in table format to condense the index, which would consume reams of pages if laid out in list format.)

Table of Drugs and Chemicals

Similar to the Table of Neoplasms, the Table of Drugs and Chemicals helps to locate codes for poisoning or allergic reactions by cross-referencing the responsible substance with six circumstances that specify whether the substance-related condition was accidental, intentional self-harm, assault, undetermined, an adverse effect, or the result of underdosing.

Index of External Causes of Injuries

ICD-10-CM external cause codes provide details explaining the events surrounding an injury, which are especially useful in collecting statistics for policy decisions concerning public health. These ICD-10-CM codes also play an important role in workers’ compensation claims. The Index of External Causes of Injuries is arranged like the Index to Diseases and Injuries, listing entries and subentries in alphabetical order along with their corresponding codes.

Tabular List

The Tabular List refers to the actual listing of ICD-10-CM codes and their descriptors. There are instructions to help in applying the codes correctly, as well as lists of additional diagnoses that a code applies to, sequencing rules, or which diagnoses codes are excluded from an ICD-10-CM code. The Tabular List is organized into 22 chapters according to body system or condition, with diagnosis codes listed alphanumerically in each chapter.

ICD-10-CM Chapters and Code Ranges

Using the ICD-10-CM Tabular List

The first three characters of an ICD-10-CM code refer to the code category and represent common traits, a disease or group of related diseases, and conditions. Once an ICD-10-CM code is found in the Alphabetic Index, the code details and instructions must be reviewed in the Tabular List to confirm that it’s the right diagnosis code and to code it properly. In the chapters listed above, the code ranges are included in the titles. These sets of alphanumeric characters further define the chapter title by telling the categories contained within it.

When coding a patient with retinopathy, for instance, the Alphabetic Index includes the entry “Retinopathy (background) H35.00.” The code details are in the chapter dedicated to diseases of the eye. But if the patient has diabetic retinopathy, the index offers several code options in the E08-E13 range, which are in Chapter 4 Endocrine, Nutritional and Metabolic Diseases (E00-E89).

The code range within each chapter in the Tabular List is broken down into subchapters called blocks, similar to the following block summary from Chapter 4.

• E00-E07 Disorders of thyroid gland

• E08-E13 Diabetes mellitus

• E15-E16 Other disorders of glucose regulation and pancreatic internal secretion

• E20-E35 Disorders of other endocrine glands

• E36 Intraoperative complications of endocrine system

• E40-E46 Malnutrition

• E50-E64 Other nutritional deficiencies

• E65-E68 Overweight, obesity and other hyperalimentation

• E70-E88 Metabolic disorders

• E89 Postprocedural endocrine and metabolic complications and disorders, not elsewhere classified

Each block contains one or more categories, many of which are divided into subcategories in the ICD-10-CM Tabular List. In the example below, E13.- is a category. E13.0-, E13.1-, E13.2-, and E13.3- are subcategories, as are E13.31- and E13.32-.

E13 Other specified diabetes mellitus:

• E13.0 Other specified diabetes mellitus with hyperosmolarity;

  • E13.00 ... without nonketotic hyperglycemic-hyperosmolar coma (NKHHC)

  • E13.01 ... with coma

• E13.1  Other specified diabetes mellitus with ketoacidosis;

  • E13.10 ... without coma

  • E13.11 ... with coma

• E13.2 Other specified diabetes mellitus with kidney complications

  • E13.21 Other specified diabetes mellitus with diabetic nephropathy

  • E13.22 Other specified diabetes mellitus with diabetic chronic kidney disease

  • E13.29 Other specified diabetes mellitus with other diabetic kidney complication

• E13.3 Other specified diabetes mellitus with ophthalmic complications

  • E13.31 Other specified diabetes mellitus with unspecified diabetic retinopathy;

    • E13.311 ... with macular edema

    • E13.319 ... without macular edema

  • E13.32 Other specified diabetes mellitus with mild nonproliferative diabetic retinopathy

    • E13.321 Other specified diabetes mellitus with mild nonproliferative diabetic retinopathy with macular edema;

      • E13.3211 ... right eye

      • E13.3212 ... left eye

      • E13.3213 ... bilateral

      • E13.3219 ... unspecified eye

Note that ICD-10-CM coding employs a decimal point following the category and preceding the subcategory. Every character to the right of the decimal point adds specific information about the diagnosis. Among the diagnosis codes listed above in the E13 category, none are the same. Each ICD-10-CM code represents a unique diagnosis. That’s why coders must always assign subdivisions until the highest level of specificity is captured when reporting ICD-10-CM codes to payers, claims clearinghouses, or billing and collection agencies.

7th Character in ICD-10-CM Coding

Not all ICD-10-CM codes require a 7th character, but when they do, the 7th character serves a particular purpose — like signifying laterality or defining whether the code represents an initial encounter for the problem, a subsequent encounter, or a sequela arising from another condition. In the E13.- category listed above, codes demonstrating laterality for E13.321- are shown with the 7th character added. The ICD-10 code book, however, may provide instructions to add the 7th character, rather than listing the complete codes, such as:

One of the following 7th characters is to be assigned to codes in subcategory E13.32 to designate laterality of the disease:

1 - right eye, 2 - left eye, 3 - bilateral, 9 - unspecified eye

Because the 7th character is required to correctly code E13.321-, option “9 - unspecified eye” is included in the event that clinical documentation doesn’t indicate which eye is affected.

Coding the 7th Character with Placeholder “X”

In some cases, codes that require a 7th character don’t have six characters of specificity. This is where placeholder “X” comes into play.

ICD-10-CM placeholder “X” was created to accommodate expansion of the code set with anticipation that characters with specific meanings will eventually occupy the designated spaces. In the interim, medical coders are required to use placeholder “X” to fill in the missing character(s), thereby preserving the diagnosis code classification structure.

For codes with fewer than six characters that require a 7th character, the placeholder “X” should be assigned for all absent characters. In other words, the 7th character must always be the 7th character. A code is invalid if the “X” isn’t added to hold the place of the 4th, 5th, or 6th character before adding the 7th.

Some code examples using ICD-10 placeholder “X” include:

• T18.1 Foreign body in esophagus becomes T18.1XXS Foreign body in esophagus, sequela.

• H40.10 Unspecified open-angle glaucoma becomes H40.10X3 Unspecified open-angle glaucoma, severe stage.

ICD-10-PCS Basics

This inpatient medical code set has a logical, consistent structure that follows a logical, consistent process down to each character level in each code. Medical coders will find constructing codes in ICD-10-PCS equally logical and consistent. While incorporating input from organizations, physicians, and researchers, the design of ICD-10-PCS follows recommendations from the National Committee on Vital and Health Statistics (NCVHS). ICD-10-PCS is completely separate from ICD-10-CM and is quite distinct from CPT^® — the other procedural code set used to report services and procedures in outpatient healthcare settings. Structural attributes of ICD-10-PCS include:  

• Standardized level of specificity: Each of the seven alphanumeric characters in an ICD-10-PCS code define procedure details such as body part, approach, and device used.

• Unique: A unique code is available for each significantly different procedure, and each code retains its unique definition. For example, the same procedure performed on a different body part has its own code, and every procedure that uses a different approach has a unique code.

• Expandability: The structure of PCS is designed to accommodate emerging procedures and technologies and allow for the creation of new codes without disrupting the system.

• Standardized terminology: Just as characters and values are defined within the system, the terminology used in ICD-10-PCS is standardized to provide precise and stable definitions of all procedures. For example, in medical operative report terminology, the word excision describes a variety of surgical procedures. When coding in PCS, excision describes a single, precise surgical objective. For example, “Cutting out or off, without replacement, a portion of a body part.” 

ICD-10-PCS System Organization

ICD-10-PCS is composed of 17 sections, represented by the numbers 0-9 and the letters B-D, F-H, and X. The broad procedure categories contained in these sections range from surgical procedures to substance abuse treatment and new technology.

ICD-10-PCS Sections - April 2024 Version

Medical and Surgical Section

All procedure codes in the Medical and Surgical section begin with the section value 0. The Medical and Surgical section contains the majority of PCS codes — all of which are used solely in U.S. inpatient hospital settings.

Medical and Surgical-Related Sections

Sections 1–9 of ICD-10-PCS comprise the Medical and Surgical-Related sections. These sections include obstetrical procedures, administration of substances, measuring and monitoring of body functions, and extracorporeal therapies.

Ancillary Sections

Sections B–D and F–H comprise the ancillary sections of ICD-10-PCS. These six sections include imaging procedures, nuclear medicine, and substance abuse treatment.

Structure of PCS Codes

All ICD-10-PCS codes consist of seven characters. Each character can be one of 34 values — the numbers 0-9 and the letters of the alphabet, minus O and I (to avoid confusion with numbers zero and one). The 34 possible character values give ICD-10-PCS vast potential, in that the same character value in a different character position carries a different meaning. Each character position in the PCS represents a category of information about the procedure. From the Medical and Surgical section, character positions represent the following categories of information:

For example, consider ICD-10-PCS code 0LB50ZZ Excision of right lower arm and wrist tendon, open approach.

Using PCS Tables

To construct complete and valid codes in ICD-10-PCS, medical coders refer to tables. PCS tables are organized in alphanumeric order in a series by section, which is the first character of a code. Tables that begin with 0 to 9 are listed first, then tables beginning with B-D, then letters F-X, are listed next. The same convention is followed within each table for the 2nd through the 7th characters — numeric values in order first, followed by alphabetical values. The operation tables consist of four columns and a varying number of rows. The values for characters 1 through 3 (typically, Section, Body System, and Operation) are provided at the top of each table, and the table itself contains columns with the applicable values for characters 4-7, as seen in the example of the root operation bypass, in the central nervous body system.

0: Medical and Surgical

0: Central Nervous System and Cranial Nerves

1: Bypass: Altering the route of passage of the contents of a tubular body part

A table may be separated into rows to specify values in characters four through seven. A code built using values from more than one row of a table is an invalid code. In the table above, there are only five choices for the 7th character of Body Part, U, Spinal Canal. The line can’t be crossed to choose a 7th character from the row above. In ICD-10-PCS, each character defines information about the procedure and all seven characters must contain a specific value. Even values such as the 6th character value Z, No Device, and the 7th character value Z, No Qualifier, provide important information about the procedure performed.

Build-a-PCS Code

Here is an example of how to find the complete ICD-10-PCS code for laparoscopic cholecystectomy to remove the entire gallbladder using the April 1, 2024 , ICD-10-PCS Addenda found on the CMS Website.

• Look in the index for main term cholecystectomy. 

  • Cholecystectomy 

    • see Excision, Gallbladder 0FB4

    • see Resection, Gallbladder 0FT4

• Refer to the Operations tables to review the definitions for excision (Cutting out or off, without replacement, a portion of a body part) and resection (Cutting out or off, without replacement, all of a body part).

• Since the scenario documents removing the entire gallbladder, refer to main term, resection, for cutting out all of a body part.

• Refer to Table 0FT.

Table OFT - April 1, 2024, ICD-10-PCS

1. The 4th character was listed in the index as 4. Confirm in the table that 4 represents gallbladder.

2. Select the 5th character, approach. Stay in the same row as the gallbladder. Crossing over into another row can cause the coder to form an invalid code. This gives two choices: 0 for open and 4 for percutaneous endoscopic. The example states the approach is laparoscopic. A laparoscope is inserted percutaneously through the abdominal wall, so the correct choice is 4 Percutaneous Endoscopic.

3. Select the 6th and the 7th characters. The 6th character has only one option to choose from: Z No Device and the 7th has G Hand-Assisted and Z No Qualifier. Assuming there is no documentation of this being a hand-assisted laparoscopic surgery, the correct 7th character is Z.

4. Put all 7 characters together to get the complete ICD-10-PCS code: 0FT44ZZ Resection of Gallbladder, Percutaneous Endoscopic Approach.

Note: If searching for the main term, laparoscopic, the ICD-10-PCS index entry would lead to: Laparoscopy see Inspection. The definition for Inspection is visually and/or manually exploring a body part. This definition does not fit our example of surgically removing the entire gallbladder using a laparoscope. The coder should therefore search for a better main term for the operation or body part affected. This illustrates that one of the first steps for PCS coding is to study the operations to understand their meaning. This will make it easier to start searching for the main term.

Resources and Training for Diagnosis Coding

Medical coders read clinical documentation to extract diagnoses from patient records and then translate those diagnoses into ICD-10 codes. While many coders use ICD-10 lookup software to help them, referring to an ICD-10 code book can be invaluable to build an understanding of the classification system.

Whether coders use software or a book, coding a medical record correctly requires ICD-10 training to follow the rules governing correct ICD-10 code assignments and to apply conventions like not elsewhere classifiable (NEC) and not otherwise specified (NOS), Excludes1, Excludes2, and what is meant by code. There are also many nuances for proper use of Z codes.

Because ICD-10 coding involves a high level of specificity, medical coders need to develop familiarity with medical terminology, human anatomy and physiology, pharmacology, disease processes, diagnostic methods, and treatment.

Knowledge in these areas will enable coders to interpret medical documentation, locate reporting errors, query providers, and assign accurate ICD-10 codes.

Coders and healthcare organizations also should be aware that ICD-11 could be on the horizon. According to the World Health Organization, countries could begin reporting health data using ICD-11 on Jan. 1, 2022. Although the transition to this code set is still far off for diagnosis coding in the United States, those involved in healthcare can begin to familiarize themselves with the basics of this new code set. This will help them prepare for the future and stay up-to-date with significant developments in the medical coding field.

Last reviewed on Jan. 29, 2024, by the AAPC Thought Leadership Team