Coding Basics: CPT, ICD-10-CM, HCPCS and Modifiers
ICD-10-CM codes say why the patient was seen (diagnosis); CPT and HCPCS codes say what was done or supplied.
The three code sets
| code set | what it describes | maintained by | format | update notes |
|---|---|---|---|---|
| ICD-10-CM | Diagnoses and reasons for visits in all health care settings | CMS and NCHS (based on WHO's ICD-10) | 3 to 7 characters; first is a letter; 'X' placeholder; some codes require a 7th character and are invalid without it | Fiscal-year update effective October 1 (FY 2027 codes apply to encounters October 1, 2026 - September 30, 2027); April 1 updates may also occur |
| CPT (HCPCS Level I) | Procedures, services, tests and E/M visits by physicians and other professionals | American Medical Association (CPT Editorial Panel) | 5 characters. Category I numeric (five digits); Category II and III alphanumeric | Category II = performance-measure tracking codes; Category III = temporary codes for new technology |
| HCPCS Level II | Products, supplies and services not in CPT, e.g., ambulance, DME, prosthetics, orthotics and supplies, drugs | CMS | 1 letter followed by 4 digits | CMS decides additions, revisions and deletions |
Preview only. The full sheet also covers: Modifiers basics; Outpatient diagnosis coding rules (ICD-10-CM Section IV).
CPT® is a registered trademark of the American Medical Association. CCMA Prep teaches how CPT is organized and does not reproduce CPT codes or descriptors.
Sources
- Healthcare Common Procedure Coding System (HCPCS): Level I (CPT) and Level II · Centers for Medicare & Medicaid Services
- CPT overview and code approval · American Medical Association
- ICD-10 Codes (2027 ICD-10-CM and PCS files) · Centers for Medicare & Medicaid Services
- ICD-10-CM Official Guidelines for Coding and Reporting FY 2027 · CMS and NCHS
- Proper Use of Modifiers 59, XE, XP, XS & XU (MLN1783722, April 2026) · Centers for Medicare & Medicaid Services
- NCCI Policy Manual for Medicare Services, 2026 (Chapter I: modifiers 25, 59, anatomic modifiers) · Centers for Medicare & Medicaid Services
- Medicare Claims Processing Manual, Chapter 30 - Financial Liability Protections (ABN, GA modifier) · Centers for Medicare & Medicaid Services