ICD-10, CPT, and HCPCS Explained: The Code Sets Medical Coders Use (2026)

Updated September 2, 2026 · Medical Billing Path Editorial Team

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Key takeaway: Every claim carries diagnosis codes (ICD-10-CM) and procedure or service codes (CPT and HCPCS Level II for physicians and outpatient facilities, ICD-10-PCS for hospital inpatient stays). Coders must know the guidelines for each set, and certification exams differ mainly in which sets they emphasize.

You will hear people say “ICD-10” as if it were one thing. It is two books with similar names and different jobs. If a program skips guidelines and only drills lookup, you will be short on every exam.

The four code sets

Code set	What it codes	Used by	Maintained by	Updated
ICD-10-CM	Diagnoses, symptoms, reasons for visit	All settings	National Center for Health Statistics (CDC)	Annually (October)
ICD-10-PCS	Inpatient procedures	Hospitals (facility claims)	Centers for Medicare and Medicaid Services	Annually
CPT	Physician and outpatient procedures and services	Physicians, outpatient facilities	American Medical Association	Annually (January)
HCPCS Level II	Supplies, drugs, equipment, and services not in CPT	Physicians, outpatient, suppliers	Centers for Medicare and Medicaid Services	Quarterly and annual updates

How they fit on a claim

A physician office claim (CMS-1500) lists ICD-10-CM diagnosis codes and CPT or HCPCS service lines, with each service pointed at the diagnoses that justify it. A hospital inpatient claim (UB-04) lists ICD-10-CM diagnoses and ICD-10-PCS procedures, which together determine the diagnosis-related group and the facility payment.

That link between diagnosis and service is called medical necessity. It is why a biller and a coder fight over the same chart.

What coders must know about each

ICD-10-CM. The official guidelines on sequencing, specificity, and combination codes; the conventions in the alphabetic index and tabular list.
CPT. Section guidelines, evaluation and management leveling, modifiers, and bundling edits.
HCPCS Level II. When a supply or drug needs its own line and which modifiers apply.
ICD-10-PCS. The seven-character structure and the root operation definitions, which take dedicated study.

You do not memorize the index. You memorize how the index lies to you if you skip the tabular list.

Which exams test which sets

The CPC centers on CPT with ICD-10-CM and HCPCS. The CCA covers all four at a fundamentals level. The CCS tests ICD-10-PCS and DRG assignment in depth. This site is not affiliated with AAPC or AHIMA.

Use the current-year manuals on exam day

New books every year are not a scam. The sets change. A 2023 CPT manual on a 2026 exam is a self-inflicted wound. Ask the program which edition is included and which edition the exam currently allows. Then read the glossary for the short definitions you will see in postings.

Frequently asked questions

What is the difference between ICD-10-CM and ICD-10-PCS?

ICD-10-CM is the diagnosis code set used in every U.S. setting. ICD-10-PCS is the inpatient procedure code set used only by hospitals for facility billing. Physician and outpatient procedures use CPT instead of PCS.

Who maintains CPT?

The American Medical Association publishes and maintains CPT and updates it annually. ICD-10-CM is maintained by the National Center for Health Statistics and ICD-10-PCS by the Centers for Medicare and Medicaid Services, which also maintains HCPCS Level II.

Do I have to memorize the codes?

No. You look them up in the manuals or an encoder. What you have to learn is the guidelines: when a code applies, how specific it must be, how to sequence, and which modifiers to append. Exams still expect speed with the books.

Which exam tests which code sets?

The CPC centers on CPT with ICD-10-CM and HCPCS. The CCA covers all four at a fundamentals level. The CCS tests ICD-10-PCS and DRG assignment in depth. Confirm current blueprints with AAPC and AHIMA.

How often do the code sets change?

ICD-10-CM typically updates each October. CPT updates each January. HCPCS Level II has quarterly and annual updates. ICD-10-PCS also updates annually. Keeping current is part of the job after you pass the exam.

What is HCPCS used for if we already have CPT?

HCPCS Level II covers supplies, drugs, equipment, and some services that CPT does not. Outpatient and physician claims use both. Hospital inpatient facility claims use ICD-10-PCS for procedures instead of CPT.

Next steps

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