The purpose of this study is to determine whether fluid administration in patients undergoing colonoscopy should affect acid-base disorder in term of the strong ion differences.

This is a prospective, double-blinded, randomized control trial. The study enrolls 90 consecutive outpatients, well-prepared bowel, scheduled to undergo routine colonoscopy for screening, surveillance, or diagnosis of colorectal diseases. At the outpatient clinic, the co-researcher invites patients who meet the inclusion criteria to join the study. The process of the project is explained to the interested patients in details before an informed consent is obtained and the first blood sample is conducted.

On the day of colonoscopy, all participants are randomized equally into three groups: Normal saline solution (NSS, n = 30) as control group, lactated Ringer's solution (LRS, n = 30) and acetated Ringer's solution (ARS, n = 30) as treatment groups. The second blood sample is obtained from patients via 20-gauge needle in either forearm immediately before an allocated intravenous (IV) fluid administered on the same site. The volume of fluid is calculated by means of Holliday and Segar formula.

The colonoscopy under total intravenous anesthesia is performed between 9:00 a.m. and 3:00 p.m. At the end of colonoscopy, the patients spontaneously wake up in the recovery room. After they gain conscious and all vital signs are stable; the intravenous fluid is off and the third blood sample is taken in the other forearm. After completion of the procedure, the patient is advised to follow the discharge instructions.

Inclusion Criteria:

• male or non-pregnant
• non-lactating female out-patient scheduled for routine colonoscopy
• over 18 years of age
• eligible for taking PEG or NaP for bowel preparation, and willing to sign informed consent.

Exclusion Criteria:

• insulin-dependant diabetes
• renal insufficiency (creatinine >2.0 mg⁄ dL)
• renal dialysis
• uncontrolled congestive heart failure (American Heart Association Classification III or IV congestive heart failure)
• unstable angina
• untreated cardiac arrhythmia
• ileus and⁄or acute obstruction or perforation
• ileostomy
• presence of a colostomy
• history of a partial colon resection
• active gastrointestinal bleeding.