By RONI CARYN RABIN

Some of the sickest patients can run up hospital charges as high as $18,000 a day, with average stays of almost three weeks, according to a new government report on the cost of hospital care.

The report, from the Agency for Healthcare Research and Quality, used data from a nationwide sampling of patients in 2008 to analyze the two million most expensive stays (5 percent of the total) in short-term nonfederal hospitals.

These patients were much more severely ill than others; the five most common diagnoses were sepsis, or blood poisoning, followed by clogged coronary arteries, heart attack, respiratory failure and complications related to a medical device, implant or graft.

Fifty-four percent of the patients were men, the average age was 58, and they were more likely to live in the Western United States.

The charges do not necessarily reflect actual fees paid; charges are significantly discounted for many insured patients, said the author of the report, Bernard S. Friedman, a senior economist with the heath care agency.