Drawing blood for medical tests could make sick patients even sicker, hints a new study that suggests taking blood for testing, over and over, may not be as innocent as doctors used to think.

Earlier research has shown that as many as half of heart attack patients who arrive at the hospital with normal levels of red blood cells go home with anemia - a deficit of the oxygen-carrying red cells - often serious enough to cause symptoms.

“People who developed moderate anemia had more symptoms - such as fatigue and shortness of breath - and actually had higher mortality after being discharged from the hospital,” said Dr. Mikhail Kosiborod of Saint Luke’s Mid America Heart and Vascular Institute in Kansas City, Missouri.

“This doesn’t prove that anemia causes the bad outcome but it certainly is a risk factor,” he told Reuters Health.

To understand what was causing the blood loss leading to anemia, Kosiborod and colleagues studied more than 17,000 heart attack patients at 57 U.S. hospitals, who were admitted with normal blood counts.

One in five patients developed moderate or severe anemia at the hospital.

Those patients turned out to have had 174 milliliters (5.9 ounces) of blood drawn, whereas their peers who didn’t experience anemia only had 84 mL (2.8 ounces) drawn.

“Drawing blood in the hospital is very important, but we never thought there was a downside,” said Kosiborod. “Now that we know there could be a potential downside, it is important for doctors to critically consider if the patients need the blood work.”

The new study, published in the Archives of Internal Medicine, didn’t look at whether the blood tests were appropriate or not. But some experts say they are used more than is medically necessary, which stretches a healthcare budget that’s already hard pressed.

Although he stressed there is no bulletproof evidence, Kosiborod said excessive blood tests now also appear to harm patients.

For every 50 mL of blood drawn, his team found, the risk of anemia climbed by 18 percent. And there was a large variation between hospitals, indicating that some might use the tests more liberally.

“What I tell patients is, they should expect to have their blood drawn in the hospital,” he said.

But, he added, “it is perfectly fine for patients to ask doctors if frequent blood work is necessary.”

His team is now looking at whether using smaller tubes to draw blood or keeping samples for reuse could have an impact on anemia.

“The smaller-sized tubes are perfectly adequate for most of the tests we do in the hospital,” Kosiborod said.

SOURCE: Archives of Internal Medicine, online August 8, 2011.