Overview of Coding & Classification Systems

Overview of Coding and Classification Systems

This section summarizes information about ICD-10 and HCPCS Level I and Level II. Appropriate parties can apply for revisions, additions, and deletions to the standard, national code sets through processes set forth by the designated code set maintainers. To increase efficiencies with intake, tracking, and processing applications submitted to CMS related to coding, CMS has designed MEARIS™, a user-focused digital platform that supports applicant submissions, communication, and revisions.  MEARIS enables CMS to electronically store, review, track, and process submissions for the ICD-10-Procedure Coding System and HCPCS Level II.

ICD-10 Codes

ICD-10 consists of codes for diagnoses and hospital inpatient procedures. In October 2015, ICD-9 was updated to ICD-10, to enable greater specificity in identifying health conditions and include terminology and disease classifications more consistent with new technology and current clinical practice. Within ICD-10 there are two types of code sets:

  • ICD-10 Procedure Coding System (ICD-10-PCS): Used for inpatient hospital procedures; developed and maintained by CMS.
  • ICD-10 Clinical Modification (ICD-10-CM): Used by all providers in every healthcare setting; codes for diagnoses; developed and maintained by the Centers for Disease Control & Prevention (CDC), National Center for Health Statistics (NCHS). For more information about how to apply for new ICD-10-CM diagnosis codes or modifications to the code set, visit the CDC’s website.

ICD-10 Code Proposals

The ICD-10 Coordination and Maintenance Committee (C&M) is a federal interdepartmental committee comprised of representatives from CMS and NCHS charged with routine maintenance, testing, enhancement, and expansion of the ICD-10 code sets. The committee is jointly responsible for approving coding changes and developing errata, addenda, and other modifications to the ICD-10 coding systems to reflect newly developed procedures and technologies and newly identified diseases. For more information visit:

ICD-10-PCS Request

Suggestions to CMS for ICD-10-PCS modification requests can be proposals for new or revised procedure codes or requests for technical coding updates including but not limited to, enhancements to existing procedure code concepts, such as adding a new body part value or a new approach value. To request an ICD-10-PCS procedure code modification, submit recommendations three months prior to a scheduled meeting. Requests for a new code should include a description of the code being requested and a rationale for why the new code is needed.

CMS will only accept ICD-10-PCS code request applications submitted via MEARIS. Once proposals are reviewed, all requestors will be contacted as to whether the proposal has been approved for presentation at the ICD-10 Coordination and Maintenance Committee meeting. For further information about the ICD-10-PCS application process, refer to the CMS Process for Requesting New/Revised ICD-10-PCS Procedure Codes.

ICD-10-CM Request

The CDC’s National Center for Health Statistics (NCHS) has lead responsibility for the ICD–10–CM diagnosis codes. For more information, please visit the NCHS ICD-10 Coordination and Maintenance Committee webpage.


HCPCS is a standard, national medical code set specified for the purpose of ensuring that claims are processed in an orderly and consistent manner. HCPCS is divided into two principal subsystems, referred to as Level I and Level II of the HCPCS.  HCPCS Level I codes are part of the Current Procedural Terminology (CPT®) code set maintained by the CPT® Editorial Panel and copyrighted by the American Medical Association (AMA).  HCPCS Level II codes are established and maintained by CMS.  Additional distinctions between Level I and Level II are discussed below.  HCPCS coding is not a methodology or system for making coverage or payment determinations, and the existence of a code does not, in itself, determine coverage or non-coverage by Medicare of an item or service.


The current version of the HCPCS Level I code set is the CPT®-4. Decisions regarding the addition, deletion, or revision of CPT® codes are made by the AMA. CPT®-4 codes primarily identify services and procedures and are primarily divided into the following six sections:

  • Evaluation and Management
  • Anesthesiology
  • Surgery
  • Radiology
  • Pathology and Laboratory
  • Medicine

For more information on revisions, updates, or modifications to the CPT® code set, refer to the CPT® process on the AMA website.


The standard, national HCPCS Level II coding system is used primarily to identify products, supplies, and services that are not included in the HCPCS Level I (CPT®) codes. Such products and services include certain drugs and biologicals, ambulance services, durable medical equipment, prosthetics, orthotics, and supplies (DMEPOS). CMS maintains a standard application process for requesting HCPCS Level II codes. Detailed information and instructions for CMS’ HCPCS Level II coding application and procedures can be found at CMS HCPCS General Information. Access to the application portal can be found on MEARIS.

Miscellaneous codes (also known as unlisted, unclassified, not otherwise specified (NOS) or not otherwise classified (NOC) codes) are incorporated throughout the HCPCS Level II code set. Historically, these codes have been used when a supplier is submitting a bill for an item or service that is not adequately described by an existing code. If a supplier or manufacturer has been advised to use a miscellaneous code, the supplier or manufacturer may submit an application to add a new HCPCS Level II code. Miscellaneous codes allow suppliers to begin billing immediately for a service or item as soon as it is allowed to be marketed by the FDA in the absence of a specific HCPCS code—including during the period when a request for a new code is being considered under the HCPCS code review process. Before using a miscellaneous code on a Medicare claim form, the supplier or provider should check with the entity that will receive the claim (the appropriate Medicare Administrative Contractors (MACs)) for coding guidance to determine whether there is a specific code that should be used rather than a miscellaneous code. MACs also meet with providers and suppliers to discuss the use of miscellaneous codes.

In some instances, a HCPCS code must be accompanied by a code modifier to provide additional information regarding the service or item identified by the HCPCS code. Modifiers are used when the information provided by a HCPCS code descriptor needs to be supplemented to identify specific circumstances that may apply to an item or service and that may have implications for the level of or conditions of payment. For example, the “UE” modifier is used when the durable medical equipment (DME) item identified by a HCPCS code is "used equipment," and the “NU” modifier is used to identify "new equipment."

For more information on HCPCS Level II codes, refer to HCPCS Level II Coding Process.

For more information on CMS HCPCS Level II Coding Procedures for codes established for pass-through payments, refer to MEARIS for Device Pass-Through and Drugs and Biological Pass-Through. Pass-through payments are temporary additional payments established for certain new devices, drugs, and biologicals.

Other Types of HCPCS Level II Codes

There are certain HCPCS codes that can be used in the absence of other national code assignment at the discretion of CMS and the MACs. Please note that G codes and C codes are considered HCPCS Level II codes, and as such, these codes, and changes to them, are included in CMS’s HCPCS Level II.

  • C codes are temporary HCPCS Level II codes created by CMS for Medicare purposes to be reported for new technology devices, drugs, biologicals, and radiopharmaceuticals that have received transitional pass-through status under the Medicare Hospital Outpatient Prospective Payment System (OPPS). 
  • G codes and M codes are used to identify professional health care services and medical services that could otherwise be coded in CPT®-4 (the current version of CPT codes) but for which CMS has determined that a Level II code should be issued. CMS does not have an application process for G codes, as they are established by CMS typically through notice and comment rulemaking specifically to support Medicare policy and claims processing needs. As G codes are part of the national HCPCS Level II code set, they may also be used by non-Medicare insurers.

HCPCS Level II Application

HCPCS Level II codes for non-Pass-Through items and services are updated four times a year for drugs and biological products and twice a year for DMEPOS and other non-drug, non-biological items. HCPCS Level II codes for Pass-Through items and services are updated four times a year via specific Pass-Through coding procedures. HCPCS Level II Pass-Through coding procedures, application forms, and submission deadlines can be found on MEARIS for HCPCS Level II Coding Procedures and the Device Pass-through and Drug and Biological Pass-through MEARIS portals.

In addition to accepting applications, CMS also may issue codes based on the needs of its programs or other federal programs. Applications can be submitted through the digital platform MEARIS.

Other Code Sets

Nearly all drugs in the United States are given a unique National Drug Code (NDC), which identifies all current manufactured drugs and is maintained by the FDA.

More information about NDC codes can be found on the FDA’s website.

Another medical code set is the Code on Dental Procedures and Nomenclature (CDT), which lists codes for billing dental procedures and supplies and is maintained by the American Dental Association (ADA).

Code Set and Classifications Table Summary

Review of code requests is dependent upon submission of timely and complete applications to the appropriate entity charged with maintaining the code set and classification system. Note that the standard medical code sets are intended to be mutually exclusive. Each has its own application format, procedures, and review process. As indicated above, each is maintained by a different entity, as follows:

Code Set and Classification Summary

Codes maintained and updated by CMS:

Codes maintained and updated by another agency/entity:


Link for more information


Maintained and updated by NCHS and CDC

Link for more information


Link to application

HCPCS Level I (CPT®-4)

Maintained and updated by AMA

Link to application



Maintained and updated by the FDA

Link to application



Maintained and updated by the ADA

Link to application

Page Last Modified:
09/06/2023 04:51 PM