Iodine contrast agents that enhance the scans can
harm vulnerable kidneys, but N-acetylcysteine taken beforehand can
protect at-risk patients
February 2008 (ANN ARBOR, Mich.) — 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.
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
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.
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.
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.
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.
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
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.
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.
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.
Creatinine levels go up when the kidneys
aren’t operating efficiently. Contrast-induced nephropathy is defined
as a 25 percent or greater increase in creatinine within 48 hours of
receiving contrast agent. The new study evaluated the impact of
pre-scan medicines by looking at their impact on patients’ creatinine
levels, as a surrogate for kidney function.
The problem of
contrast-induced nephropathy is a relatively recent one — a byproduct
of the dramatic increase in the use of CT scans and X-ray guided
procedures such as angioplasty.
The benefit of these scans
is not in question: Intense X-rays can reveal valuable information
about injuries, diseases, and patients’ response to treatment.
Iodine-containing agents allow doctors to improve the scans even
further, giving better “contrast” between the blood vessels and tissue,
because X-rays are scattered differently by iodine. But as the iodine
is carried by the blood to the kidneys, and filtered out into the
urine, the iodine can upset the delicate balance that keeps the kidneys
In recent years, manufacturers of contrast
agents have begun to offer different formulations that may pose less
risk to patients’ kidneys. More expensive low-iodine and iodine-free
agents are available to especially vulnerable patients, and to people
who are allergic to iodine. But for everyone else, medical imaging
specialists have tried to find ways to prevent the damage.
is already widely used to clear mucus in cystic fibrosis patients, and
to treat overdoses of acetaminophen. It’s also being studied for other
Tablets of prescription-strength N-acetylcysteine are
inexpensive – about 25 cents for a 500 milligram tablet – and stocked
by most pharmacies. It has few side effects. Over-the-counter
supplement forms of the drug should not be used for pre-scan kidney
Source : University of Michigan Health System