Both reports are published in the July 23 issue of the Archives of Internal Medicine..

One expert thinks these studies shed light on an important and pervasive problem.

"What happens in the hospital doesn't stay in the hospital," said Dr. Samuel Z. Goldhaber, of Brigham and Women's Hospital in Boston, and author of an accompanying editorial in the journal. "There are important ramification for the 90 days following hospital discharge in terms of developing pulmonary embolism," he said.

Goldhaber said the studies have important implications for treatment.

"There is a unique opportunity to improve patient safety by aggressively implementing prophylaxis in the hospital that will yield a return by not only lowering the rate of inpatient thromboembolism, but also prevent the development of thromboembolism in the first few months after hospital discharge," he said.

Blood clots include deep vein thrombosis -- clots in the deep veins of the legs and pelvis -- and pulmonary embolism -- clots in the lungs -- and they are a major cause of complications and death in hospitalized patients. In fact, 10 percent of deaths in the hospital are due to pulmonary embolism.

In one study in the journal, lead researcher Dr. Frederick A. Spencer, of McMaster University Medical Center in Hamilton, Ontario, Canada, and colleagues studied the medical records of patients from Worcester, Mass., who were diagnosed with venous thromboembolism in 1999, 2001 and 2003.

Spencer's team found a total of 1,897 cases of venous thromboembolism. Among these patients, 73.7 percent developed their clot after leaving the hospital. Among these patients, 23.1 percent had undergone surgery, and 36.8 percent had been hospitalized in the three months before their clot developed.

Among patients who developed clots, 67 percent had the condition within one month of their hospitalization. Other risk factors for clots were having cancer and having had a previous blood clot.

For patients with venous thromboembolism, 59.7 percent received therapy to prevent clots while in the hospital, the researchers found.

"Because most of the cases of venous thromboembolism occurred within 29 days of hospital discharge (and 41 percent occurred within 14 days), it is not unreasonable to assume that some of these cases may have been prevented simply by increased use of appropriate in-hospital deep vein thrombosis prophylaxis (e.g., compression stockings, pneumatic compression devices and, in high-risk patients, anticoagulants)," the authors wrote.

The second study in the journal looked at 36 published research papers dealing with medications used to prevent venous thromboembolism. The studies compared the blood-thinner unfractionated heparin to a placebo, or low-molecular-weight heparin with a placebo, or both types of heparin with each other. One study compared the blood-thinning drug fondaparinux sodium with a placebo.

"Although these agents reduce thrombotic events, they do not reduce all-cause mortality, perhaps because many events are not life-threatening or because of other mortality risks or because adverse events of these agents offset their benefits," said lead researcher Dr. Henry Krum, a professor of epidemiology at Monash University, in Melbourne, Australia.

Compared with a placebo, unfractionated heparin was linked to a 67 percent lower risk of venous thrombosis and a 36 percent lower risk of pulmonary embolism. Low-molecular-weight heparin was associated with a 44 percent lower risk of deep vein thrombosis and 63 percent lower risk of pulmonary embolism, the researchers found.

When compared with each other, low-molecular-weight heparin was associated with a 32 percent reduced risk of deep vein thrombosis and a 53 percent lower rate of hematoma at the injection site. Fondaparinux sodium also prevented venous thromboembolism. But, using these drugs wasn't associated with lower death rates, according to the study results.

"These medications should be given to reduce thrombotic events, but do not expect to reduce mortality," Krum said.

Goldhaber believes that patients need to take an active role in making sure they are receiving treatment to prevent blood clots. "Patients should demand, during hospitalization, to know what is being done to prevent thromboembolism during this period of risk," he said.

More information

For more information on deep vein thrombosis, visit the U.S. National Heart, Lung, and Blood Institute.