CT scan can harm kidneys, but cheap drug can help

Published on February 20, 2008 at 4:12 AM · No Comments

As more and more Americans undergo CT scans and other medical imaging scans involving intense X-rays, a new study suggests that many of them should take a pre-scan drug that could protect their kidneys from damage.

The inexpensive drug, called N-acetylcysteine, can prevent serious kidney damage that can be caused by the iodine-containing "dyes" that doctors use to enhance the quality of such scans.

That "dye," called contrast agent, is usually given intravenously before a CT scan, angiogram or other test. But the new study shows that taking an N-acetylcysteine tablet before receiving the contrast agent can protect patients - and that it works better than other medicines that have been proposed for the same purpose.

People whose kidneys are already vulnerable, including many older people and those with diabetes or heart failure, are the most at risk from contrast agents, and have the most to gain from taking the drug.

Researchers from the University of Michigan Health System performed the study, which is published in the Annals of Internal Medicine. It is a meta-analysis of data from 41 randomized controlled studies that evaluated various drugs for their kidney-protecting effects. It was led by Aine Kelly, M.D., M.S., an assistant professor in the Department of Radiology at the U-M Medical School.

Only N-acetylcysteine clearly prevented contrast-induced nephropathy, the medical name for kidney damage caused by contrast agents. Theophylline, another drug that has been seen as a possible kidney-protecting agent, did not reduce risk significantly. Other drugs had no effect, and one, furosemide, raised kidney risk.

"Our goal is to improve the safety and quality of these common tests by studying drugs that reduce the risk of kidney failure," says senior author Ruth Carlos, M.D., associate professor of radiology.

Mild to moderate kidney damage occurs in one in four high-risk people who have CT scans, and in as many as one in ten people with normal kidney function. In some cases, it causes acute kidney failure.

"Millions of people receive contrast agent each year, including most heart patients who have angioplasties and stents, as well as those having a CT scan. Contrast agent helps physicians see the things we need to see, but it also does pose a hazard to some people," says Kelly. "This drug, which is quick, convenient, inexpensive and widely available, with no major side effects, appears to be the best choice to protect those whose kidneys are most at risk."

Only studies that involved intravenous iodine-containing contrast agents, and compared a drug with a water or saline control, were included in the analysis. Oral "milkshake" barium contrast agents, used in CT scans of the digestive system, do not cause kidney damage, and were not included.

The study also did not assess potential ways to protect against kidney damage from gadolinium contrast agents used in MRI (magnetic resonance imaging) scans. Since May 2007, those contrast agents have carried a warning from the U.S. Food and Drug Administration about risk to kidneys.

Kelly, Carlos and their colleagues performed the study to try to get a firm answer to a question that has puzzle medical imaging specialists for years.

Although many drugs have been tried for prevention of iodine-related contrast-induced nephropathy, contradictory evidence has emerged from studies of how well they work. The result has been widespread variation in what hospitals and medical imaging centers do before scanning a patient.

Although a prospective trial comparing N-acetylcysteine directly to other drugs should be conducted to verify the U-M team's findings, the team hopes its new study will help guide both clinicians and patients.

In fact, Kelly says, patients who know they have weakened kidneys - also called impaired renal function - should speak up when their doctor orders a CT scan, angiogram or angioplasty, and make sure they get a tablet of N-acetylcysteine beforehand.

And, since most kidney problems cause no symptoms, even healthy people might want to ask their doctors to test their blood creatinine levels before sending them for a scan.

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