Consecutive asymptomatic individuals who underwent both screening colonoscopy and abdominal ultrasonography as part of their medical checkup between July 2010 and April 2011 at the Health Promotion Center of Kyung Hee University Hospital in Gang Dong, Seoul, Korea were recruited. In Korea, healthy individuals undergo medical screening programs for early detection of diseases. Participants in our program undergo a variety of examinations including a physical examination, chest radiography, electrocardiography, blood laboratory tests, urine analysis, abdominal ultrasonography, upper endoscopy, and colonoscopy. Exclusion criteria in the present study were age <30 or ≥75 years, incomplete colonoscopy due to poor bowel preparation or failure to achieve cecal intubation, personal history of colonic neoplasia or inflammatory bowel disease, colorectal surgery or cholecystectomy, polyposis syndromes or hereditary non-polyposis colorectal cancer, poor general condition of worse than American Society of Anesthesiologists grade III, symptoms or signs indicating the need for a colonoscopy (such as abdominal pain, diarrhea, hematochezia, anal bleeding, or positive results on a fecal occult blood test), and inability to provide informed consent. To avoid potential interfering effects of medications, individuals who had been regularly taking NSAIDs, aspirins, or statins for more than 1 year were also excluded from the analysis.

All participants were interviewed by well-trained nurses regarding their smoking habits; diabetes mellitus (DM) and hypertension status; family history of CRC in first-degree relatives; history of regular use of aspirins, NSAIDs, or statins; and history of colorectal surgery or cholecystectomy. Current smoking was defined as the usage of at least one pack per week for more than 12 months. Regular use of medications was defined as taking such medications for more than 12 months. Hypertension was defined as a blood pressure of ≥140 mmHg or the use of anti-hypertension medication. DM was defined as a fasting glucose level of ≥126 mg/dL or the use of insulin or hypoglycemic agents. Height and body weight, used to calculate BMI, were measured by trained nurses. Informed consent was obtained from each participant. The present study was conducted according to the guidelines outlined in the Declaration of Helsinki and was approved by the institutional review board of Kyung Hee University Hospital at Gang Dong (KHNMC IRB 2010-053).

All participants underwent dietary restriction for 3 days and a bowel preparation process by drinking 4 L of polyethylene glycol solution during the 12 hours prior to colonoscopy until clear rectal fluid was evacuated. Signs and symptoms of potential complications of colonoscopy were explained to each participant, and all participants provided verbal and written consent prior to the procedure. For conscious sedative colonoscopy, an individualized dose of midazolam and/or propofol was administered to each patient by a gastroenterologist according to his/her age, weight, and general condition. An antispasmodic (10 mg hyoscine methobromide, intravenously) was administered to participants with no contraindications for the drug. All colonoscopic examinations were performed by experienced gastroenterologists using a standard video colonoscope (EC-590ZW/L, Fujinon Inc., Saitama, Japan), by following a standardized protocol. All detected colorectal polyps were biopsied or polypectomized for histopathologic examination, except for multiple polyps in the rectosigmoid area that appeared to be hyperplastic polyps based on endoscopic features consistent with a hyperplastic histology such as small size, sessile shape, pale color, and a type 1 or 2 pit pattern.12 In addition, all polyps were photographed, and their characteristics such as size, number, and location were documented. The size of each polyp was estimated by comparing it with open biopsy forceps (Olympus FB-28U-1, Aomori Olympus Co., Ltd., Aomori, Japan); forceps of a larger size were used for multiple polyps. The proximal colon was defined as the colonic region from the cecum to the splenic flexure, and the distal colon, as the colonic region distal to the splenic flexure. Polyps were classified as single or multiple (≥2). All polyps, pathologically evaluated by two gastrointestinal pathologists, were classified in accordance with the World Health Organization criteria.13 A colorectal adenoma was defined as one in the colon and rectum regardless of its grade or villous component. An advanced adenoma was defined as one with a diameter of ≥10 mm, an adenoma with high-grade dysplasia, or an adenoma with villous features in >25% of its area.

Abdominal ultrasonography after fasting was performed by two certified radiologists as a routine screening procedure using one of three ultrasound units (iU22 xMATRIX, Philips Electronics Ltd., Amsterdam, the Netherlands). The liver, GB, bile duct, pancreas, spleen, and kidneys were routinely examined. The diagnostic criterion for GB polyps was a hyperechoic immobile echo protruding from the GB wall into the lumen without an acoustic shadow, regardless of its histology. The radiologists and colonoscopists were blinded to the results of the examinations of the other group.

The primary endpoint was the prevalence of colorectal adenoma according to GB polyp status. Secondary outcomes included characteristics of detected lesions (number, multiplicity, location, and size) in both groups, with or without GB polyps. All data are presented as mean±SD for continuous variables and as number (percentage) of participants for categorical variables. For intergroup comparisons, continuous variables were analyzed using the Student's t test and categorical variables were analyzed using the χ² test or Fisher's exact test, as appropriate. Logistic regression analysis was performed to control for confounding variables and to identify independent risk factors for colorectal adenoma. Statistical adjustments were applied for potentially relevant variables that differed between the two groups in univariate analysis with a P value of <0.1. We computed the OR and 95% CI using logistic regression analysis. A P value of <0.05 was considered statistically significant, and a P value of <0.1 was considered indicative of a statistical trend. All statistical analyses were performed using Statistical Package for the Social Sciences (SPSS) software version 13.0 for Windows (SPSS Inc., Chicago, IL, USA).

As expected, participants of old age, male gender, and a high BMI were more likely to be in the colorectal adenoma group than the non-colorectal adenoma group (Table 1). Furthermore, those with colorectal adenomas tended to showed a trend toward a higher prevalence of GB polyps than those without colorectal adenomas (12.3% vs. 7.5%, P=0.051). However, there was no statistically significant difference in any other clinical or GB characteristics between the two groups.

The clinical and pathologic characteristics of the participants with colorectal adenoma according to the presence of GB polyps are described in Table 2. Participants with colorectal adenoma in the non-GB polyp group were older than those in the GB polyp group (P=0.039). There was no statistically significant difference in any other clinical characteristics between the two groups. Furthermore, the groups did not differ in characteristics of the detected colorectal adenoma either.

To determine independent predictors of colorectal adenoma, we performed logistic regression analysis after adjusting for age, gender, BMI, and GB polyp status, which were potentially relevant variables that differed between the two groups in univariate analysis with a P value of <0.1 (Table 3). In this analysis, age and gender, which are well-known risk factors for colorectal adenoma, were found to be independent risk factors of colorectal adenoma. However, GB polyps as a risk factor for colorectal adenoma only showed a statistical trend (OR=1.796, 95% CI=0.986-3.269, P=0.055).