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What is ICD (International Classification of Diseases)?

Created: October 1, 2015 Last Modified: October 29, 2015

This articles is a good introduction to all things ICD. First we provide a high-level overview of ICD-9 and ICD-10 then touches on some of the finer technical details of each.

What is ICD and when is it used? International Classification of Diseases (ICD) was developed by the World Health Organization and adapted for use in the American health care system. ICD codes are used to capture medical diagnosis and procedure information about patients. Accurate coding is essential for statistical analysis and billing purposes.

ICD codes apply only to in-patient care, observations over 12 hours, and same-day surgery (same-day surgery is usually coded with both ICD and CPT codes). In order to bill for a hospital visit, there must be at least one Diagnosis code assigned. There may be more Diagnosis codes and there may be one or more Procedure codes.

In FY 2016, the United States health care system made the long-overdue switch from ICD version 9 to ICD version 10. There are also two code sets within ICD 9 and 10 – procedure and diagnosis codes. These code sets are explained in more depth below.


ICD-9-CM (Clinical Modification) is a medical coding standard used in the United States from 1979 to October 1, 2015. ICD-9-CM is based on the international ICD specification created by the World Health Organization (WHO).

ICD-9-CM Volume 1 and Volume 2

ICD-9-CM is divided into 3 volumes. Volumes 1 and 2 represent that same data in two different formats. Volumes 1 and 2 contain Diagnosis codes. Volume 1 is known as the tabular format and organizes codes based on the code number (i.e. starts with 872.00, 872.01, etc.). ICD-9-CM volume 2 organizes codes into an index, allowing you to look up codes alphabetically by their description.

ICD-9 Volume 1 codes are 4 or 5 digits and appear in the format WXX.YZ where the ‘W’ represents a digit or a letter (‘E’ or ‘V’) and the final digit is optional.

ICD-9-PCS Volume 3

ICD-9-CM volume 3 contains Procedure codes which map to ICD-10-PCS codes. Procedure codes describe the type of treatment an individual received while in the hospital. Every hospital visit will have at least one Procedure code assigned as long as some sort of treatment was given to the patient.

ICD-9 Volume 3 codes are 3 or 4 digits and appear in the format XX.YZ where the final digit is optional.


ICD-10 is the 10th version of the International Classification of Diseases and replaced ICD-9 medical codes used to describe patient diagnoses and level of care in hospitals across the United States. All patients discharged after October 1, 2015 will be coded and billed using ICD-10. ICD-10 is broken into two types – ICD-10-CM contains Diagnosis codes and ICD-10-PCS contains Procedure codes. Like ICD-9, ICD-10 codes are only used for inpatient care. There are over 70,000 ICD-10 codes – approximately 5 times more codes than in ICD-9. ICD-10 codes are 3 to 7 characters long while ICD-9 codes are 3 to 5 digits in length.

ICD-9-CM Volumes 1 and 2 = ICD-10-CM (Diagnostic codes)
ICD-10-CM Volume 3 = ICD-10-PCS (Procedure codes)

Conversion Tool: Use our ICD-9 to ICD-10 Conversion Tool to convert between ICD-9 and ICD-10 codes. The tool supports converting both CM (diagnosis codes) and PCS (procedure codes). The conversion tool is based on the Center for Medicare & Medicaid's official General Equivalence Mapping (GEM) files.


ICD-10-CM (diagnosis) codes are 3 to 7 characters long. The first 3 characters represent the code category. The category can then be expanded to four, five, six, or seven characters to increase the level of detail. A decimal point is optionally placed between the category and the 4th digit. Digits four, five, and six specify the Etiology, Anatomic Site, Severity, or other Vital Details. The seventh character is the Extension. There is usually a space placed between the 6th and the 7th digits.

Let’s break down the follow code: S86.011D. The category is ‘S86’. ‘S’ represents “Injuries, poisoning and certain other consequences of external causes related to single body regions”. ‘S86’ is “Injury of muscle, fascia and tendon at lower leg”. Now let’s dive deeper:

  • S68.0 = Injury of Achilles tendon
  • S68.01 = Strain of Achilles tendon
  • S68.011 = Strain of right Achilles tendon
  • Finally, the extension is used to document episodes of care for injuries and other conditions with external causes. ‘A’ is “Initial encounter”, ‘D’ is “Subsequent encounter”, and ‘S’ is for Sequela.

Sidebar: What is Sequela?


ICD-10-PCS codes are always 7 digits long and are alphanumeric. Unlike the ICD-10-CM codes, PCS does do not have a period. Here are several examples of ICD-10-PCS codes:

  • 7W02X3Z - Osteopathic Treatment of Thoracic Region using High Velocity-Low Amplitude Forces
  • XR2H021 - Monitoring of Left Knee Joint using Intraoperative Knee Replacement Sensor, Open Approach, New Technology Group 1
  • GZ72ZZZ - Family Psychotherapy

You will notice these codes describe medical procedures, rather than diagnoses.

ICD-10-PCS Axes

ICD-10-PCS codes are composed of seven characters. Each character is an axis of classification that specifies information about the procedure performed. Within a defined code range, a character specifies the same type of information in that axis of classification.

Example: The fifth axis of classification specifies the approach in sections 0 through 4 and 7 through 9 of the system.

New Technology ("X" codes)

Section X codes are standalone codes. They are not supplemental codes. Section X codes fully represent the specific procedure described in the code title, and do not require any additional codes from other sections of ICD-10-PCS. When section X contains a code title which describes a specific new technology procedure, only that X code is reported for the procedure. There is no need to report a broader, non-specific code in another section of ICD-10-PCS.

Example: XW04321 Introduction of Ceftazidime-Avibactam Anti-infective into Central Vein, Percutaneous Approach, New Technology Group 1, can be coded to indicate that Ceftazidime-Avibactam Anti-infective was administered via a central vein. A separate code from table 3E0 in the Administration section of ICD-10-PCS is not coded in addition to this code.

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Source: http://icd.codes/articles/what-is-icd