CPT vs ICD-10 Codes: What’s the Difference?

CPT codes describe what you did: the visit, test or procedure. ICD-10-CM codes describe why you did it: the diagnosis, symptom or other reason for the visit. A physician claim needs both, and each service line points to the diagnosis that justifies it. If the two don't fit together, the payer can deny the line.

The two code sets come from different organizations and change on different schedules. In the US, "ICD-10" on a physician claim means ICD-10-CM, the clinical modification for diagnoses. Hospitals report inpatient procedures with a third code set, ICD-10-PCS, instead of CPT. Payers also add their own coverage rules on top of both code sets, so a valid pair of codes isn't a guarantee of payment.

What are CPT codes?

Current Procedural Terminology (CPT) is the American Medical Association's code set for medical services and procedures performed by physicians and other qualified health care professionals. The AMA's CPT Editorial Panel maintains it. CPT has three categories:

• Category I: five-digit codes for procedures and services, from 00100 to 99499. These are the codes used for billing most visits and procedures.
• Category II: supplemental tracking codes for performance measurement.
• Category III: temporary codes for new and emerging technology and services.

Some familiar Category I codes: 99213 is an established-patient office visit, 93000 is a routine electrocardiogram with the physician's interpretation, and 95816 is a routine EEG.

Under HIPAA, CPT together with HCPCS (the Healthcare Common Procedure Coding System maintained by HHS) is the standard code set for physician and other health care services. HCPCS Level II codes cover items CPT doesn't, such as medical supplies, durable medical equipment, and orthotic and prosthetic devices.

What are ICD-10-CM codes?

ICD-10-CM is the US clinical modification of the World Health Organization's ICD-10. The WHO owns ICD-10. The CDC's National Center for Health Statistics (NCHS) develops the US clinical modification, and CMS and NCHS publish the official coding guidelines together. ICD-10-CM codes report diseases, injuries, symptoms and other reasons for care, and HIPAA has required them for diagnoses since October 1, 2015.

Codes are 3 to 7 characters long, for example:

• I10: essential (primary) hypertension.
• M54.50: low back pain, unspecified. The older code M54.5 was split into more specific codes and is no longer valid.
• R56.9: unspecified convulsions (a symptom code).
• G40.909: epilepsy, unspecified, not intractable, without status epilepticus.

A code is invalid unless it has every character it requires. The official guidelines also say to code to the highest level of specificity the record supports. The code set is updated each October 1, so check that you're using the files for the date of service.

How do CPT and ICD-10-CM codes differ?

How do the two codes work together on a claim?

On a professional claim (the CMS-1500 or its electronic equivalent), diagnoses are listed in item 21, in priority order. Each service line then points, in item 24E, to the diagnosis that explains why that service was performed. The payer reads each pair.

When the diagnosis doesn't support the procedure, the payer can deny the line. Two reason codes cover this: CARC 11 (the diagnosis is inconsistent with the procedure) and CARC 50 (the payer doesn't consider the service medically necessary). For Medicare, the billing and coding articles that accompany local coverage determinations (LCDs) list the diagnosis codes for which a service is, or isn't, covered.

A worked example

Hypothetical, with no real patient. A patient comes to the office after a first episode that looked like a seizure. The physician does an evaluation and orders a routine EEG.

• The visit is reported with an E/M code at the level the documentation supports.
• The EEG is reported with its CPT code, 95816.
• The diagnosis depends on what the physician documented. If the note says "possible epilepsy" and the diagnosis hasn't been confirmed, the outpatient guidelines say not to code epilepsy. Code the symptom instead, here R56.9. If a later visit confirms epilepsy, that visit is coded with a G40 code at the specificity the record supports.

This is one of the most important differences between inpatient and outpatient diagnosis coding. In the outpatient setting and in the physician office, uncertain diagnoses ("probable", "suspected", "rule out") are not coded as if they were confirmed.

What coding rules should physicians know?

A few rules from the official ICD-10-CM outpatient guidelines explain many denials and audit findings:

• List first the reason chiefly responsible for the visit. Then add the coexisting conditions that affect care.
• Code symptoms when there's no confirmed diagnosis. Don't code a diagnosis described as probable, suspected or rule out.
• Code all documented conditions that coexist and require or affect care at that visit. Don't code conditions that were treated before and no longer exist.
• Use every character the code requires, and code to the specificity the record supports.

And on the CPT side:

• Report what the record documents. Choose the code, and the E/M level, that the documentation supports: not higher, not lower. Undercoding and overcoding are both errors.
• Use modifiers only when the record supports them. A modifier describes a real circumstance, such as a separate site or a separate encounter. It isn't a way to get a line paid. The CO-97 guide explains why.
• Use the code set in effect on the date of service. Codes are added, revised and deleted every year.

Where do these codes fit in billing?

Coding is one step in the billing process, between documentation and claim submission. For the full process, see what medical billing is. For the claim details that most often cause denials, see what a clean claim is.

If you'd like a second look at how your visits are coded and linked on your claims, Highview's medical billing team can start with a free call.

Sources

• AMA, CPT overview and code approval: what CPT is, the CPT Editorial Panel, Category I, II and III codes.
• CDC/NCHS, ICD-10-CM: the relationship between WHO's ICD-10 and the US clinical modification.
• CDC/NCHS, FY 2027 ICD-10-CM code descriptions: code titles for I10, M54.50, R56.9 and G40.909.
• CMS and NCHS, ICD-10-CM Official Guidelines for Coding and Reporting, FY 2027 (October 1, 2026 to September 30, 2027): Section IV, outpatient services.
• CMS, ICD-10: ICD-10-CM and ICD-10-PCS, the October 1, 2015 transition, current code files.
• CMS, Medicare Coverage Database: LCDs and billing and coding articles.
• eCFR, 45 CFR 162.1002: HIPAA standard medical data code sets.
• CMS, Medicare Claims Processing Manual, Chapter 26: CMS-1500 items 21 and 24E.
• X12, Claim Adjustment Reason Codes: CARC 11 and CARC 50.