MARCH 2008


Cheap drug shields kidneys from CT dye damage: study

N-acetylcysteine beats other meds at fighting common scan side effect

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.

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.

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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