Medical Billing Codes: A Beginner’s Guide to CPT, ICD-10, and HCPCS

If you are new to healthcare administration, the medical coding world can feel like a different language. But once you understand the basics, it becomes predictable.

Medical billing codes are the standardized labels used on claims to explain two things:

• What condition was treated (the diagnosis)

• What service or item was provided (the procedure, test, supply, or equipment)

If the codes are accurate and supported by documentation, claims tend to process faster. If they are incomplete or mismatched, denials and delays are more likely to occur.

What Are Medical Billing Codes?

Medical billing codes are standardized identifiers that translate a clinical encounter into a claim that payers can process. Understanding the three primary coding systems, CPT codes, ICD-10 codes, and HCPCS codes, is the foundation of effective medical billing.

• CPT codes: What service was provided

• ICD-10-CM: Why the service was medically necessary

• HCPCS Level II: What supplies, equipment, or special items were used

“Medical billing codes are the shared language between providers and insurance companies.”

What Is a CPT Code?

If you’re looking for the CPT full form in medical billing, it means Current Procedural Terminology. A CPT code is a five-digit number used to indicate the specific medical service or procedure a provider provided to a patient.

These codes are created and managed by the American Medical Association (AMA) and are used across the U.S. by doctors, insurance companies, and government payers.

In simple terms, a CPT code in medical billing tells the insurer exactly what service was performed during the patient’s visit.

How CPT Codes Work

A CPT code in medical billing acts as a precise description of a medical service. Each code tells payers exactly what type of care was provided. These codes are divided into three main categories:

Category I (10000–99999)

These codes cover commonly performed and well-established medical services. They include evaluation and management visits, surgical procedures, radiology services, laboratory tests, and anesthesia services.

Category II (0001F–9999F)

These are optional tracking codes used to measure the quality of care and performance. Category II codes are not typically used for payment but support reporting and quality improvement programs.

Category III (0001T–9999T)

These temporary codes are designed for emerging technologies, new procedures, and experimental services. They allow providers to report new treatments while clinical data is collected for future permanent coding.

List of cpt codes

The CPT code list is very broad, but in day-to-day practice, most providers rely on a small set of commonly used codes. Below are some commonly used CPT codes: 

CPT Code	Description
99213	Office/outpatient visit
93000	Electrocardiogram (ECG)
36415	Blood draw (venipuncture)
97110	Therapeutic exercises
90834	Psychotherapy, 45 minutes

For a complete list, visit the AMA CPT Code Search.

ABA Billing Codes

For therapy and behavioral health teams, ABA billing codes are an area where even small errors can cause claim delays or denials.

Most Applied Behavior Analysis services are billed using CPT codes in the 97151–97158 range, though payer rules may vary. Commonly used ABA CPT codes include:

• 97151: Behavior identification assessment

• 97152: Supporting assessments

• 97153: Adaptive behavior treatment by protocol

• 97155: Treatment with protocol modification

• 97156: Family or caregiver guidance

• 97158: Group adaptive behavior treatment

ABA billing runs smoothly when documentation, authorizations, and time-based billing rules are reviewed and aligned before the claim is submitted. Use this detailed guide on how to bill CPT code 97151 for ABA assessments.

Occupational Therapy CPT Codes

Occupational therapy coding typically focuses on evaluations and treatment services. Some commonly used OT-related CPT codes include:

97110: Therapeutic exercises

97530: Therapeutic activities

97112: Neuromuscular reeducation

While these codes are widely used, what gets paid still depends on payer policies, documentation standards, and authorization limits. For a detailed OT reference, read our guide on essential CPT codes for accurate occupational therapy billing

CPT Codes for Physical Therapy

Physical therapy coding usually follows a clear flow: evaluation, re-evaluation, and then treatment services. Common examples include:

• 97162: Physical therapy evaluation

• 97140: Manual therapy techniques

• 97535: Self-care/home management training

A frequent beginner mistake is mixing up timed and untimed codes. Even when the CPT code is correct, incorrect unit calculation can result in downcoding or denials. For PT evaluations, check out our guide on how to use and bill CPT code 97162 in physical therapy.

CPT Codes for Mental Health

Mental health coding often includes assessment and psychotherapy services.

Examples of commonly used codes:

• 90791: Psychiatric diagnostic evaluation

• 90832 / 90834 / 90837: Psychotherapy (30, 45, 60 minutes)

• 90846 / 90847: Family psychotherapy

• 96127: Brief emotional or behavioral assessment

For time-based psychotherapy codes, documentation needs to match the billed time thresholds and payer-specific rules.

What are ICD-10 codes?

ICD-10-CM codes are diagnosis codes. They explain the medical condition, symptom, or reason for the visit. 

While CPT codes tell us what was done, ICD-10 codes explain why it was done. These codes document patient diagnoses, symptoms, and medical conditions, providing the medical justification for the services billed. Check out our guide about ICD-10 codes.

Common ICD-10 Codes

ICD-10 Code	Description
E11.9	Type 2 diabetes mellitus
J06.9	Acute upper respiratory infection
I10	Essential (primary) hypertension
M54.5	Low back pain
F41.1	Generalized anxiety disorder

Why ICD-10 Replaced ICD-9

ICD-9 had limited room to capture modern clinical detail. ICD-10 expanded the structure to support more specificity and a much larger set of diagnoses. CMS training materials highlight the shift in code structure and character capacity between ICD-9 and ICD-10.

“ICD-10 is not harder for fun. It is designed to describe healthcare with more detail, and payers increasingly expect that specificity.”

What are HCPCS codes?

HCPCS stands for the Healthcare Common Procedure Coding System. In simple words, these codes are used in medical billing to describe procedures, supplies, products, and healthcare services so insurance companies know exactly what was provided.

According to CMS, HCPCS Level II is mainly used to identify items and services that don’t fall under CPT codes.

The Two Levels of HCPCS

HCPCS is divided into two main levels:

• HCPCS Level I: These are the familiar CPT codes, the five-digit procedure codes used by providers to report medical procedures and professional services.

• HCPCS Level II: These are alphanumeric codes used for supplies, equipment, certain medications, ambulance services, and other items that are not covered by CPT.

Because of this setup, CPT/HCPCS codes refer to the complete procedural coding used in medical billing, covering both services provided and related supplies or equipment.

Common HCPCS Level II Codes

HCPCS Code	Description
A0428	Ambulance service, basic life
E0114	Crutches, underarm, pair
J3490	Unclassified drugs
G0439	Annual wellness visit
L3908	Wrist hand orthosis

What is the difference between CPT and HCPCS codes?

CPT and HCPCS codes are both used in medical billing, which is why they’re often confused. But they don’t compete with each other, they work together.

If you’re billing for a professional service such as a patient visit, procedure, or interpretation, you’re usually using CPT codes.

If you’re billing for a product or supply like equipment, medical devices, ambulance transport, or many drugs, you’re typically using HCPCS Level II codes. For an ABA-focused breakdown, read HCPCS vs CPT codes in ABA therapy.

FAQ

1. How do CPT and HCPCS codes differ from one another?

ICD-10 explains why a patient needs care by listing diagnoses, while CPT explains what service or procedure the provider actually performed.

2. Is ICD-10 Used for Billing?

Yes. ICD-10 codes support billing by justifying medical necessity, but they cannot be billed alone without CPT or HCPCS procedure codes.

3. What CPT codes are used for ABA therapy?

ABA therapy commonly uses CPT codes like 97151, 97153, 97155, and 97156 to report assessments, direct treatment, and caregiver training.

Conclusion

Medical coding is not about memorizing thousands of entries. It is about understanding the system: medical billing codes translate care into claims through diagnosis(ICD-10), services (CPT codes in medical billing), and items/supplies (CPT/HCPCS codes).

Ready to take control of your medical billing? If you need expert help with coding, claims, or revenue management, explore our Cube Therapy Billing services to simplify medical billing and reduce costly errors.