Historically, bowel preparations were administered as a single dose on the day or evening before colonoscopy. A body of literature has now emerged in support of split-dose bowel preparations, in which half of the preparation is given the day before the procedure and the second half is given the morning of the procedure. Studies have shown that the split-dose schedule increases adenoma detection rate, possibly due to improved patient tolerance and willingness to take the entire preparation [14, 15]. ASGE recommends administration of the second dose between 3 and 8 h before colonoscopy to allow sufficient time to achieve the desired response and avoid potential aspiration with sedation. Split-dose administration remains the preparation schedule of choice in both inpatient and outpatient settings. This recommendation stands regardless of whether the scope occurs in the morning or afternoon, although endoscopists may consider full-dose administration on the morning of the procedure for afternoon colonoscopies.

It has been well established that adequate bowel cleansing is essential to achieve high rates of cecal intubation and adenoma detection [16]. While no one regimen has shown superiority to this end, it seems clear that all established bowel preparations work in the majority of patients who are compliant with the instructions [4]. The reasons for inadequate preparation are numerous and likely multifactorial. Interestingly, failure to follow prep instructions only contributes to approximately 20% of inadequate preparations, which suggests a significant number of patients have an inherent difficulty with bowel cleansing [3]. Patient factors associated with poor preparation include inpatient status, older age, male gender, or history of cirrhosis, stroke, colorectal surgery, colonic inertia, dementia, obesity, diabetes, or active use of narcotics, calcium channel blockers, tricyclic antidepressants [2–4]. One model used a number of patient factors (e.g., cirrhosis, Parkinson’s, diabetes, male gender, BMI, age, positive fecal occult blood test, and prior colorectal surgery) to predict inadequate preparation, and was able to predict 60% of inadequate preparations. This model could theoretically reduce the rate of inadequate cleansing from 33 to 13% if widely utilized to identify patients in whom a more aggressive preparation would be beneficial [2]. To avoid the waste associated with repeat colonoscopy, it is important that we utilize such models and ensure there is a plan in place to optimize bowel cleansing prior to committing to endoscopy . At a minimum, patients with a history of inadequate preparation should be considered for a more aggressive bowel cleansing regimen.

Patient education has been found to be a contributor to inadequate bowel preparation in approximately 20% of patients [3]. Proper teaching is perhaps the easiest and most cost-effective method of ensuring an adequate bowel cleansing . While it is of critical importance to all bowel preparations, it should be a special point of emphasis in those patients who are deemed high risk for inadequate bowel cleansing , to include those who are non-English-speaking, cognitively impaired, or who have risk factors for inadequate preparation. Thus, appropriate education is the first component of any intensive bowel cleansing regimen.

Several aggressive adjunctive measures have been proposed for patients at high risk of inadequate preparation, although the evidence to support these practices is predominantly anecdotal. Most would support a full 4-L PEG solution in split doses (the evening before and the day of colonoscopy) rather than a 2-L dose. Two full days of clear liquids prior to examination or double administration of the preparation over 2 days are often utilized in this setting, although there is no evidence to support these practices. Other options are the addition of magnesium citrate (300 mL) or bisacodyl (10 mg) to a standard PEG preparation [17].