Medical Coding Basics for Medical Assistants: ICD-10 and CPT

Medical Coding Basics

Medical coding translates patient diagnoses, procedures, and services into standardized codes. As a medical assistant, you won't typically assign codes yourself — that's the job of certified coders — but you will enter codes into EHR systems, verify insurance authorizations, and catch obvious errors before claims go out. Understanding the three main code sets makes you a stronger link in the billing chain.

ICD-10-CM: Diagnosis Codes

The International Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM) codes patient diagnoses, symptoms, and reasons for a visit. The U.S. adopted ICD-10-CM in October 2015, replacing ICD-9.

ICD-10-CM codes are alphanumeric and range from 3 to 7 characters:

• First character: Always a letter (A through Z)
• Second and third characters: Numbers forming the 3-character category
• Characters 4 through 7: Add specificity such as laterality, severity, or episode of care

For example, J06.9 is acute upper respiratory infection, unspecified. S52.501A is an unspecified fracture of the lower end of the right radius, initial encounter. The 7th character "A" signals the initial encounter; "D" means subsequent encounter; "S" means sequela.

Common ICD-10-CM categories you will see as an MA:

• Z codes: Encounters for reasons other than illness (Z00.00 = routine adult exam, Z23 = immunization encounter)
• E codes: External causes of injury (used as secondary codes)
• V, W, X, Y codes: Accidents and adverse effects

CPT: Procedure Codes

Current Procedural Terminology (CPT) codes describe what was done to the patient. Published by the American Medical Association, CPT codes are 5-digit numeric codes grouped into three categories:

• Category I: The main set of codes covering established procedures across Evaluation and Management, Surgery, Radiology, Pathology, and Medicine. Example: 99213 = established patient office visit, low complexity.
• Category II: Supplemental tracking codes for performance measurement. Optional and not used for payment.
• Category III: Temporary codes for new and emerging technologies. Format: four digits followed by the letter T.

Evaluation and Management (E&M) codes (99202 through 99215) are the ones you will encounter most in outpatient settings. The level is determined by medical decision-making complexity or total time the provider spent with the patient.

HCPCS Level II: Supplies and Equipment

Healthcare Common Procedure Coding System (HCPCS) Level II codes cover items and services not found in CPT, mainly durable medical equipment, supplies, and medications administered in the office. They start with a letter (A through V) followed by four numbers. Common examples: A6216 = gauze bandage; J0696 = ceftriaxone injection. HCPCS Level I is simply another name for CPT codes.

Where MAs Fit Into Coding

Your role in coding is supportive, not diagnostic. Common MA tasks include:

• Entering ICD-10 and CPT codes into the EHR from the provider's documentation
• Confirming the diagnosis code matches the procedure code (called code linking) so the claim makes clinical sense to the payer
• Verifying that prior authorization was obtained for the specific CPT code being billed
• Flagging incomplete documentation before a claim is submitted
• Working with encounter forms (superbills) listing the practice's most common codes

Never select or alter a diagnosis code on your own. The provider documents the diagnosis; you transcribe or verify it.

Common Code Categories by Specialty

Different practice types use different code sets heavily. A family practice MA works with E&M codes (99202-99215), preventive visit codes (99381-99397), and immunization codes (90460-90461). A dermatology MA sees destruction codes (17000s), biopsy codes (11100s), and skin lesion removal codes. An orthopedic MA encounters fracture care codes (20000s-29999s) and injection codes (20600-20611). Knowing the typical codes for your specialty helps you catch errors before they cause denials.

Practice Questions