Improper fee-for-service payment rates in the Medicare program dropped from 11% in 2016 to 9.5% in 2017 — the lowest rate since 2013, the Centers for Medicare & Medicaid Services reported.

In a blog post released Thursday, CMS shared that Medicare fee-for-service improper pay rates decreased by $4.9 billion over the past year. That drop represents the first time since 2013 the improper FFS payment rate has reached below the 10% compliance threshold laid out in the Improper Payments Elimination and Recovery Act of 2010.

CMS was quick to note improper payments aren’t always an indication of fraud, or claims that should not have been reimbursed. The majority (17%) of the payments that led to monetary loss to the program stemmed from medical necessity, followed by incorrect coding at 10%. A total of 66% of improper payments made were “unknown,” or were not supported with proper documentation to prove whether they resulted in monetary loss, according to CMS.

“Although documentation errors are the largest cause of improper payments, CMS employs multi-layered efforts to target all root causes of improper payments, with an emphasis on prevention-oriented activities,” the blog post reads.

The agency said it plans to continue to address vulnerabilities within the Medicare program “while minimizing burden for our partners.”