The percentage of working-age Americans with moderate to middle incomes who lacked health insurance for at least part of the year rose to 41 percent in 2005, a dramatic increase from the 28 percent in 2001 without coverage, a study released on Wednesday found.

Moreover, more than half of the uninsured adults said they were having problems paying their medical bills or had incurred debt to cover their expenses, according to a report by the Commonwealth Fund, a New York-based private, health care policy foundation.

The study of 4,350 adults also found that people without insurance were more likely to forgo recommended health screenings such as mammograms than those with coverage and were less likely to have a regular doctor than their insured counterparts.

The report paints a bleak health care picture for the uninsured. "It represents an explosion of the insurance crisis into those with moderate incomes," said Sara Collins, a senior program officer at the Commonwealth Fund.

Collins said the study also illustrates how more employers are dropping coverage or are offering plans that are just too expensive for many people.

About 45.8 million Americans did not have health insurance in 2004, according to the U.S. Census Bureau.

The percentage of individuals earning less than $20,000 a year without insurance rose to 53 percent, up from 49 percent in 2001. Overall, the percentage of people without insurance rose to 28 percent in 2005 from 24 percent in 2001.

The study also found that 59 percent of uninsured with chronic conditions such as asthma or diabetes either skipped a dose of their medicine or went without it because it was too expensive.

One-third of those in that group visited an emergency room or stayed in a hospital overnight or did both, compared to 15 percent of their insured counterparts.

Collins said those statistics are significant because giving up medicines typically leads to more expensive health problems later. Treating people in expensive settings such emergency rooms places a financial burden on the health care system, she added.

"People not being able to take care of themselves should send out a big red flag," said Collins.

HCA Inc. hoisted a red flag on Tuesday, when the nation's largest for-profit hospital operator said its earnings fell 8.5 percent in the first quarter after an increase in the uninsured admissions cut into revenue gains. Uninsured admissions rose 13 percent during the quarter, and the company said its provision for "doubtful accounts" rose to $852 million from $683 million a year earlier.

The Commonwealth Fund's study was bolstered by analysis of government data funded and released by the Robert Wood Johnson Foundation, a private organization that provides health care grants.

That study found that cost prevented 41.1 percent of uninsured adults from seeing a doctor, compared to 9.2 percent of individuals with coverage.

Meanwhile, 51 percent of women without health insurance haven't had a mammogram in two years, compared to 22.8 percent of women with insurance.

And 76.3 percent of uninsured men between the ages of 40 to 64 haven't had the PSA test, which detects prostate cancer, in two years. That compares to 52.2 percent of their insured counterparts.

Researchers at the University of Minnesota School of Public Health used data from the U.S. Centers for Disease Control and Prevention to reach the study's conclusion.