CPT vs ICD-10 Codes: Understanding the Difference
Learn how CPT procedure codes and ICD-10 diagnosis codes work together in medical billing and why both are essential.
Dr. Chukwuma Onyeije
Founder, CodeCraftMD
October 8, 2024
6 min read
Table of Contents
The Fundamental Difference
CPT Codes
Answer: "What procedure was performed?"
Identify the treatment, service, or procedure
ICD-10 Codes
Answer: "Why was the procedure performed?"
Identify the diagnosis or reason for the service
Both code types are required for every medical claim. CPT codes justify what you charged for, while ICD-10 codes justify why the procedure was medically necessary. Insurance companies use both to determine payment and verify medical necessity.
CPT Codes: What Was Done?
Current Procedural Terminology (CPT) codes are numeric and descriptive codes maintained by the American Medical Association. They describe the specific procedures, services, and treatments provided to patients.
CPT Code Structure:
- • Format: 5-digit numeric codes (e.g., 99213, 70450)
- • Range: 10001 to 99607
- • Updated: Annually on January 1st
- • Use: Billing and reimbursement
Common CPT Code Categories:
Evaluation & Management (E/M)
Office visits, consultations, emergency room visits
Surgery
Operative procedures, repairs, removals
Pathology & Laboratory
Blood tests, tissue analysis, diagnostic tests
Radiology
X-rays, ultrasounds, MRIs, CT scans
ICD-10 Codes: Why Was It Done?
International Classification of Diseases, 10th Revision (ICD-10) codes identify diagnoses, symptoms, diseases, injuries, and conditions that justify the medical procedures performed.
ICD-10 Code Structure:
- • Format: Alphanumeric (letter + 2 digits + decimal + 1-2 digits, e.g., E11.9, M79.3)
- • Total codes: Over 70,000 specific diagnosis codes
- • Updated: Annually on October 1st
- • Use: Clinical documentation and medical necessity
Side-by-Side Comparison
| CPT Codes | ICD-10 Codes |
| Purpose: Describe procedure/service | Purpose: Describe diagnosis/condition |
| Example: 99213 (Office visit) | Example: E11.9 (Type 2 diabetes) |
| Format: 5 digits, numeric | Format: Alphanumeric with decimals |
| Quantity: ~10,000 codes | Quantity: ~70,000 codes |
| Maintained by: American Medical Association (AMA) | Maintained by: World Health Organization (WHO) |
| Updated: January 1 annually | Updated: October 1 annually |
| Determines: Amount to bill | Determines: Medical necessity |
Real-World Examples
Example 1: Office Visit for Hypertension
CPT: 99213
Established patient office visit (15 min)
ICD-10: I10
Essential hypertension
Meaning: The provider spent 15 minutes with an established patient for a hypertension check-up
Example 2: Knee X-Ray
CPT: 71020
Chest X-ray, 2 views
ICD-10: R06.02
Shortness of breath, exertional
Meaning: Took 2 views of the chest because patient had shortness of breath during exercise
How They Work Together
A complete medical claim requires both codes working together. Insurance companies validate medical necessity by matching the diagnosis to the procedure.
The Complete Claim Equation:
CPT Code
"What was done"
ICD-10 Code
"Why it was done"
Valid Claim
Reimbursement
Without proper ICD-10 coding, even the correct CPT code will be denied because the insurance company won't understand why the procedure was medically necessary. Similarly, without the correct CPT code, you can't bill for the service at all.