As the duration of the interval between the time of last preparation-agent dose and the start of the colonoscopy is increasing, the quality of bowel preparation will be worse.

The aim of the present studies is to determine how the duration of the interval of the time of last preparation dose and the start of colonoscopy affects the quality of the bowel preparation and when the proper timing of the colonoscopy is after completion of last preparation regimen

Inclusion Criteria:

• Adult outpatients(18-85 years of age) who undergo colonoscopy for screening, cancer surveillance or with gastrointestinal symptoms, other symptoms

Exclusion Criteria:

• age under 18 years, pregnancy, breast feeding, prior history of surgical large bowel resection, severe renal failure (creatinine ≥ 3.0mg/dL (normal 0.8-1.4)), drug addiction or major psychiatric illness; allergy to PEG, refusal of consent to participate in the study.

Adult outpatients who undergo colonoscopy