For even more resources, check out MLN Matters SE1217. When Part B practices perform self-audits, they often report that their top errors include Medicare Secondary Payer (MSP) claims being billed as primary. You can avoid this common error, and other MSP denial issues, if you take heed of CMS's MSP instructions, presented in MLN Matters SE1217. 1. Ask Patients About All of Their Insurers. 2. Bill Primary Payer First, Not Simultaneously. To read the complete MLN Matters article, visit www.cms.gov/Outreach-and-Education/Medicare-Learning-Network-MLN/MLNMattersArticles/downloads/SE1217.pdf.