According to this study:
"Oral saline hydration was as efficient as intravenous saline hydration for the prevention of contrast nephropathy in patients with stage 3 renal diseases."
Researchers prospectively studied 312 patients with chronic renal failure who were undergoing radiological procedures with a non-ionic, low osmolality IV contrast. In arm A of the study, patients received 1 g/10 kg of body weight/day of sodium chloride PO for 2 days before the procedure. In arm B, patients received 0.9% saline IV at a rate of 15 ml/kg for 6 hr before the procedure.
A randomized trial of saline hydration to prevent contrast nephropathy in chronic renal failure patients. Nephrology Dialysis Transplantation 2006 21(8):2120-2126; doi:10.1093/ndt/gfl133.
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