Common Medical Billing and Coding Errors (And How to Avoid Them)

These are the coding-specific mistakes that cause the most problems. Each one results in either denied claims, reduced reimbursement, or compliance risk.

Upcoding. Assigning a higher-level code than the documentation supports. For example, billing a level 4 E/M visit (99214) when the provider's notes only support a level 3 (99213). Upcoding can be accidental (misreading documentation) or intentional (trying to increase revenue), but either way it's a compliance violation. Payers use automated edits to flag patterns of upcoding, and consistent upcoding can trigger a formal audit.

Undercoding. The opposite problem: assigning a lower code than what's documented. This is technically compliant but costs your employer money. Undercoding often happens when coders are unsure about a code and default to the safer, lower option. It's especially common with new coders who are cautious about overcoding.

Unbundling. Billing separately for procedures that should be billed as a single bundled code. The National Correct Coding Initiative (NCCI) maintains edit tables that define which procedure codes can and can't be billed together. Unbundling is one of the most common reasons for claim denials and can raise fraud flags if it happens repeatedly.

Incorrect modifier use. Modifiers tell payers that a procedure was altered in some way (different anatomical site, reduced service, distinct procedure). Using the wrong modifier, or forgetting one entirely, leads to denials. Modifier 25 (significant, separately identifiable E/M service) and modifier 59 (distinct procedural service) are among the most commonly misapplied.

Many claim denials have nothing to do with coding accuracy. They're caused by basic data entry errors that are entirely preventable.

Wrong patient information. Misspelled names, incorrect dates of birth, wrong insurance ID numbers, and mismatched subscriber information cause immediate rejections. These errors happen most often during patient registration and check-in. A simple verification step (asking patients to confirm their information at each visit) prevents most of them.

Incorrect date of service. Billing for the wrong date is a straightforward error that triggers automatic denials. It's most common when batching claims for multiple dates of service or when there's a lag between the patient visit and claim entry.

Missing or expired authorization. Many procedures and specialist visits require prior authorization from the insurance company. If the authorization isn't obtained before the service, or if the auth number isn't included on the claim, the claim gets denied. Authorization tracking is a billing function that requires coordination between the front desk, providers, and billing staff.

Duplicate billing. Submitting the same claim twice, usually because the first submission didn't show as received in the clearinghouse. Payers automatically reject duplicates, but repeated duplicate submissions can flag your practice for unusual billing patterns.

You can't code what isn't documented. One of the most frustrating challenges for medical billers and coders is receiving incomplete or ambiguous documentation from providers.

Insufficient documentation for the code level. A provider may perform a comprehensive exam but only document a brief note. You can't assign a high-level E/M code based on what you assume happened. You can only code what's written in the medical record. This is where provider education and query processes become essential.

Missing linkage between diagnosis and procedure. Every procedure needs a diagnosis that establishes medical necessity. If a provider documents a knee X-ray but doesn't document the clinical reason (pain, injury, follow-up for fracture), the claim will be denied for lack of medical necessity. Coders should query the provider for clarification rather than guessing at the diagnosis.

Late or missing documentation. Claims have timely filing deadlines (typically 90 days to one year depending on the payer). When providers don't complete their notes promptly, the billing cycle stalls and claims risk hitting filing limits. Building documentation completion into the daily workflow prevents this.

Use encoder and scrubbing software. Tools like 3M CodeFinder, Optum EncoderPro, and claim-scrubbing platforms (Waystar, Availity) catch bundling errors, modifier mismatches, and medical-necessity failures before claims reach the payer. Running every claim through a scrubber is the single most effective error-prevention step.

Build a pre-submission checklist. Before submitting any batch of claims, verify: patient demographics match, dates of service are correct, authorization numbers are included where required, diagnosis codes support the procedures billed, modifiers are applied correctly, and no duplicate claims are in the batch.

Stay current on code updates. ICD-10-CM codes update every October 1. CPT codes update every January 1. Using outdated codes is an easy way to generate denials. Set calendar reminders to review updates from CMS and the AMA, and attend your AAPC chapter's annual code-update workshops.

Maintain your continuing education. AAPC requires 36 CEUs every two years for CPC holders. AHIMA has similar requirements. These aren't just bureaucratic checkboxes. CEU activities keep you current on guideline changes, payer policy updates, and emerging compliance issues. Staying current is how you prevent errors before they happen.

Query providers, don't guess. If documentation is unclear or insufficient, send a coding query to the provider. Never assign a code based on assumptions about what was done or diagnosed. A compliant query process protects both you and the provider.

Modern billing offices rely on technology to catch errors before claims go out the door. Claim scrubbing software (Waystar, Availity, ClaimRemedi) automatically checks every claim against NCCI edits, LCD/NCD rules, and payer-specific requirements. These tools flag bundling issues, missing modifiers, and diagnosis-procedure mismatches in real time.

Encoder software (3M CodeFinder, Optum EncoderPro, TruCode) helps coders select accurate codes by providing guideline references, code descriptions, and cross-references during the coding process. Computer-assisted coding (CAC) tools are increasingly used to suggest codes based on clinical documentation, though human review remains required for accuracy.

AI-powered tools are expanding in this space, but they complement human coders rather than replacing them. The most effective error-reduction strategy combines technology (scrubbers, encoders) with human processes (audits, checklists, provider queries). For more on how AI is affecting medical coding, see our dedicated article.