Risk factors for inadequate bowel preparation before colonoscopy: A retrospective cohort study

Abstract

BACKGROUND

Bowel preparation is a critical step in colonoscopy and endoscopic surgery. Adequate and effective bowel cleansing significantly improves lesion detection rates while reducing operative risks and complications.

AIM

To investigate the current state of inadequate bowel preparation in patients undergoing colonoscopy, identify the contributing factors, and develop interventions.

METHODS

This study was designed as a retrospective cohort study. A convenience sampling method was used to select 484 patients who underwent colonoscopy at Beijing Chao-Yang Hospital, Capital Medical University, from October 2023 to October 2024. General patient data, disease-related variables, and the Boston bowel preparation scale were collected. Logistic regression analysis was conducted to identify the factors associated with inadequate bowel preparation.

RESULTS

Among the 484 patients, the rate of inadequate bowel preparation was 25.8% (125/484). Influential factors for poor bowel preparation included history of colorectal surgery [odds ratio (OR) = 5.814], low-residue diet 1 day prior (OR = 0.145), time interval from last dose to start of examination (OR = 1.447), total exercise time after medication (OR = 0.992), and total number of bowel movements after medication (OR = 0.900) (all P < 0.05).

CONCLUSION

This study highlights several modifiable and non-modifiable factors influencing bowel preparation, such as surgical history and behavioral adherence. The findings support implementing dietary adjustments, optimized laxative timing, physical activity guidance, and tailored strategies for high-risk patients to improve bowel cleansing and enhance the diagnostic accuracy of colonoscopy.

Key Words: Colonoscopy; Bowel preparation; Colorectal surgery; Low-residue diet; Exercise; Defecation frequency; Risk factors; Retrospective study

Core Tip: This retrospective study analyzed 484 patients undergoing colonoscopy and identified key factors influencing inadequate bowel preparation. History of colorectal surgery and a shorter time interval from the last dose to the start of examination were identified as risk factors, while a 1-day low-residue diet, longer total exercise time after medication, and greater defecation frequency after medication served as protective factors. These findings can help guide targeted interventions to improve bowel preparation quality and enhance diagnostic accuracy during colonoscopy.

INTRODUCTION

Colonoscopy is widely recognized as the gold standard and most effective screening method for detecting colonic malignancies, significantly contributing to the reduction in both the incidence and mortality of colorectal cancer[1]. The success of colonoscopy is closely tied to the quality of bowel preparation. However, inadequate bowel preparation remains a common issue, as highlighted in multiple studies across different populations[2-4]. Poor bowel preparation adversely impacts colonoscopy outcomes by reducing adenoma detection rates and cecal intubation success, increasing procedural duration, and elevating healthcare costs[5-7].

While previous studies have explored factors influencing bowel preparation, many focused on single variables such as dietary modifications or laxative timing and often lacked quantitative metrics or failed to account for interactions among clinical and behavioral factors. Furthermore, these findings have been inconsistent, and no consensus has been reached on optimal strategies for patients with specific risks[8-11].

Recent meta-analyses have helped clarify this landscape by identifying consistent risk factors, including comorbidities such as diabetes, constipation, and a history of abdominal surgery, as well as behavioral factors such as poor dietary adherence, insufficient physical activity, and incomplete laxative intake[2,12]. These findings support the need for individualized strategies, but further real-world studies are needed to clarify the relative impact of these factors.

Consequently, this study aimed to identify the influencing factors of inadequate bowel preparation, in order to identify early patients at risk of poor bowel preparation and implement appropriate interventions in advance, ensuring the effectiveness of subsequent colonoscopy and avoiding economic losses.

MATERIALS AND METHODS

Study design and participants

A total of 484 patients who underwent colonoscopy at the Digestive Endoscopy Center of Beijing Chao-Yang Hospital, Capital Medical University, between October 2023 and October 2024 were selected through convenience sampling as the study subjects.

Inclusion criteria: (1) Age 18-65 years; (2) Scheduled for colonoscopy; (3) Polyethylene glycol electrolytes powder (II) was used as the bowel preparation agent; (4) Signed and dated informed consent form; and (5) Commitment to comply with the study procedures and cooperate with implementation of the study.

Exclusion criteria: (1) Severe heart failure (New York class III or IV); (2) Recent acute myocardial infarction or unstable angina; (3) Uncontrolled hypertension; (4) Chronic kidney disease; (5) Suspected intestinal obstruction or perforation; (6) Scheduled for intestinal resection; (7) Gastroparesis; (8) Diarrhea > 3 times per day; (9) Decompensated liver cirrhosis or clinically significant ascites; (10) Pregnancy or nursing; (11) Mental illness with an inability to cooperate with the examination; and (12) Allergies to bowel preparation drugs.

Based on previous literature analysis and expert experience, 24 influencing factors were identified[9,12,13]. According to Kendall’s sample size estimation criterion for multifactor analysis, the required sample size should be at least 10 times the number of independent variables. Since this study included 24 independent variables, a minimum of 240 samples was necessary. To account for potential sample loss, the sample size was increased by at least 20%, resulting in a final calculation of: 24 × 10 × (1 + 20%) = 288 samples.

Survey tools and data collection methods

Based on a review of the literature and expert discussions, a custom-designed questionnaire was developed to assess bowel preparation in patients undergoing colonoscopy. The questionnaire includes the following components.

General patient information included: (1) Age; (2) Sex; (3) Body mass index (BMI); (4) Education level; (5) History of constipation, hypertension, diabetes, coronary heart disease, hyperlipidemia, cerebrovascular disease, hypothyroidism, or liver cirrhosis; (6) Use of psychotropic medication; (7) History of colorectal surgery or other abdominal/pelvic surgeries; and (8) Walking ability.

Intestinal-preparation-related information included: (1) Whether the patient followed a low-residue diet 1 day before the procedure; (2) Consumed high-fiber foods within 24 hours prior to the procedure; frequency of bowel movements after taking the preparation solution; (3) Use of anesthesia; (4) Whether this was their first colonoscopy; (5) Whether the sodium potassium magnesium sulfate solution was fully consumed; (6) Interval between the first bowel movement and the start of the procedure; (7) Time between the last dose and start of the procedure; and (8) Duration of exercise after taking the preparation solution.

Holden functional ambulation classification[14] was used to evaluate the patients’ walking ability, categorizing it into grades 0 to 5 based on the degree of independence in walking or need for assistance. Grades 0 to 3 indicated difficulty in walking, while grades 4 and 5 represented normal walking function.

Constipation was assessed using the Rome IV diagnostic criteria[15], which require the presence of at least two of the following in ≥ 25% of defecations: Straining, lumpy or hard stools (Bristol stool form scale types 1-2), sensation of incomplete evacuation, sensation of anorectal obstruction or blockage, use of manual maneuvers (e.g., digital evacuation, pelvic floor support), and fewer than three spontaneous bowel movements per week. Loose stools should rarely occur without laxative use, and criteria for irritable bowel syndrome must not be met. Symptoms should be present for the past 3 months, with onset at least 6 months before diagnosis. For research purposes, patients with opioid-induced constipation were not classified as having functional constipation due to diagnostic overlap. On the day of the colonoscopy, a researcher collected data through a one-on-one interview format, accurately recording the patients’ responses.

The quality of bowel preparation was assessed using the Boston bowel preparation scale, a validated scoring system developed by Lai et al[16]. The colon was divided into three segments, each scored from 0 to 3 based on cleanliness, with a total score ranging from 0 to 9. A total score ≥ 6, with each segment scoring ≥ 2, was defined as adequate preparation. The assessment was jointly performed by an experienced endoscopist and a senior endoscopy nurse.

Statistical analysis

The data were analyzed using SPSS 26.0 statistical software. Measurement data following a normal distribution were presented as mean ± SD and compared using the t test. Data not conforming to a normal distribution were expressed as the median (quartile) and analyzed using the rank-sum test. Categorical data were expressed as case numbers and percentages, with group comparisons conducted using the χ² test. Fisher’s exact test was applied when the expected case number was < 5.

For multivariate analysis, binary logistic regression was performed with bowel preparation adequacy as the dependent variable (poor bowel preparation = 1, successful bowel preparation = 0). Independent variables included: (1) Demographic characteristics (such as age, sex, BMI); (2) Comorbidities (such as hypertension, diabetes, coronary heart disease, hyperlipidemia, liver cirrhosis) coded as 0 = absent, 1 = present; (3) Surgical history (colorectal surgery = 1, no = 0; other abdominal surgery = 1, no = 0); and (4) Preparation-related factors (low-residue diet adherence = 1, non-adherence = 0; exercise duration after medication in minutes; number of bowel movements after medication; time interval from last dose to examination in hours). Variables with P < 0.05 in univariate analysis were included in a backward stepwise (likelihood ratio) multivariate logistic regression model to identify independent factors influencing bowel preparation quality. Results were expressed as adjusted odds ratios (ORs) with 95% confidence intervals, where OR > 1 indicates increased risk of inadequate preparation and OR < 1 indicates protective effect. Model fit was assessed using the Hosmer-Lemeshow goodness-of-fit test (P > 0.05 indicating adequate fit). P < 0.05 was considered statistically significant.

RESULTS

Baseline characteristics

A total of 484 patients were included in this study, comprising 359 in the successful bowel preparation group and 125 in the poor bowel preparation group. Table 1 presents the baseline characteristics of both groups, including general demographic information, medical history, and surgical history.

Table 1 Univariate analysis of patients’ basic data (n = 484), mean ± SD/n (%).

	Successful bowel preparation (n = 359)	Poor bowel preparation (n = 125)	t/χ2 value	P value
Age	50.23 ± 1.27	53.77 ± 1.19	-1.558	0.120
BMI (kg/m2)	24.53 ± 0.23	24.85 ± 0.28	-0.744	0.458
Gender			7.767	0.005a
Male	175 (48.7)	79 (63.2)
Female	184 (51.3)	46 (36.8)
Educational level			5.174	0.395
Primary school or below	9 (2.5)	2 (1.6)
Middle school	55 (15.3)	21 (16.8)
High school/vocational school	59 (16.4)	31 (24.8)
Associate degree	51 (14.2)	16 (12.8)
Bachelor’s degree	140 (39.0)	41 (32.8)
Graduate degree or above	45 (12.5)	14 (11.2)
Constipation	81 (22.6)	28 (22.4)	0.001	0.970
Hypertension	84 (23.4)	41 (32.8)	4.278	0.039a
Diabetes mellitus	39 (10.9)	29 (23.2)	11.685	< 0.001a
Coronary artery disease	29 (8.1)	25 (20.0)	13.295	< 0.001a
Hyperlipidemia	70 (19.5)	44 (35.2)	12.695	< 0.001a
Cerebrovascular disease	10 (2.8)	7 (5.6)	2.167	0.141
Hypothyroidism	6 (1.7)	6 (4.8)	3.754	0.053
History of psychiatric medication use	5 (1.4)	3 (2.4)	0.579	0.431
Liver cirrhosis	0 (0)	3 (2.4)	8.670	0.017a
History of colorectal surgery	10 (2.8)	16 (12.8)	18.292	< 0.001a
History of abdominopelvic surgery (excluding colorectal surgery)	76 (21.2)	22 (17.6)	0.732	0.392
Difficulty walking	7 (1.9)	5 (4.0)	1.612	0.204

^aP < 0.05.

BMI: Body mass index.

Compared to the successful bowel preparation group, the poor preparation group had a significantly higher proportion of male patients (P < 0.05), while no significant differences were observed in age, BMI, or educational level.

Regarding comorbidities, patients in the poor preparation group had higher rates of hypertension, diabetes mellitus, coronary artery disease, and liver cirrhosis (P < 0.05). However, the prevalence of constipation, cerebrovascular disease, and other conditions showed no significant differences between the groups.

Additionally, a significantly higher number of patients in the poor preparation group had a history of colorectal surgery (P < 0.05), whereas other types of abdominopelvic surgeries not involving the colorectum did not differ significantly. Lastly, no significant difference was found between the groups in terms of difficulty walking.

Univariate analysis of factors related to poor bowel preparation in colonoscopy

The univariate analysis of factors associated with poor bowel preparation in colonoscopy patients is presented in Table 2. The poor bowel preparation group had significantly lower ratios in low-residue diet 1 day prior to the procedure and first-time colonoscopy, and shorter times in total exercise time after medication and total number of bowel movements after medication compared to the successful bowel preparation group. In the poor preparation group, the time interval from last dose to start of examination was significantly longer than in the successful preparation group. These differences were significant (all P < 0.05). In contrast, no significant differences were observed for other factors.

Table 2 Analysis of factors related to intestinal preparation in patients (n = 484), mean ± SD/n (%).

	Successful bowel preparation (n = 359)	Poor bowel preparation (n = 125)	t/χ2 value	P value
Anesthesia and surgery	305 (85.0)	98 (78.4)	2.862	0.091
Low-residue diet 1 day prior	286 (79.7)	40 (32.0)	95.805	< 0.001a
High-fiber food intake within 24 hours	86 (24.0)	47 (37.6)	8.662	0.003a
First-time colonoscopy	242 (67.4)	68 (54.4)	6.815	0.009a
Complete intake of sodium potassium magnesium sulfate	336 (93.6)	116 (92.8)	0.095	0.759
Time interval from first dose to first bowel movement (minutes)	59.54 ± 2.98	70.92 ± 7.23	-1.723	0.086
Time interval from last dose to start of examination (hours)	6.53 ± 0.10	8.24 ± 0.26	-6.192	< 0.001a
Total exercise time after medication (minutes)	87.76 ± 3.65	46.04 ± 5.80	5.502	< 0.001a
Total number of bowel movements after medication (times)	10.52 ± 0.19	9.09 ± 0.31	3.799	< 0.001a

^aP < 0.05.

Analysis of influencing factors for poor bowel preparation in colonoscopy

The multivariate logistic regression analysis results are presented in Table 3. As shown in the table, history of colorectal surgery (OR = 5.814) and time interval from last dose to start of examination (hours) (OR = 1.447) were identified as independent risk factors for inadequate bowel preparation, while low-residue diet 1 day prior (OR = 0.145), total exercise time after medication (minutes) (OR = 0.992), and total number of bowel movements after medication (times) (OR = 0.900) were identified as independent protective factors (all P < 0.05). The model showed good fit based on the Hosmer-Lemeshow test (P > 0.05).

Table 3 Logistic regression analysis of factors influencing inadequate bowel preparation for colonoscopy (n = 484).

Independent variable	β coefficient	SE	Wald χ2	OR	95%CI	P value
History of colorectal surgery	1.762	0.586	9.054	5.814	1.848-18.519	0.003
Low-residue diet 1 day prior	-1.928	0.287	45.215	0.145	0.083-0.255	< 0.001
Time interval from last dose to start of examination (hours)	0.369	0.074	25.189	1.447	1.252-1.669	< 0.001
Total exercise time after medication (minutes)	-0.008	0.003	8.962	0.992	0.986-0.997	0.003
Total number of bowel movements after medication (times)	-0.105	0.04	7.011	0.900	0.832-0.973	0.008

OR: Odds ratio; CI: Confidence interval.

DISCUSSION

This study identified several modifiable and non-modifiable factors influencing bowel preparation quality, providing important insights for clinical practice. Notably, behavioral factors such as dietary adherence, post-medication physical activity, and the time interval between laxative administration and the examination were independently associated with preparation adequacy, suggesting these should be emphasized in patient education programs. Furthermore, even among patients who adhered to standard bowel preparation protocols, those with a history of colorectal surgery remained at significantly higher risk of inadequate preparation, indicating that this population may require a more intensive regimen. The quality of bowel preparation is crucial for the accuracy of colonoscopy results. Previous studies have reported that the incidence of inadequate bowel preparation ranges from 15% to 40%[4,9,17,18]. Inadequate preparation increases the risk of missed colorectal cancer diagnoses, procedural complications, and the need for repeat colonoscopies, thereby exacerbating patient discomfort and financial burden. Therefore, identifying risk factors for poor bowel preparation is essential to improve cleansing quality[19,20]. These findings highlight the importance of individualized preparation strategies based on patient risk profiles to enhance colonoscopy effectiveness and overall outcomes.

The findings of this study revealed that the interval between the last dose of medication and the initiation of colonoscopy is an independent risk factor for poor bowel preparation. This finding corroborates the research conducted by Kim et al[21] that demonstrated a negative correlation between the time interval and intestinal cleanliness, indicating that a longer interval may result in decreased bowel cleanliness. Ideally, this interval should be minimized to achieve optimal bowel cleansing results. According to the consensus opinion on intestinal preparation related to the diagnosis and treatment of digestive endoscopy in China, it is recommended that this interval should not exceed 4 hours, and generally no more than 6 hours for anesthetized colonoscopies[22]. European guidelines also advise commencing the final bowel preparation no later than 5 hours before the procedure, recommending a wait of no longer than 4 hours. To mitigate the adverse effects of prolonged intervals on bowel cleanliness, healthcare providers can schedule patients accordingly and guide them on the timing of the last dose of bowel cleansing medication relative to their scheduled examination[4].

Additionally, this study found that total exercise time following medication was an independent protective factor for poor bowel preparation. The results indicate the longer the total exercise time after taking the laxative, the more effective the bowel cleansing. A domestic study confirmed this correlation, suggesting that longer exercise durations during bowel preparation are associated with improved intestinal cleanliness[11]. This may be attributed to exercise promoting the secretion of digestive enzymes and enhancing gastrointestinal motility, thus facilitating bowel emptying[23]. We recommend light exercise, such as walking, which is an effective method for stimulating intestinal motility. Patients are encouraged to take a relaxing walk after taking laxatives to stimulate intestinal activity and promote defecation. Light housework may also be beneficial, but patients should listen to their bodies and cease any activity if they experience discomfort, such as dizziness, nausea, or abdominal pain. Strenuous exercise should be avoided, as it may exacerbate diarrhea and lead to dehydration.

The total number of defecation events following medication emerged as an independent protective factor for inadequate bowel preparation. Specifically, a higher frequency of bowel movements was associated with more effective bowel clearance, which aligns with our findings[21,24,25]. The number of defecation events after taking a purgative can be influenced by various factors. However, individual differences among patients, dietary habits, and compliance with bowel cleansing protocols can create special clinical scenarios, particularly in elderly or diabetic patients[26]. These patients may require longer adherence to low-residue and low-fiber diets and adjustments in their purgative intake timing to improve the efficacy of bowel cleansing. If patients do not achieve optimal stool consistency, they should not excessively use laxatives, as this may lead to intestinal dysfunction. It is crucial that patients adhere to their physician's guidance in adjusting their cleansing regimen.

In this study, a history of colorectal surgery was identified as an independent risk factor for poor bowel preparation. A negative correlation was observed between a history of colorectal surgery and intestinal cleanliness, indicating that patients with such a history exhibited lower levels of intestinal cleanliness, which has also been found in previous studies[27,28]. The Chinese guide for intestinal preparation related to digestive endoscopic diagnosis and treatment (Shanghai, 2019)[29] noted that patients with a history of colorectal surgery are at increased risk for inadequate bowel preparation. Post-surgery changes in intestinal anatomy may impair peristalsis and emptying function, leading to stool retention and a decline in bowel preparation quality[30]. To enhance bowel preparation in patients with a history of colorectal surgery, supplementary measures are needed, including optimizing dietary guidance, improving patient education, adjusting bowel cleanser dosages, and implementing remedial measures as necessary.

Our findings indicated that the proportion of patients in the control group adhering to a 1-day low-residue diet was significantly higher than that of the observation group. Consequently, dietary compliance plays a critical role in enhancing bowel cleanliness by minimizing food residue through a low-residue diet[31]. It is important to note that a restricted diet does not equate to complete fasting but rather involves adherence to a low-residue diet. A low-residue diet is preferable to a clear liquid diet, as it can reduce adverse reactions such as nausea, vomiting, hunger, headaches, abdominal distension, and abdominal pain[32]. A low-residue diet provides patients with more energy, facilitates tolerability of laxatives, and stimulates natural bowel movements, thereby improving both compliance and intestinal cleansing[33]. Enhanced tolerance and acceptance of dietary restrictions have also been shown to increase patients’ willingness to undergo repeat colonoscopies, with no significant differences noted in the quality of bowel preparation or incidence of adverse reactions[34]. Regarding the timing of dietary restrictions before colonoscopy, previous studies indicate that a 3-day low-residue diet does not significantly enhance bowel preparation outcomes compared to a 1-day low-residue diet, a finding corroborated by our study[10,31].

This study has several strengths. We conducted a relatively comprehensive analysis of potential factors influencing bowel preparation quality, including not only general demographic and clinical variables but also more detailed indicators such as the distinction between colorectal and non-colorectal abdominal surgeries, as well as exercise duration and bowel movement frequency following medication. These variables are highly relevant to real-world clinical practice. By incorporating these factors, our findings offer more practical and actionable insights, which may support the development of targeted interventions to improve bowel preparation effectiveness.

However, this study had some limitations. Firstly, this was a single-center retrospective study with a relatively small sample size. Since bowel preparation protocols and definitions of inadequate preparation may vary across institutions, the risk factors identified in this study may not be universally applicable. Secondly, we employed a convenience sampling method, as not all eligible patients were willing or available to participate in the study. It may have introduced selection bias and limited the generalizability of the findings. Moreover, due to the limitations inherent in retrospective data collection, specific information on the types of colorectal surgeries was missing for some patients, making it difficult to determine which surgical procedures are more strongly associated with inadequate bowel preparation. Future research should adopt prospective, multicenter designs with randomized sampling to validate our findings and collect more detailed medical histories to better explore potential risk factors.

CONCLUSION

This study identified both modifiable and non-modifiable factors associated with bowel preparation adequacy. A history of colorectal surgery and a prolonged interval between the last dose of laxative and the examination were independent risk factors, while strict adherence to a low-residue diet, adequate post-medication physical activity, and a higher number of bowel movements were protective factors. These findings underscore the need for targeted and individualized strategies, including structured dietary plans, optimized timing and dosing of bowel preparation, and personalized protocols for high-risk patients, to improve bowel cleanliness and enhance the diagnostic efficiency of colonoscopy.