Abstract
Background
The quality of bowel preparation significantly affects the outcome of colonoscopy. Adequate bowel cleansing improves diagnostic accuracy while reducing the need for repeat procedures. Currently, evidence regarding factors associated with inadequate bowel preparation in outpatients and inpatients remains limited. Therefore, this study aimed to compare the quality of bowel preparation between outpatients and inpatients undergoing colonoscopy and to identify risk factors for inadequate bowel preparation.
Methods
A total of 20,688 patients undergoing sedated colonoscopy were included in this study. Bowel preparation quality was assessed using the Boston Bowel Preparation Scale (BBPS). Propensity score matching (PSM) was used to adjust the baseline differences between the outpatients and inpatients, followed by univariate and multivariate logistic regression analyses to assess risk factors for inadequate bowel preparation. Subgroup analyses were conducted to analyze the differences in bowel preparation quality between outpatients and inpatients.
Results
After matching, outpatients had higher BBPS scores than inpatients (7.29 ± 1.06 vs. 6.97 ± 1.19, P < 0.001), and had a higher rate of adequate bowel preparation compared with inpatients (95.95% vs. 93.07%, P < 0.001). Univariate analysis showed that aged ≥ 65 years, ethnic minorities, inpatient status, diabetes, and a history of abdominal surgery were significantly related to inadequate bowel preparation (all P < 0.001). Multivariate analysis showed that aged ≥ 65 years (OR: 1.90; 95% CI: 1.52–2.39; P < 0.001), ethnic minorities (OR: 4.22; 95% CI: 2.91–6.12; P < 0.001), inpatient status (OR: 1.52; 95% CI: 1.23–1.89; P < 0.001), diabetes (OR: 1.86; 95% CI: 1.13–3.06; P < 0.001) and a history of abdominal surgery (OR: 1.55; 95% CI: 1.25–1.93; P < 0.001) were independent risk factors for inadequate bowel preparation. Subgroup analysis showed that rates of bowel preparation adequacy were significantly higher in outpatients than in inpatients.
Conclusions
Although bowel preparation quality was statistically lower in inpatients than in outpatients, the absolute difference was modest. Aged ≥ 65 years, ethnic minorities, inpatient status, diabetes, and a history of abdominal surgery were independent risk factors for inadequate bowel preparation. Clinical attention should be directed toward patients who present with a combination of these risk factors, which tend to occur more frequently in inpatient settings, to implement individualized preparation strategies.
Supplementary Information
The online version contains supplementary material available at 10.1186/s12876-026-04674-z.
Keywords: Colonoscopy, Bowel preparation quality, Boston bowel preparation scale, Propensity score matching, Outpatients, Inpatients
Introduction
Colorectal cancer (CRC) is one of the most prevalent malignant tumors, ranking among the top three cancers globally in terms of incidence and mortality [1]. Its increasing prevalence poses a significant public health burden, particularly as aging populations and lifestyle factors continue to change [2]. Colonoscopy is the most important means of screening, surveillance, or diagnostic colorectal diseases [3]. This procedure allows for direct visualization of the colon and rectum, enabling the detection of early-stage malignancies, precancerous lesions, and other abnormalities. Early detection through screening has been shown to significantly improve survival outcomes by allowing for the removal of precancerous polyps before progression to cancer [4]. International guidelines recommend starting colonoscopy-based CRC screening at age 50 for individuals at average risk [5, 6]. In China, national health guidelines also recommend incorporating colonoscopy into the routine health checkup for individuals aged 40 and above [7]. These guidelines aim to reduce the overall burden of CRC by identifying and removing polyps or tumors at an early, more treatable stage.
However, adequate bowel preparation is essential for successful implementation of colonoscopy in CRC screening [8]. The quality of bowel preparation is critical for ensuring optimal visualization of the colonic mucosa and enhancing lesion detection, both of which directly affect diagnostic accuracy and procedural safety [9]. In contrast, inadequate bowel preparation has been linked to missed lesions [10], prolonged procedure times [11], incomplete examinations, and the need for repeat colonoscopies [12]. Currently, 20–30% of colonoscopies still have inadequate bowel preparation [13, 14]. The degree of bowel cleanliness is significantly correlated with the detection rate of colorectal adenomas [11, 15]. A previous study has shown that inadequate bowel preparation can result in the ignoring of flat adenomas, which could be up to 27% due to limited visualization [16]. Failure to detect these lesions during endoscopy not only increases the risk of CRC development but also imposes additional psychological, physiological, and economic burdens on patients.
The quality of bowel preparation can be influenced by various factors, including patient-related physiological or psychological characteristics, the bowel preparation regimen, and the clinical environment [17–19]. Besides, inadequate bowel preparation is often attributed to poor patient adherence [20]. The bowel preparation process is relatively complex, requiring patients to follow dietary restrictions and adhere to the timing, method, and dosage of laxatives [11]. Traditionally, most studies have focused on single populations undergoing elective colonoscopy [21, 22]. In outpatient settings, patients generally have greater flexibility in following preparation instructions but often lack the direct supervision and assistance compared to inpatient settings [23]. By contrast, inpatients may receive more medical care but are accompanied by more complex comorbidities, more concomitant medications, and more risk factors [24, 25].
Despite advances in bowel preparation regimens, the differences in bowel preparation quality between inpatients and outpatients have not been thoroughly investigated. Understanding these differences is crucial for developing targeted interventions to improve preparation quality and enhance diagnostic accuracy. Therefore, this study aims to assess the current status and differences in bowel preparation quality between outpatients and inpatients undergoing screening, surveillance, or diagnostic colonoscopy.
Methods
Data source and sample
This study retrospectively analyzed the data of patients who underwent colonoscopy at the Digestive Endoscopy Center of the Third People’s Hospital of Chengdu from January 2024 to February 2025. The inclusion criteria for the study were as follows: (1) patients who underwent a complete colonoscopy (which was defined as the arrival of the endoscope to the ileocecal region with thorough mucosal inspection during withdrawal) during the study period; (2) aged ≥ 18 years; (3) absence of severe cardiovascular or cerebrovascular diseases, such as advanced heart failure, acute myocardial infarction, or cerebrovascular accidents. Patients were excluded if they did not complete the full colonoscopy due to bowel lesions or inadequate bowel preparation. In our center, some inpatients undergo scheduled diagnostic or surveillance colonoscopy during hospitalization. Written informed consent was obtained from all patients before inclusion in the study. The study protocol was approved by the Ethics Committee of the Third People’s Hospital of Chengdu (No. 2024-S-324).
Bowel preparation methods
According to the Guidelines for Intestinal Preparation Related to Digestive Endoscopy Diagnosis and Treatment in China [26], all patients included in this study received a 2–3 L polyethylene glycol electrolyte solution (PEG-ELS) regimen for intestinal preparation. The day before the colonoscopy, patients were instructed to follow a low-residue diet (e.g., noodles, cake, and milk) while avoiding meat, high-fiber vegetables, and seeded fruits. The details of the bowel preparation regimen can be found in Supplementary Table 1. The patient would continue to take 20 mL of Dimethicone Powder after taking PEG-ELS. They were also encouraged to walk around and gently massage the abdomen in a clockwise direction to promote intestinal motility. Bowel preparation was considered adequate when the patient passed stool that was light yellow or clear and transparent.
All colonoscopies were performed by endoscopists who were certified with more than 3 years of experience and had completed at least 500 colonoscopies (Supplementary Table 2). High-resolution adult endoscopes (FUJIFILM EC-760R/ZP-V/M or Olympus CF-H290I/Q260JI) were used in our study. Sedation was achieved using standard doses of propofol or ciprofol, with the dose adjusted by an anesthesiologist according to each patient’s individual health status to ensure optimal sedation and safety.
Bowel preparation quality assessment criteria
The primary outcome was bowel preparation quality, assessed by endoscopists using the Boston Bowel Preparation Scale (BBPS) in all patients undergoing screening, surveillance, or diagnostic colonoscopy. The BBPS is a commonly used tool for assessing the quality of bowel preparation for colonoscopy [27]. It assesses the cleanliness of three colonic segments: the right colon, the transverse colon, and the left colon, with each segment scored from 0 to 3. Bowel preparation quality was assessed after adequate cleansing of the colon during colonoscopy using methods such as suction and irrigation. Scores for each segment were summed to a total score ranging from 0 to 9, with higher scores indicating better bowel preparation. A total score of ≥ 6 was considered adequate bowel preparation, whereas a score of <6 was deemed inadequate bowel preparation [27].
Statistical analysis
Data were analyzed using SPSS 26.0 (IBM Corp., Armonk, NY, USA) and R version 4.2.0 (R Foundation for Statistical Computing, Vienna, Austria). Continuous variables (for normal distributions) were expressed as means and standard deviation (SD) and compared using independent sample t test. Categorical variables were expressed as percentages and compared using the Chi-square test or Fisher’s exact test. To adjust for confounding bias in the baseline characteristics between the groups, we used propensity score matching (PSM) [28] with a 1:1 matching ratio and the nearest-neighbor strategy with a caliper of 0.2. PSM was calculated by logistic regression models and estimated with the following variables: age, gender, nationality, bowel preparation, time for colonoscopy, season, chief complaint, comorbidity, and history of abdominal surgery. The selection of the above covariates was mainly based on the P-value of the baseline data and the advice of clinical experts. A standardized mean difference (SMD) of < 0.2 between the groups was considered a good balance of the matched cohort [29]. Univariate and multivariate logistic regression analyses based on PSM data were performed to identify independent predictors of inadequate bowel preparation. Variables demonstrating statistical significance (P < 0.05) in univariate analysis were included in the multivariate model. Results are expressed as odds ratios (OR) with corresponding 95% confidence intervals (CI). A P-value < 0.05 was considered statistically significant. In subgroup analyses, factors associated with bowel preparation quality were further compared between outpatients and inpatients.
Results
Participants characteristics
During the study period, a total of 21,881 patients completed colonoscopy. Among them, 60 patients were excluded due to age < 18 years, 156 for missing demographic data, 215 for incomplete colonoscopy, and 762 for missing BBPS data (Fig. 1). Finally, 20,688 patients were included for analysis in this study, including 14,803 outpatients and 5,885 inpatients (Table 1). Compared with outpatients, inpatients were associated with more cases of male, more colonoscopies in the afternoon, and more cases of hematochezia, abdominal pain or abdominal discomfort, and abdominal surgery history (all P < 0.05). After PSM, 3,759 matched pairs of outpatients and inpatients (n = 7,518) were identified. All variables of SMD were less than 0.2, showing a good balance between the groups.
Fig. 1.
Flowchart of patients included in the present study. Abbreviations: BBPS: Boston bowel preparation scale; PSM: Propensity score matching
Table 1.
Basic characteristics of outpatients and inpatients
| Variable | Before matching | After matching | ||||||
|---|---|---|---|---|---|---|---|---|
| Outpatients (n = 14803) |
Inpatients (n = 5885) |
P-value | SMD | Outpatients (n = 3759) |
Inpatients (n = 3759) |
P-value | SMD | |
| Age, mean ± SD | 48.39 ± 14.21 | 58.86 ± 13.69 | <0.001 | 0.765 | 53.80 ± 12.94 | 54.66 ± 13.73 | 0.004 | 0.063 |
| Gender, n (%) | <0.001 | -0.056 | 0.711 | -0.009 | ||||
| Male | 6588(44.50) | 2784(47.31) | 1704(45.33) | 1720(45.76) | ||||
| Female | 8215(55.50) | 3101(52.69) | 2055(54.67) | 2039(54.24) | ||||
| Nationality, n (%) | <0.001 | 0.159 | 0.058 | 0.038 | ||||
| Han nationality | 14,612(98.71) | 5612(95.36) | 3644(96.94) | 3614(96.14) | ||||
| ethnic minority | 191(1.29) | 273(4.64) | 115(3.06) | 145(3.86) | ||||
| Dietary adherence, n (%) | 11,910(80.46) | 4802(81.60) | 0.06 | 0.029 | 3090(82.20) | 3048(81.08) | 0.211 | -0.029 |
| Medication adherence, n (%) | 13,645(92.18) | 5436(92.37) | 0.64 | 0.007 | 3480(92.58) | 3464(92.15) | 0.487 | -0.016 |
| Bowel preparation, n (%) | <0.001 | -0.110 | 0.049 | -0.045 | ||||
| 2 L PEG-ELS | 5310(35.87) | 2429(41.27) | 1409(37.48) | 1492(39.69) | ||||
| 3 L PEG-ELS | 9493(64.13) | 3456(58.73) | 2350(62.52) | 2267(60.31) | ||||
| Time for colonoscopy, n (%) | <0.001 | -1.506 | 0.414 | -0.019 | ||||
| Morning | 11,282(76.21) | 955(16.23) | 986(26.23) | 955(25.41) | ||||
| Afternoon | 3521(23.79) | 4930(83.77) | 2773(73.77) | 2804(74.59) | ||||
| Season, n (%) | ||||||||
| Spring | 3380(22.83) | 1534(26.07) | <0.001 | 0.075 | 951(25.30) | 870(23.14) | 0.029 | -0.050 |
| Summer | 3568(24.10) | 1435(24.38) | 0.670 | 0.007 | 802(21.33) | 896(23.84) | 0.010 | 0.060 |
| Autumn | 3395(22.93) | 1509(25.64) | <0.001 | 0.063 | 808(21.50) | 975(25.94) | <0.001 | 0.104 |
| Winter | 4460(30.13) | 1407(23.91) | <0.001 | -0.140 | 1198(31.87) | 1018(27.08) | <0.001 | -0.105 |
| Chief complaint, n (%) | ||||||||
| Constipation | 621(4.2) | 240(4.08) | 0.704 | -0.006 | 165(4.39) | 132(3.51) | 0.051 | -0.045 |
| Hematochezia | 613(4.14) | 357(6.07) | <0.001 | 0.090 | 183(4.87) | 222(5.91) | 0.046 | 0.046 |
| Abdominal discomfort | 1047(7.07) | 482(8.19) | 0.006 | 0.041 | 274(7.29) | 306(8.14) | 0.167 | 0.032 |
| Abdominal pain | 2030(13.71) | 1082(18.39) | <0.001 | 0.128 | 481(12.79) | 733(19.50) | <0.001 | 0.183 |
| Diarrhea | 1237(8.36) | 273(4.64) | < 0.001 | -0.151 | 197(5.24) | 189(5.03) | 0.676 | -0.010 |
| Health examination | 4611(31.15) | 1222(20.76) | <0.001 | -0.239 | 1107(29.45) | 892(23.73) | <0.001 | -0.129 |
| Others | 4644(31.37) | 2229(37.88) | <0.001 | 0.137 | 1352(35.97) | 1285(34.18) | 0.105 | -0.038 |
| Comorbidity, n (%) | ||||||||
| Hypertension | 1083(7.32) | 834(14.17) | <0.001 | 0.223 | 376(10.00) | 400(10.64) | 0.363 | 0.021 |
| Diabetes | 233(1.57) | 197(3.35) | <0.001 | 0.115 | 89(2.37) | 91(2.42) | 0.880 | 0.003 |
| Others | 60(0.41) | 39(0.66) | <0.001 | 0.036 | 20(0.53) | 14(0.37) | 0.302 | -0.024 |
| History of abdominal surgery, n (%) | 4564(30.83) | 2347(39.88) | <0.001 | 0.185 | 1433(38.12) | 1405(37.38) | 0.505 | -0.015 |
Abbreviations: PEG-ELS Polyethylene glycol electrolyte solution, SMD Standardized mean difference
Colonoscopy results of outpatient and inpatient patients
For the primary outcome, outpatients had higher BBPS scores than inpatients (7.29 ± 1.06 vs. 6.97 ± 1.19, P < 0.001), and had a higher rate of adequate bowel preparation than inpatients (95.95% vs. 93.07%, P < 0.001) (Table 2). For secondary outcomes, outpatients were associated with a lower polyp detection rate (45.65% vs. 60.41%, P < 0.001), less cecal intubation time (5.04 ± 4.16 min vs. 5.66 ± 4.87 min, P < 0.001), less withdrawal time (7.86 ± 5.43 min vs. 9.89 ± 7.36 min, P < 0.001), and less total colonoscopy examination time (12.09 ± 6.97 min vs. 15.55 ± 8.98 min, P < 0.001) compared with inpatients. The results of PSM cohort did not differ from the results before matching, except for cecal intubation time (P > 0.05). The cecal intubation rate was 100% in both groups.
Table 2.
Effect of different sources on patient bowel Preparation and colonoscopy results
| Outcomes | Before matching | After matching | ||||
|---|---|---|---|---|---|---|
| Outpatients (n = 14803) |
Inpatients (n = 5885) |
P-value | Outpatients (n = 3759) |
Inpatients (n = 3759) |
P-value | |
| Total BBPS score, mean ± SD | 7.29 ± 1.06 | 6.97 ± 1.19 | < 0.001 | 7.27 ± 1.06 | 7.04 ± 1.14 | < 0.001 |
| Adequate bowel preparation, n (%) | 14,204(95.95%) | 5477(93.07%) | < 0.001 | 3614(96.14%) | 3533(93.99%) | < 0.001 |
| Polyp detected rate, n (%) | 6757(45.65%) | 3555(60.41%) | < 0.001 | 1974(52.51%) | 2144(57.04%) | < 0.001 |
| Successful cecal intubation, n (%) | 14,803(100%) | 5885(100%) | - | 3759(100%) | 3759(100%) | - |
| Cecal intubation time (min), mean ± SD | 5.04 ± 4.16 | 5.66 ± 4.87 | < 0.001 | 5.32 ± 4.46 | 5.51 ± 4.77 | 0.062 |
| Withdrawal time (min), mean ± SD | 7.86 ± 5.43 | 9.89 ± 7.36 | < 0.001 | 8.37 ± 5.79 | 9.62 ± 7.20 | < 0.001 |
| Colonoscopy time (min), mean ± SD | 12.09 ± 6.97 | 15.55 ± 8.98 | < 0.001 | 13.70 ± 7.36 | 15.13 ± 8.72 | < 0.001 |
Abbreviations: SD standard deviation, BBPS Boston bowel preparation scale
Univariate analysis: risk factors for inadequate bowel preparation
Univariate analysis was performed to compare inter-group differences between patients with adequate and inadequate bowel preparation (Table 3). Aged ≥ 65 years, ethnic minorities, inpatient status, diabetes, and a history of abdominal surgery were significantly related to inadequate bowel preparation (P < 0.001) except for dietary adherence, season, colonoscopy in the afternoon, constipation, hematochezia, abdominal discomfort, abdominal pain, hypertension, and other chronic diseases (P > 0.05). In addition, female, medication adherence, and the 3 L PEG regimen were significantly associated with adequate bowel preparation (P < 0.001).
Table 3.
Univariable analysis for inadequate bowel Preparation as the primary outcome
| Variables | β | S.E | OR (95%CI) | P-value |
|---|---|---|---|---|
| Age | 0.02 | 0.00 | 1.02 (1.01–1.03) | < 0.001 |
| Age | ||||
| <65 | 1.00 (Reference) | |||
| ≥ 65 | 0.67 | 0.11 | 1.96 (1.58–2.44) | < 0.001 |
| Gender | ||||
| Male | 1.00 (Reference) | |||
| Female | -0.48 | 0.11 | 0.62 (0.50–0.76) | < 0.001 |
| Nationality | ||||
| Han nationality | 1.00 (Reference) | |||
| Ethnic minority | 1.30 | 0.18 | 3.68 (2.57–5.26) | < 0.001 |
| Dietary adherence | -0.07 | 0.13 | 0.93 (0.71–1.21) | 0.592 |
| Medication adherence | -0.52 | 0.17 | 0.60 (0.43–0.83) | 0.002 |
| Bowel preparation | ||||
| 2 L PEG-ELS | 1.00 (Reference) | |||
| 3 L PEG-ELS | -0.84 | 0.11 | 0.43 (0.35–0.53) | < 0.001 |
| Time for colonoscopy | ||||
| Morning | 1.00 (Reference) | |||
| Afternoon | -0.09 | 0.12 | 0.91 (0.72–1.15) | 0.450 |
| Season | ||||
| Spring | 1.00 (Reference) | |||
| Summer | -0.00 | 0.16 | 1.00 (0.73–1.36) | 0.992 |
| Autumn | 0.11 | 0.15 | 1.12 (0.83–1.51) | 0.451 |
| Winter | 0.02 | 0.15 | 1.02 (0.76–1.36) | 0.887 |
| Hospitalization | ||||
| Outpatient | 1.00 (Reference) | |||
| Inpatient | 0.47 | 0.11 | 1.59 (1.29–1.97) | < 0.001 |
| Chief complaint | ||||
| Constipation | 0.35 | 0.24 | 1.42 (0.88–2.29) | 0.148 |
| Hematochezia | 0.07 | 0.24 | 1.07 (0.67–1.71) | 0.766 |
| Abdominal discomfort | -0.22 | 0.23 | 0.81 (0.52–1.26) | 0.342 |
| Abdominal pain | -0.03 | 0.16 | 0.97 (0.71–1.33) | 0.854 |
| Diarrhea | -0.09 | 0.26 | 0.91 (0.55–1.51) | 0.725 |
| Health examination | -0.10 | 0.14 | 0.90 (0.69–1.18) | 0.453 |
| Comorbidity | ||||
| Hypertension | 0.17 | 0.17 | 1.19 (0.86–1.65) | 0.302 |
| Diabetes | 0.93 | 0.24 | 2.53 (1.57–4.09) | < 0.001 |
| Others | -0.49 | 1.02 | 0.61 (0.08–4.49) | 0.631 |
| History of abdominal surgery | 0.42 | 0.11 | 1.52 (1.23–1.87) | < 0.001 |
Abbreviations: PEG-ELS Polyethylene glycol electrolyte solution, OR Odds ratios, CI Confidence intervals
Multivariate analysis: risk factors for inadequate bowel preparation
Variables with P < 0.05 in univariate analysis were further included in the multivariate logistic regression model. The results showed that aged ≥ 65 years (OR: 1.90; 95% 1.52–2.39; P < 0.001), ethnic minorities (OR: 4.22; 95% CI: 2.91–6.12; P < 0.001), inpatient status (OR: 1.52; 95% CI: 1.23–1.89; P < 0.001), diabetes (OR: 1.86; 95% CI: 1.13–3.06; P < 0.001) and a history of abdominal surgery (OR: 1.55; 95% CI: 1.25–1.93; P < 0.001) were independent risk factors for inadequate bowel preparation (Table 4).
Table 4.
Multivariate logistic regression analysis for independent risk factors for inadequate bowel Preparation
| Risk factor | β | S.E | OR (95%CI) | P- value |
|---|---|---|---|---|
| Female | -0.52 | 0.11 | 0.59 (0.48–0.74) | < 0.001 |
| Age(≥ 65) | 0.64 | 0.12 | 1.90 (1.52–2.39) | < 0.001 |
| Ethnic minority | 1.44 | 0.19 | 4.22 (2.91–6.12) | < 0.001 |
| Medication adherence | -0.59 | 0.17 | 0.56 (0.40–0.78) | < 0.001 |
| 3 L PEG-ELS | -0.82 | 0.11 | 0.44 (0.36–0.55) | < 0.001 |
| Hospitalization (Inpatient) | 0.42 | 0.11 | 1.52 (1.23–1.89) | < 0.001 |
| Diabetes | 0.62 | 0.25 | 1.86 (1.13–3.06) | 0.014 |
| History of abdominal surgery | 0.44 | 0.11 | 1.55 (1.25–1.93) | < 0.001 |
Abbreviations: PEG-ELS Polyethylene glycol electrolyte solution, OR Odds ratios, CI Confidence intervals
Subgroup analysis
The results of the subgroup analyses were presented in Fig. 2. Overall, inpatients had a significantly higher risk of inadequate bowel preparation compared with outpatients (OR: 1.59; 95% CI: 1.29–1.97; P < 0.001). In most subgroups, the rate of inadequate bowel preparation was significantly higher in inpatients than in outpatients, except for cases of poor medication adherence. Notably, bowel preparation quality among inpatients warranted particular attention, especially those who were male (OR: 1.71; 95% CI: 1.28–2.28; P < 0.001), 2 L PEG regimen (OR: 2.07; 95% CI: 1.55–2.79; P < 0.001), colonoscopy in the afternoon (OR: 1.60; 95% CI: 1.25–2.06; P < 0.001), dietary adherence (OR: 1.76; 95% CI: 1.39–2.24; P < 0.001), medication adherence (OR: 1.71; 95% CI: 1.36–2.15; P < 0.001), or with a history of abdominal surgery (OR: 1.90; 95% CI: 1.38–2.61; P < 0.001).
Fig. 2.
Subgroup analysis of factors associated with the rate of inadequate bowel preparation. Abbreviations: PEG-ELS: Polyethylene glycol electrolyte solution; OR: Odds ratios; CI: Confidence intervals
Discussion
This retrospective study investigated risk factors associated with bowel preparation quality among outpatients and inpatients, revealing significant differences between these two populations. This finding demonstrated that inpatients were at a substantially higher risk of inadequate bowel preparation than outpatients. Aged ≥ 65 years, ethnic minorities, inpatient status, diabetes, and a history of abdominal surgery were independent risk factors of inadequate bowel preparation.
This study found that the quality of bowel preparation was significantly lower in inpatients than in outpatients, which was consistent with previous studies [23–25]. This suggests that the clinical setting itself (inpatient vs. outpatient) may serve as an independent predictor of inadequate bowel preparation, potentially mediated through environmental and operational differences in care delivery. Previous studies have confirmed that there was a nearly two-fold increase in the risk of unsuccessful bowel preparation among inpatients compared with outpatients, and less than 50% of inpatients had adequate bowel preparation [24, 30]. In our study, the rate of adequate bowel preparation was higher in outpatients (95.95%) than in inpatients (93.07%), with the statistically significant difference (P < 0.05), which may be attributed to the poor health condition and more comorbidities of inpatients. Inpatients tend to have more severe or complex conditions, and comorbidities such as diabetes and chronic kidney disease may lead to decreased gastrointestinal motility and affect the absorption of laxatives [22]. Chronic conditions like hypertension and diabetes likely contribute to the failure of bowel preparation through autonomic neuropathy or medication effects [31]. Our findings differ from the perspective of Rotondano et al. [21], who observed no statistically significant difference in bowel preparation quality between the right colon and left colon both in the inpatients and outpatients. This phenomenon may be attributed to differences in bowel preparation regimens and study populations. It is noteworthy that although inpatients had a slightly lower rate of adequate bowel preparation than outpatients, the rate exceeded 90% in both groups. This observation suggests that both outpatients and inpatients can generally achieve high-quality preparation and benefit from current preparation protocols and health education systems. However, although this difference reached statistical significance, the absolute magnitude was small (approximately 2%-3%), which is likely attributable in part to the large sample size, and its clinical impact at the individual patient level may therefore be limited. Interestingly, inpatients also had higher polyp detection rates (60.41% vs. 45.65%) despite lower preparation quality, which may be related to differences in patient characteristics, such as older age or specific diagnostic indications rather than superior examination quality, highlighting that detection rates are influenced by multiple factors beyond bowel preparation alone [32, 33].
The strong association between advanced age and inadequate bowel preparation is consistent with previous literature linking aging to delayed colonic transit and reduced adherence to preparation protocols [34, 35]. A greater proportion of elderly patients in the inpatient setting results in greater vulnerability to age-related decreases in colonic motility [36]. Furthermore, sensory impairments and cognitive decline in older adults may compromise their ability to comprehend and execute complex preparation protocols, particularly when combined with polypharmacy-induced gastrointestinal dysregulation commonly observed in geriatric inpatients [36]. Generally speaking, older adults represent a high-risk group for chronic diseases, functional disability, and other adverse factors [37]. This vulnerability is exacerbated by the high prevalence of comorbidities in geriatric inpatients. It has been confirmed that diabetes, walking difficulties, or reduced activities of daily living are related to inadequate bowel preparation in patients over 65 years old [38]. Additionally, inadequate bowel preparation is the main reason for the failure of colonoscopy in the elderly population aged 90 and above [39]. Amitay et al. found that age ≥ 70 years, smoking, and abdominal symptoms were significantly associated with inadequate bowel preparation, further confirming this finding [40].
Bowel preparation regimen is a key factor affecting colonoscopy quality [41]. Studies have shown that compared with single-dose administration, split-dose bowel preparation regimens can improve both bowel preparation quality and patient medication adherence [42]. Previous studies at our center have demonstrated that the quality of bowel preparation in patients with split dosing was superior to that of single dosing, along with greater patient acceptance and a lower incidence of adverse effects [43]. These findings are consistent with those reported by Avalos DJ et al. [44]. In this study, patients undergoing morning colonoscopy received the split-dose regimen, while most afternoon colonoscopy patients were treated with a single-dose regimen. The results showed that patients examined in the morning had better bowel preparation quality, aligning with the findings of the previous studies [45, 46]. Notably, this study observed an amplified disadvantage in patients with a history of abdominal surgery. This implies that changes in previous abdominal anatomy may exacerbate the difficulty of bowel preparation, consistent with the previous report [47]. This subgroup’s vulnerability underscores the need for tailored bowel cleansing protocols, possibly incorporating extended preparation durations or prokinetic adjuvants. Additionally, this study found that ethnic minority patients exhibited a disproportionately high risk of inadequate bowel preparation. This finding may reflect barriers to effective patient education caused by cultural, linguistic, or socioeconomic factors and warrants further investigation. Furthermore, the observed differences in gender also highlight the importance of personalized approaches. For instance, male inpatients had a significantly higher risk compared to females, possibly due to differences in health-seeking behaviors or physiological responses to laxatives [47, 48].
While the findings highlight the healthcare setting as a potentially modifiable factor, the following limitations should be acknowledged. First, the single-center design of this study may limit its generalizability. Second, the Boston Bowel Preparation Scale, while validated, relies on subjective endoscopic assessments, which may vary among practitioners. Third, the study did not adequately consider the possibility that laxatives during bowel preparation may cause patients to experience adverse effects such as nausea and vomiting, which could in turn affect bowel preparation quality. While PSM was applied, residual confounding including illness severity, functional status, mobility, polypharmacy, and nursing assistance, were not accounted for in this study and may have impacted the observed outcome. Future studies should incorporate objective metrics and prospective designs to validate these findings.
Conclusion
Inpatients had significantly poorer bowel preparation than outpatients. Aged ≥ 65 years, ethnic minorities, inpatient status, diabetes, and a history of abdominal surgery were independent risk factors for inadequate bowel preparation, highlighting the need for targeted strategies and enhanced education, especially for high-risk inpatients.
Supplementary Information
Acknowledgements
None.
Authors’ contributions
XBS and ZYC conceived and designed the study. CD, YL, JLL, TXC, PZ, QD and JR performed all statistical analyses and interpreted the data. LJ, MW, ZYC and XBS revised all parts of the manuscript. All authors contributed to the article and approved the submitted version.
Funding
This study was funded by the Scientific Research Special Project of Sichuan Association of Integrated Traditional Chinese and Western Medicine (ZXY2025001), the General Project of Sichuan Provincial Administration of Traditional Chinese Medicine (25MSZX221) and the Sichuan Science and Technology Program (2025ZNSFSC1800).
Data availability
The data for this study was obtained from the colonoscopy examination system of the Department of Gastroenterology at the Third People’s Hospital of Chengdu, but the availability of these data is limited. However, information may be obtained from the corresponding author on reasonable request.
Declarations
Ethics approval and consent to participate
This study has been approved by the Ethics Review Committee of the Third People’s Hospital of Chengdu. Informed consent was obtained from all participants.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Lin Jiang and Man Wu They are co-first authors and contribute equally to this article.
Contributor Information
Zhiyao Chen, Email: zhiyao_chen@yeah.net.
Xiaobin Sun, Email: xbsun1197@163.com.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
The data for this study was obtained from the colonoscopy examination system of the Department of Gastroenterology at the Third People’s Hospital of Chengdu, but the availability of these data is limited. However, information may be obtained from the corresponding author on reasonable request.