Medical Billing Codes Explained

CPT (Current Procedural Terminology)

Five-digit codes that identify clinical procedures and services performed by clinicians.

CPT is the main code set used to bill outpatient procedures, office visits, surgeries, lab tests, and imaging. Insurers use the code to decide reimbursement, and providers use it to describe what was done.

Example: CPT 99213: established patient office visit, 20 to 29 minutes.

Lookup: AMA CPT overview

Watch for: Upcoding (a more complex code than the visit warranted), unbundling (billing component services separately when they should be combined), and codes that do not match the documented diagnosis.

HCPCS Level II

Codes for medical equipment, supplies, drugs, ambulance services, and other items not covered by CPT.

HCPCS Level II covers things like wheelchairs, crutches, injectable medications administered in a clinical setting, durable medical equipment, and ambulance transport. CPT (HCPCS Level I) covers the procedures themselves; Level II covers the items.

Example: HCPCS J3490: unclassified drug, used to bill medications that do not have a specific code.

Lookup: CMS HCPCS Level II

Watch for: Equipment billed at a purchase price when it should have been a rental, and unspecified-drug codes (J3490, J3590) that obscure what was actually administered.

ICD-10-CM

Diagnosis codes that describe the patient's condition or the reason for the visit.

ICD-10-CM is the diagnosis side of the bill. It tells the insurer why a service was needed. CPT says what was done; ICD-10-CM says why. Insurers cross-check the two to decide whether a service was medically necessary under your plan.

Example: ICD-10-CM E11.9: type 2 diabetes mellitus without complications.

Lookup: CMS ICD-10

Watch for: Diagnosis codes that do not match the procedures billed, which can lead to denied claims or accusations of upcoded diagnoses to justify higher-paying procedures.

ICD-10-PCS

Inpatient procedure codes used by hospitals for services delivered during an inpatient stay.

ICD-10-PCS is hospital-only. If you were admitted as an inpatient, the procedures performed during the stay are coded in ICD-10-PCS rather than CPT. The codes are seven characters long and describe the procedure in structured detail.

Example: ICD-10-PCS 0FT44ZZ: laparoscopic resection of the gallbladder.

Lookup: CMS ICD-10-PCS

Watch for: These codes drive DRG assignment for inpatient billing, so a single different character can change the dollar amount substantially. Request your itemized inpatient bill if anything looks off.

NDC (National Drug Code)

A unique 10 or 11-digit identifier for a specific drug, including manufacturer, product, and package size.

Pharmacies and clinics use NDC codes to bill medications. The code identifies the exact product, not just the active ingredient, so two NDCs can refer to the same drug from different manufacturers.

Example: NDC 0002-7510: a specific manufacturer-and-package code for a particular medication.

Lookup: FDA National Drug Code Directory

Watch for: Brand-name NDCs billed when a generic equivalent was approved and dispensed, and quantity errors that multiply the per-unit price.

DRG (Diagnosis-Related Group)

A classification used to set a fixed payment amount for an inpatient hospital stay, based on diagnoses and procedures.

Medicare and many private insurers pay hospitals a single DRG-based amount per admission rather than itemizing every charge. The DRG is determined by the principal diagnosis, secondary diagnoses, procedures, age, sex, and discharge status.

Example: MS-DRG 470: major hip and knee joint replacement without major complications.

Lookup: CMS MS-DRG

Watch for: Hospital itemized bills can show charges that look enormous individually because the insurer pays a flat DRG amount regardless of the line-item totals. Compare the EOB to the itemized bill to see what the plan actually paid.

Modifiers

Two-character additions to a CPT or HCPCS code that change its meaning without changing the underlying procedure code.

Modifiers describe circumstances that affect payment: which side of the body, whether the service was bilateral, whether it was a separately identifiable service from another billed on the same day, and so on.

Example: Modifier 50: bilateral procedure. Modifier 25: significant, separately identifiable evaluation and management service on the same day as a procedure.

Lookup: CMS modifier reference

Watch for: Inappropriate use of modifier 25 to bill an evaluation and management visit on top of a procedure when the visit was not separately identifiable. This is a frequent source of overbilling.

Place of Service codes

Two-digit codes that indicate where the service was delivered.

Insurers reimburse the same procedure differently depending on the setting. Place of Service 11 is an office; 22 is on-campus outpatient hospital; 21 is inpatient hospital. Hospital-based clinics often bill at facility rates even when the visit looks like an office visit.

Example: POS 22: a clinic visit at an outpatient department of a hospital, billed at facility rates rather than office rates.

Lookup: CMS Place of Service code set

Watch for: Facility-rate billing for a visit at a hospital-owned clinic that you assumed was a regular office visit. The same CPT code paid at POS 22 can cost substantially more than at POS 11.

Revenue codes

Four-digit codes used on the UB-04 hospital claim form to group charges by department or category.

Revenue codes describe the type of service or department the charge came from: room and board, pharmacy, operating room, lab, and so on. They are paired with HCPCS or CPT codes that describe the specific service.

Example: Revenue code 0450: emergency room, general classification. Revenue code 0250: pharmacy.

Lookup: NUBC overview (publishers of UB-04)

Watch for: Charges grouped under revenue codes that do not match the visit you actually had, and pharmacy revenue codes with no NDC detail to verify what was administered.

EOB and billing abbreviations

These acronyms appear repeatedly across bills, EOBs, and insurance correspondence. Knowing what each one means makes the rest of the document readable.

EOB

Explanation of Benefits, the insurer's record of how a claim was processed. Not a bill.

COB

Coordination of Benefits, the rules that determine which plan pays first when you have more than one insurance.

AOB

Assignment of Benefits, an authorization that lets the provider receive payment directly from the insurer.

DOS

Date of Service, the date the care was actually delivered.

POS

Place of Service code, indicating where care was delivered (office, outpatient hospital, inpatient hospital, etc.).

PCP

Primary Care Provider, your designated lead doctor. Some plans require referrals through the PCP.

PPO

Preferred Provider Organization, a plan that pays out-of-network providers at a reduced rate without requiring referrals.

HMO

Health Maintenance Organization, a plan that requires care from in-network providers and usually a referral from a PCP.

EPO

Exclusive Provider Organization, similar to an HMO but typically no PCP referral required.

POS plan

Point of Service plan, a hybrid that lets you go out of network at a higher cost.

Deductible

What you pay out of pocket before the insurer starts paying covered expenses.

Copay

A fixed dollar amount you pay per visit or service.

Coinsurance

A percentage of the allowed amount you owe after the deductible is met.

OOP max

Out-of-pocket maximum, the cap on what you can be required to pay in covered costs in a plan year.

Allowable / Allowed Amount

The maximum the insurer treats as eligible for payment for a given service. Charges above the allowable are typically the provider's write-off (in network) or your responsibility (out of network).

Adjustment / Write-off

A reduction the provider agreed to under the network contract. You do not owe this amount.

N/C (Non-covered)

The service is not covered under your plan, in part or in full.

N/A

Not applicable, often used in EOB columns where a value would not make sense for that line.

Pending

The claim has not finished processing yet.

Paid

The insurer has paid its portion of the claim.

Denied

The insurer has refused to pay, with a reason code that should appear on the EOB.

Appealed

A formal request to the insurer to reconsider a denial. Most plans give you 180 days from the EOB date.