According to http://www.ahrq.gov

In general, poor populations have reduced access to high-quality care. While people with low incomes are more likely to be uninsured, income-related differences in quality of care that are independent of health insurance coverage have also been demonstrated (Brown, et al., 2003).

Also this metric has increased over the years:

People without a usual source of care who indicated a financial or insurance reason for not having a source of care.