Medical bills have been hitting U.S. consumers so hard partly because the consumers are so short on cash.

Analysts at the Center for Studying Health System Change, Washington, have published figures supporting that conclusion in a report based on survey data on U.S. residents ages 64 and younger gathered in 2003 and 2007.

All the people included in the survey had employer-sponsored health coverage. The sample size was 27,000 in 2003 and 9,600 in 2007.

The researchers wanted to see whether the “affordability threshold for medical care”– the amount of spending that leads to problems paying medical bills–changes over time, the center says.

The proportion of participants who said they had medical bill problems increased to 15.4% in 2007 from 12.5% in 2003.

Family out-of-pocket spending on health services increased 37% over that period, to $1,318, and family income increased 10%, the researchers found.

The families spent about 2.8% of family income on out-of-pocket health care spending in 2007, up from 2.2% in 2003.

When researchers looked more closely, they found that the percentage of families with medical bill problems would have increased to 14.1% in 2007, even if the ratio of out-of-pocket spending to income had not increased.

The increase in medical expenses had even less effect on the lower-income participants’ medical bill problems, the researchers say.

The researchers did not use a method that identified factors other than medical expense cost increases that contributed to the spread of medical bill problems, but the researchers say increases in fuel prices and the start of the housing market slump may have played a role.

-ab