According to a recent report,

the percentage of adults with medical debt broken down by income level is:

• 32.5% of middle-income adults (those with incomes of 138% or below federal poverty level)
• 25.6% of low-income adults (those with incomes of 139-399% of federal poverty level)
• 15.4% of high-income adults. (those with incomes at or above 400% of federal poverty level)

Source: "Most Adults with Medical Debt Had Health Insurance at the Time the Debt Was Incurred," The Urban Institute, May 21, 2015,
http://hrms.urban.org/briefs/Most-Adults-with-Medical-Debt-Had-Health-Insurance-at-the-Time-the-Debt-Was-Incurred.html