# medicalbilling101.com

> Practical medical billing education from an RCM specialist with 10+ years of experience: denial codes (CARCs), CPT and unlisted-code billing, NCCI edits, global surgery rules, corrected claims, and Medicare/commercial payer guidelines for US billing teams.

All articles are written by Manikandan (RCM Specialist, 10+ years). Content is educational and not legal or payer-specific advice; always confirm against the current CMS manual or payer policy.

## Core guides

- [What Is Medical Billing? Definition, Steps & Key Terms](https://medicalbilling101.com/what-is-medical-billing/): Medical billing is how providers turn care into paid insurance claims: the claim cycle, forms, code sets, remittance advice and denials, with CMS sources
- [What Are CPT Codes? Categories, Structure & Updates](https://medicalbilling101.com/what-are-cpt-codes/): CPT codes are the AMA's five-character codes for medical services and procedures: categories, code format, who maintains them and how they're updated

## Key guides

- [CO 97 Denial Code: What It Means and How to Work It (2026 Guide)](https://medicalbilling101.com/co-97-denial-code/): CARC 97 bundling denials, NCCI PTP edits, modifier 59/X{EPSU}, step-by-step rework and appeal
- [Global Surgical Package Guide: 10-Day and 90-Day Global Period Billing Rules](https://medicalbilling101.com/global-surgical-package-guide-reference-page-on-10-day-and-90-day-global-period-billing-rules/): What's bundled, 0/10/90-day periods, modifiers 24, 25, 57, 58, 78, 79
- [When to Use Unlisted CPT Codes Without Getting Audited](https://medicalbilling101.com/when-to-use-unlisted-cpt-codes-without-getting-audited/): When unlisted codes are appropriate, documentation, audit risk
- [Medical Necessity Documentation: What Payers Actually Look For](https://medicalbilling101.com/medical-necessity-documentation-what-payers-actually-look-for/)
- [Corrected Claim Resubmission Guide by Payer Type](https://medicalbilling101.com/corrected-claim-resubmission-guide-step-by-step-instructions-for-corrected-claims-by-payer-type/)
- [Step-by-Step Guide to Searching NCCI PTP Tables on CMS.gov](https://medicalbilling101.com/step-by-step-guide-to-searching-ncci-ptp-tables-on-cms-gov/)

## Denial codes

- [Denial Codes List: CARC & RARC Lookup With Fixes (hub)](https://medicalbilling101.com/denial-codes/): Look up any claim denial code; how group codes, CARCs and remark codes work together, with step-by-step fixes for common denials
- [CO-16 Denial Code: Claim Lacks Information or Has a Billing Error](https://medicalbilling101.com/denial-codes/co-16/): CO-16 means the claim lacks information or has a billing error; official definition, the remark codes that explain it and how to fix it
- [CO-45 Denial Code: Charge Exceeds Fee Schedule or Contracted Amount](https://medicalbilling101.com/denial-codes/co-45/): CO-45 means the charge was higher than the allowed or contracted amount; what it means, when to dispute it and how to post it
- [OA-18 Denial Code: Exact Duplicate Claim or Service](https://medicalbilling101.com/denial-codes/oa-18/): OA-18 means the payer thinks the claim is an exact duplicate; official definition, the N522 remark code and how to stop duplicate denials
- [CO-4 Denial Code: Procedure Code Inconsistent With the Modifier](https://medicalbilling101.com/denial-codes/co-4/): CO-4 means the modifier doesn't fit the procedure code, or a required one is missing; definition, the N519 remark code and how to fix it
- [CO-50 Denial Code: Not Deemed a Medical Necessity by the Payer](https://medicalbilling101.com/denial-codes/co-50/): CO-50 means the payer decided the service wasn't medically necessary; official definition, N115, ABN rules and how to appeal
- [CO-22 Denial Code: Care May Be Covered by Another Payer (Coordination of Benefits)](https://medicalbilling101.com/denial-codes/co-22/): CO-22 means another payer may be primary; official definition, the MA92 and N598 remark codes, and how to rebill in the right order
- [CO-29 Denial Code: The Time Limit for Filing Has Expired](https://medicalbilling101.com/denial-codes/co-29/): CO-29 means the claim was filed after the payer's deadline; official definition, Medicare's 12-month rule and its four exceptions
- [PR-1, PR-2 and PR-3: Deductible, Coinsurance and Copay (Patient Responsibility)](https://medicalbilling101.com/denial-codes/pr-1-pr-2-pr-3/): Patient responsibility group codes for the deductible, coinsurance and copay; what each means and how to post them
- [N290 Remark Code: Missing, Incomplete or Invalid Rendering Provider Identifier](https://medicalbilling101.com/denial-codes/n290/): N290 means the rendering provider's NPI is missing or invalid; official definition, where the NPI goes and how to fix CO-16 + N290
- [MA130 Remark Code: Unprocessable Claim, No Appeal Rights](https://medicalbilling101.com/denial-codes/ma130/): MA130 means the claim was returned as unprocessable, with no appeal rights; why appealing wastes time and how to correct and resubmit
- [CO-96 Denial Code: Non-Covered Charges in Cosmetic Billing](https://medicalbilling101.com/co-96-denial-code-what-non-covered-charges-mean-in-cosmetic-billing/)
- [CO-197 Criteria: UnitedHealthcare, Cigna, and Aetna Compared](https://medicalbilling101.com/co-197-denial-unitedhealthcare-cigna-aetna/)
- [Guide to Building a CO-97 Denial Tracking Dashboard in Your PM System](https://medicalbilling101.com/guide-to-building-a-co97-denial-tracking-dashboard-in-your-pm-system/)
- [Clearinghouse NCCI Scrubbing Setup by PM Platform](https://medicalbilling101.com/clearinghouse-ncci-scrubbing-setup-guide-to-enabling-pre-submission-edit-checking-by-pm-platform/)
- [CPT 64772 vs 64999: Thumb CMC Joint Denervation Coding Guide](https://medicalbilling101.com/cpt-64772-vs-64999-for-selective-thumb-cmc-joint-denervation-which-code-is-actually-correct/)
- [All denial code articles](https://medicalbilling101.com/category/denial-codes/)

## CPT, ICD-10 and modifiers

- [Category III CPT Codes vs Unlisted Codes: Which One Should You Bill](https://medicalbilling101.com/category-three-cpt-codes-vs-unlisted-codes-which-one-should-you-bill/)
- [How to Write a Comparable Procedure Narrative for Unlisted CPT Codes](https://medicalbilling101.com/how-to-write-a-comparable-procedure-narrative-for-unlisted-cpt-codes/)
- [What to Put in Box 19 on the CMS-1500 for Unlisted Procedures](https://medicalbilling101.com/what-to-put-in-box-19-on-the-cms-1500-for-unlisted-procedures/)
- [CPT Unlisted Code Denials: How to Appeal and Win](https://medicalbilling101.com/cpt-unlisted-code-denials-how-to-appeal-and-win/)
- [GY Modifier on Medicare Claims: Cosmetic Procedures Billing Guide](https://medicalbilling101.com/gy-modifier-on-medicare-claims-cosmetic-procedures-billing-guide/)
- [ICD-10 Z41.1 Encounter for Cosmetic Surgery: When to Use It](https://medicalbilling101.com/icd-10-z41-1-encounter-for-cosmetic-surgery-when-to-use-it-and-when-not-to/)
- [CPT 90839 and 90840 Denied by Medicaid](https://medicalbilling101.com/cpt-90839-and-90840-denied-by-medicaid-what-it-means-and-how-to-fix-it/)
- [All medical code articles](https://medicalbilling101.com/category/medical-codes/)

## Insurance guidelines

- [How to Handle a Medicare MAC Prepayment Review](https://medicalbilling101.com/how-to-handle-a-medicare-mac-prepayment-review/)
- [CPT 15771 and 15772 Denial: "Primary Service Code Was Not Submitted"](https://medicalbilling101.com/cpt-15771-and-15772-denial-how-to-fix-primary-service-code-was-not-submitted/)
- [CPT Code for Juvederm Breast Injections (11950-11954)](https://medicalbilling101.com/cpt-code-for-juvederm-breast-injections-11950-11954-what-billers-need-to-know/)
- [Patellofemoral Arthroplasty Coding: 27438 vs 27599 (three-payer comparison)](https://medicalbilling101.com/i-called-three-commercial-payers-about-patellofemoral-arthroplasty-coding-27438-vs-27599/)
- [All insurance guideline articles](https://medicalbilling101.com/category/insurance-guidelines/)

## RCM guides

- [Prior Authorization for Surgical Procedures: A Billing Team Checklist](https://medicalbilling101.com/prior-authorization-for-surgical-procedures-a-billing-team-checklist/)
- [Billing Medicare Secondary for Global Imaging Code 74181: CMS Anti-Markup Rule](https://medicalbilling101.com/billing-medicare-secondary-when-global-imaging-code-74181-was-paid-by-primary-insurance-a-complete-guide-to-the-cms-anti-markup-rule/)
- [CPT 87798 Denied for 13 Units: Why It Happens and How to Fix It](https://medicalbilling101.com/cpt-87798-denied-for-13-units-why-it-happens-and-how-to-fix-it/)
- [All RCM guides](https://medicalbilling101.com/category/rcm-guides/)

## Medical billing news

- [OIG Audit Triggers in Medical Billing: The 2026 Work Plan Explained](https://medicalbilling101.com/oig-audit-triggers-medical-billing/)
- [All medical billing news](https://medicalbilling101.com/category/medical-billing-news/)

## About

- [About the author: Manikandan, RCM Specialist](https://medicalbilling101.com/about/): Medical billing, denial management, CPT codes, ICD-10, revenue cycle management
- [Contact](https://medicalbilling101.com/contact/)
- [Editorial Policy: How Medical Billing 101 Checks Its Facts](https://medicalbilling101.com/editorial-policy/): How guides are checked against X12, CMS, MAC and AMA sources, how code wording is quoted exactly and how errors are fixed
- [Disclaimer](https://medicalbilling101.com/disclaimer/)