Oral phosphate purgative is the preferred bowel regimen on the basis of better tolerability, cost effectiveness, and efficacy. There are also numerous reports of patients with even previously normal renal function developing acute and chronic kidney disease after use of oral phosphate purgative. Several uncontrolled case reports and case series suggest a potential link between oral phosphate and acute kidney injury and/or chronic kidney disease1. Its use is contraindicated in patients with preexisting renal disease because of the risk for developing acute renal failure, so called acute phosphate nephropathy, or electrolyte disturbance. Since most of the outpatients who are going to undergo a colonoscopy are exposed to this agent, it is important to detect or prevent vulnerable patients. We would seek a sensitive and rapid diagnosis method of acute kidney injury following sodium phosphate bowel preparation. Within a few hours, NGAL mRNA is highly upregulated after kidney injury, such as renal ischemia-reperfusion and cisplatin nephropathy, NGAL induction precedes the elevation of classical markers for kidney damage such as serum creatinine. The investigators will investigate the change of NGAL following sodium phosphate bowel preparation.