ICD represents patient information on claims records, data collection for use in performance measurement, reimbursement for medical claims, and more. In the United States, data submitted to CMS transitioned from ICD-9-Clinical Modification (CM) to ICD-10-CM/Procedure Coding System (PCS) beginning October 1, 2015. It is generally a good practice not to change originally captured and coded patient information (i.e., prior to October 1, 2015). There is not a simple method to crosswalk from ICD-9-CM to ICD-10-CM/PCS, so most legacy data using ICD-9-CM should remain archived in that form. The ICD-10 classification systems provide significant improvements through more detailed information and the ability for providers to capture additional advancements in clinical medicine, but the transition does create difficulties for monitoring trends when capturing data using both code systems.

ICD-10-CM/PCS consists of two parts:

• ICD-10-CM—Diagnosis classification system developed by the Centers for Disease Control and Prevention (CDC) National Center for Health Statistics (NCHS) for use in health care treatment settings in the United States. Diagnosis coding under this system uses three to seven alphanumeric characters and full code titles, but the format is the same as ICD-9-CM.
• ICD-10-PCS—Procedure classification system developed by CMS for use in the United States for inpatient hospital settings. The PCS uses seven alphanumeric characters, whereas the ICD-9-CM coding system uses three or four numeric digits.

Measure developers should not include codes not valid for clinical coding (e.g., ICD-10-CM Group Codes) in clinically-related value sets. Specifically, they should not use codes associated with sections or groups of codes in value sets, for example

• Codes for dermal burns.
  • Use the fourth digit (0–9) to identify the percentage of body surface, e.g., T31.0, Burns involving less than 10% of body surface. 
  • Use the fifth digit to indicate the percentage with third degree burns, e.g., T31.11, Burns involving 10-19% of body surface with 10-19% third-degree burns
• Unspecified codes such as A09, Infectious Gastroenteritis and Colitis, unspecified—a standalone code and does not require additional digits to be valid.
• Non-billable codes, e.g., A08, Viral and Other Specified Intestinal Infections.

When a measure developer submits ICD-10-CM/PCS codes for consideration by the CMS consensus-based entity (CBE), the CBE has additional requirements. The CBE outlines these requirements in the document Inclusion of ICD‐10 Codes in Measures Using ICD‐9 Codes.

You can find processes for requesting changes to ICD-10-PCS on the CMS website and processes for requesting changes to ICD-10-CM on the CDC NCHS website. Measure developers should account for contractual timelines when considering applying for new concepts.

There are different standardized groupings of ICD-10 codes such as the Agency for Healthcare Research and Quality (AHRQ) Clinical Classification Software Refined (CCSR) for the Healthcare Cost and Utilization Project (HCUP) and the Medicare Severity Diagnostic Related Groups (MS-DRG) used for Medicare hospital payments. Measure developers may use these groupings in lieu of individual ICD-10 codes.