Polyethylene glycol (PEG) solution is the most widely used bowel preparation agent for colonoscopy because of its safety and efficacy in colon cleansing. It has been hypothesized that the timing of colon preparation may influence the quality of colon cleansing, and therefore affect the diagnostic yield of colonoscopy. The aim of this study was to determine the optimal interval of time between complete ingestion of PEG and performing colonoscopy.

We prospectively enrolled 1,355 patients who had undergone a PEG-based bowel preparation on the day of colonoscopy in 11 tertiary hospitals in Korea between March 2008 and February 2009. All colonoscopies were performed in the afternoon. The start time of PEG ingestion, completion time of PEG ingestion, dose of PEG ingested, start time of colonoscopy, and the quality of bowel cleansing were recorded.

There was no difference of cleansing quality of bowel preparation between three groups of less than 7 hours of elapsed time (2≤ and <3 hours, 3≤ and <5 hours, 5≤ and <7 hours). However, group of more than 7 hours of elapsed time presented poor cleansing quality than others ( P <0.01).

If>7 hours elapses after ingestion of PEG, the quality of bowel preparation is poor. Therefore, depending on the time colonoscopy is scheduled, the start time of PEG ingestion may need to be adjusted.