Patients with impaired renal function
should not accept a CT scan unless they are first given
a pill to protect their kidneys, say researchers writing
in the February 19 issue of the Annals of Internal
Medicine.
Even patients with healthy kidneys
could benefit from taking the nephroprotective drug
N-acetylcysteine, also known as NAC or acetylcysteine,
before undergoing scans involving iodine-based radio-contrast
agents, the study finds. N-acetylcysteine is
cheap, safe, widely available, and could prevent serious
kidney damage often caused by iodine.
The use of contrast agents has
brought enormous medical benefits, especially in angioplastic
surgery, but has also heralded a steep rise in kidney
failure, by far the most common and serious side effect
of powerful scanners. As many as one in ten healthy
people undergoing CT scans suffer acute kidney failure
— an increase of at least 50% in blood creatinine
— typically peaking two to five days after exposure.
Patients considered at-risk for kidney problems fare
worse — about one in four suffer mild to moderate
kidney damage. A Danish study in 2003 identified contrast
media assisted x-ray as the third leading cause of hospital-acquired
acute renal failure.
A
CLEAR WINNER
Various approaches have been tried to ameliorate this
problem, hydration being the commonest. But hydration
alone is often not enough. A whole battery of drugs
have been tried, including N-acetylcysteine,
theophylline, fenoldopam, bicarbonate, dopamine, iloprost,
statins, furosemide, and mannitol. Each has had its
advocates, and some have shown efficacy in trials. But
the research is skimpy, the studies are small and these
options have rarely been compared head-to-head.
The answer for such circumstances
is a meta-analysis, and that has now been performed
by researchers from the University of Michigan Health
System. They compared 41 studies involving 3622 patients.
Their conclusion will please any radiologist on a tight
budget — humble N-acetylcysteine, at about
25 cents for a 500mg tablet or two dollars for a 48-hour
prophylactic course (Canadian prices run in the same
range), was the clear winner. Relative risk of contrast-induced
nephropathy, defined as a 25% increase in serum creatinine,
was 0.62 for patients who took this drug.
"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," says lead author radiologist
Dr Aine Kelly.
In fact, theophylline beat that
result slightly, with a relative risk of 0.49, but the
data supporting this finding was much weaker, and the
95% confidence intervals even allowed the possibility
of some increased kidney risk. The other drugs studied
— fenoldopam, dopamine, iloprost, statin, furosemide,
and mannitol — mostly had no effect, except furosemide,
which was found to sharply increase danger to the kidney,
with a relative risk of 3.27.
CHEAP
AND HANDY
None of the findings in this study apply to contrast
mediums that don't contain iodine, like barium enemas,
or the gadolinium contrast agents used with MRI scan,
typically in multiple sclerosis. A few centres offer
low-iodine or no-iodine contrast agents with CT scan,
but it's expensive and generally reserved for those
with known kidney problems.
N-acetylcysteine is generally
used as a mucolytic, and also as a treatment for acetaminophen
overdose. Any facility sophisticated enough to offer
radio-contrast x-rays should have some handy. It can
be had as a supplement, boasting clinical effects similar
to mugwort, but these formulations are useless for kidney
protection, the Michigan researchers warn.
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