The Federal Reserve Bank's report said that the effect of Medicaid expansion really began being seen in the first quarter of 2015.
Before then, from 2009 until early 2015, there was no statistically significant difference between "counties with high uninsurance rates in expansion and non-expansion states in the average level of debt sent to collection agencies."
"Collections in counties that did not expand Medicaid grew along similar trends as in counties that did," the authors wrote.
But beginning in early 2015, a pronounced difference was seen.
By the fourth quarter of 2015, "on average, collections declined by more than $100 per capita in the counties most affected by Medicaid expansion relative to the less affected counties," the report said.
"This is a sizeable decline, given that the mean of collection balances over our sample period is $280," the report said.
The authors said that it was not surprising that the decline was not seen as soon as the ACA began being fully implemented, in early 2014, because "bills can take many months to enter collections."
The report also found some evidence, not nearly as strong, that suggests that Medicaid expansion has helped lower average per capita credit card balances in the affected counties in the past three quarters. And there is also evidence that suggests credit risk scores have improved in such counties.