ICD codes, aka International Classification of Diseases (ICD) is a medical coding system used to classify illnesses, diseases, injuries, and diagnoses. Developed and maintained by the World Health Organization (WHO), this system is currently in its 10th revision (ICD-10). All healthcare facilities and insurance payers in the US use ICD codes for preparing, submitting, and processing medical reimbursement claims.
What is the US Version of ICD-10?
The United States version of ICD-10 was created by the Centers for Medicare & Medicaid Services (CMS) and the National Center for Health Statistics (NCHS) to provide more comprehensive data on the severity of patient conditions, improving healthcare management and analytics.
There are two components to ICD-10 in the United States. These are 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).
The ICD-10-CM is a diagnosis code set, used to create and submit medical claims, used in all healthcare facilities.
The ICD-10-PCS is a procedure code set, used strictly in hospital inpatient settings.
- ICD-10-CM codes are for patient diagnoses, showing that procedures are medically necessary. ICD-10-CM is used by all medical facilities in the US.
- ICD-10-PCS codes are for procedures, and used for inpatient facilities only.
The United States created their own version of ICD in 1979, the ICD-CM-9, the predecessor to ICD-10-CM.
In the United States the Health Insurance Portability and Accountability Act (HIPAA) decreed all medical institutions and facilities in the US must use ICD-10-CM for medical coding of patient diagnoses as of October 1, 2015.
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Where Did ICD-10 Come From?
In the mid-1800s, medical statisticians William Farr and Marc d’Espine compiled lists of diseases and causes of mortality in an attempt to better understand and classify diseases. This built on work that was done by Australian medical statistician François Bossier de Lacroix in the early 1700s. In 1891, the International Statistical Institute, led by Jacques Bertillon, prepared a classification of causes of death, compiled from classifications from English, Swiss, and German medical statisticians. The first official ICD-1 was adopted in 1900, with future revisions expanding the entries and numbering system to add more detail and clarification.
By the time the International Classification of Diseases, Revision 6 was published in 1948, the codes had expanded to four digits, adding more levels of detail and classification, including three auxiliary lists of morbidity and mortality. This was the first ICD revision entrusted to the World Health Organization, who began working on this revision in a 1946 committee.
Subsequent revisions added more levels of clarification and information, along with adoption of the ICD as a worldwide standard for healthcare coding. These included ICD-7 (1955), ICD-8 (1965), ICD-9 (1975), ICD-10 (1989), and ICD-11 (2019).
Revisions of ICD for Oncology were also published in 1975 (Revision 1), 1990 (Revision 2), and 2000 (Revision 3).
Today, the ICD is used worldwide by healthcare providers, medical researchers, and health organizations as a standardized language to classify diseases and conditions.
Structure of ICD-CM-10 Codes
Each ICD-10-CM code consists of three to seven characters. Each character adds more specific information about the disease, illness, or condition. The first alphanumeric character in the code denotes the top-level group (called “chapters”) of the classification.
The second and third characters are numbers, and these create a category within the chapter, a cluster of similar diagnoses, conditions, and illnesses.
The subsequent fourth through seventh alphanumeric characters add specificity in order to define and classify a distinct disease or condition.
Each character adds more information about a disease or condition. The chapter, the category, and then the specific diagnosis.
It is important to note, insurance payers will not accept an incomplete or truncated code on a claim! You must fill out the most specific condition for the appointment to receive reimbursement. You cannot simply put the category (three characters)—payers will not accept this claim. ICD-10-CM is organized in such a way that billers and coders can find the most specific code for a patient’s condition.
How Often are ICD Codes Updated?
New ICD codes come out on an irregular, as-needed basis, with minor updates published every one to four years.
Major revisions to the ICD occur much less frequently, as these signify massive changes to the structure of medical coding.
ICD-10-CM entered common use in the healthcare industry in 1994. ICD-11 was published in May of 2019, entering common use on January 1, 2022.
ICD-CM is still the healthcare industry standard at this time (as of early 2026).
How to Find a List of ICD-10 Codes
The World Health Organization has a list of ICD-10 codes (2019 edition) on the official site. This can be a handy reference if you need to research the entire set of codes.
What Is ICD-11?
ICD-11 is a newer revision, published in 2019 by the World Health Organization, but it is not in full adoption in the United States. ICD-10-CM is still the standard used, and will be for the foreseeable short-term future. It takes many years for new revisions of the ICD to be fully adopted as a healthcare industry standard in the US, to maintain consistency.
What’s the Difference Between ICD Codes and CPT Codes?
Like ICD codes, CPT® codes are used for medical coding. The main difference between Current Procedural Terminology (CPT®) codes and ICD codes is the information they encode. CPT codes describe procedures and treatments, and ICD-10-CM codes are for recording diagnoses. CPT are procedural codes, ICD are diagnostic codes.
Anything that is a treatment, procedure or service is a CPT code. Anything that is a diagnosis or reason for a doctor’s appointment is an ICD code. Putting these codes together in a medical reimbursement claim or electronic health record tells the story of why the patient visited the medical office, the diagnosis, treatments, and procedures.
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