By Naomi Foster, Healthcare WriterLook up any common billing code, check a staffing ratio, or estimate a bill, free.
This overview is educational, not billing advice. For an actual claim or billing dispute, loop in a qualified biller or coder.
A medical billing code is a standardized number or alphanumeric string that identifies a diagnosis, procedure, or supply used in a healthcare encounter. Insurers require these codes to process claims: they allow consistent, auditable comparison across millions of transactions.
CPT (Current Procedural Terminology) codes are five-digit numbers that describe what was done: office visits, surgeries, lab tests, imaging, physical therapy, and thousands of other services. The AMA maintains CPT and updates it every January 1. See how to look up a CPT code.
ICD-10-CM codes are alphanumeric (a letter followed by up to six characters) and describe the patient's diagnosis or reason for the visit. CMS and CDC maintain ICD-10-CM, which updates every October 1. See ICD-10 vs CPT codes for how the two sets work together.
HCPCS Level II codes (a letter followed by four digits) cover items and services CPT does not address: durable medical equipment, ambulance services, drugs given in a clinical setting, and more. For example, A6216 is a gauze dressing and J0696 covers cefazolin injection.
A standard CMS-1500 claim form (used for physician billing) includes fields for the diagnosis codes (ICD-10) and procedure codes (CPT or HCPCS) for each service line. The diagnosis code establishes medical necessity; the procedure code tells the payer what to pay. A claim can have multiple diagnosis codes and multiple procedure lines.
A patient visits a provider. The provider documents the encounter. A coder (or the provider themselves) translates the documentation into codes. The codes are submitted on a claim. The payer adjudicates the claim (checking codes against coverage rules, fee schedules, and medical necessity criteria) and either pays, denies, or requests more information. Understanding this flow helps explain why a small coding error can delay payment for weeks.
Medicine changes. New procedures are developed, old ones are retired, and coding guidelines are refined. CPT updates each January 1; ICD-10 updates each October 1. Using a code from the wrong year on a service date is a common source of claim denial, particularly in the weeks after each update. Practices must update their code libraries on schedule.
Upcoding (billing a higher complexity than documented), undercoding (billing less than what was done to avoid scrutiny), unbundling (billing components separately that should be combined), and using an incorrect or outdated code are the most frequent issues. Documentation drives everything: if the chart does not support the code, the code is wrong. Use the Medical Billing Calculator to estimate the dollar impact of a CPT code before a claim goes out.
Look up any common billing code, check a staffing ratio, or estimate a bill, free.
For office visits, 99213 (established patient, low-to-moderate complexity) and 99214 (established patient, moderate complexity) are among the most frequently billed CPT codes. For diagnoses, common ICD-10 codes include Z00.00 (routine annual exam), I10 (hypertension), and E11.9 (Type 2 diabetes). See the common CPT codes article for a broader list.
CPT codes are five-digit numbers (example: 99214). ICD-10-CM codes start with a letter followed by one to six more characters (example: K21.0 for gastroesophageal reflux with esophagitis). HCPCS Level II codes start with a letter followed by four digits (example: E0601 for a CPAP device).
Your itemized hospital bill and the Explanation of Benefits (EOB) from your insurer both list the codes billed. The EOB shows the CPT and ICD-10 codes, the billed amount, what the insurer paid, and what you owe. See the article on how to read an EOB for a step-by-step walkthrough.
A diagnosis code (ICD-10) describes the medical reason for the visit. A billing code, in common usage, often refers to a procedure code (CPT or HCPCS) that describes what was done and what the provider is requesting payment for. Both appear on a claim together.
