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Bowel Preparation

Editor: Amit Mori Updated: 4/17/2023 4:26:19 PM


Clinicians can use a range of methods and efficacies to prepare the bowel for colonoscopy. Colonoscopy is currently the gold standard for imaging the mucosa of the colon to identify any concerning lesions for excision or biopsy. Reviews have shown that rates of incomplete colonoscopies, defined as the inability to achieve cecal intubation and mucosal visualization effectively, have ranged between 10% to 20%.[1][2][3] Poor bowel preparation can lead to failed detection of cancerous lesions and has been associated with an increased risk of procedural adverse events.[3][4]

Many studies have identified risk factors for poor bowel preparation. These risk factors include previous poor bowel preparation, non-English speaking, Medicaid insurance, single, inpatient status, polypharmacy, suffering from obesity, advanced age, male gender, and comorbidities such as diabetes, stroke, dementia, and Parkinson disease.[5][6][7][8][9] Ideal preparation reduces patient discomfort and reduces shifts in fluid and electrolytes. Preparation should be safe, tolerable, and inexpensive.[10]

Anatomy and Physiology

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Anatomy and Physiology

Adequate bowel preparation increases the likelihood of identifying lesions during colonoscopy. Colonoscopies are intended to examine the rectum, colon (sigmoid colon, descending colon, transverse colon, ascending colon, and the cecum), and the distal part of the small intestine, the terminal ileum.


Adequate bowel preparation is indicated when a colonoscopy is needed.


There are certain contraindications to specific bowel preparations, which include but are not limited to:

  • Low volume 2-L PEG-ELS with ascorbic acid should be avoided in those with glucose-6-phosphate dehydrogenase deficiency.
  • PEG-3350 should be avoided in those with electrolyte abnormalities.
  • Magnesium citrate should be avoided in those with electrolyte abnormalities such as kidney disease and those with a greater risk of magnesium toxicity.[3]
  • Oral sodium sulfate in a single dose has been found to cause increased gastrointestinal (GI) events.[11]
  • Sodium phosphate is not recommended as a bowel preparation regimen due to its side effects. It should be avoided in those with renal dysfunction, dehydration, hypercalcemia, hypertension treated with an angiotensin-converting enzyme inhibitor or an angiotensin receptor blocker because they have developed phosphate nephropathy.[12] Hyperphosphatemia, elevated blood urea nitrogen, increased plasma osmolality, hypocalcemia, hyponatremia, and seizures have also been documented.[13][14][15][16][17][3]
  • The bowel preparation combination of sodium picosulfate/magnesium citrate is known to have a GI side-effect profile that includes abdominal pain, nausea, and vomiting.[3]
  • The bowel preparation combination of sodium sulfate and sulfate-free PEG-ELS has been associated with vomiting.[18]


The primary care physician or gastroenterologist determines the most appropriate bowel preparation for a patient. Often, the decision can be collaborative because the primary care physician may have greater knowledge regarding medical history that may contraindicate certain bowel regimens. The gastroenterologist is most likely to decide on care because they have more in-depth knowledge about bowel regimens and can educate and instruct the patient on properly administering the regimen. In the inpatient setting, the nursing staff plays a pivotal role in monitoring the patient for complaints, side effects and ensuring complete and proper administration of the regimen.


Bowel preparation can be divided into 3 categories: isosmotic, hypoosmotic, and hyperosmotic agents.

Isosmotic agents include high-volume polyethylene glycol (PEG) preparations, low-volume PEG preparations, and sulfate-free PEG-electrolyte solutions (ELS).

  • High-volume PEG preparations are osmotically balanced with nonfermentable electrolyte solutions to help prevent fluid and electrolyte shifts. PEG is an inert polymer of ethylene oxide that is designed to pass through the bowel without absorption. High volume PEG preparations consist of 4 L of solution and can be taken either as a single dose or split dose preparation. There is growing evidence that split-dose regimens are superior in bowel preparation.[3][19] High-volume PEG preparations are typically well tolerated; however, nearly 5% to 15% of patients do not complete preparation due to poor palatability and/or large volume.[20] Moreover, it does not change histological features of mucosa and can be used in those patients thought to have inflammatory bowel disease.[21] It can also be used in those with preexisting electrolyte imbalances and those who cannot tolerate high sodium loads, such as those with renal failure, heart failure, or cirrhosis.[22]
  • Low-volume PEG preparations were designed to have the same efficacy as high-volume preparations but in a more tolerable amount. The only FDA-approved. low-volume PEG preparation is low -volume, 2-L, PEG-ELS with ascorbic acid.[3] Since this preparation includes ascorbic acid, it must be used with caution in those with glucose-6-phosphate dehydrogenase deficiency as the ascorbic acid can exacerbate hemolysis.[23]
  • Sulfate-free PEG-ELS was created to improve the smell and taste of PEG-ELS.[24] It is less salty, more palatable, and importantly comparable to PEG-ELS in regards to colonic cleansing, overall tolerance, and safety.[3][25]

Hyposmotic agents include a low-volume PEG preparation called PEG-3350 (PEG-SD) that requires an additional electrolyte solution (sports drink) and is often combined with bisacodyl.[3] The combination of PEG-3350 and an electrolyte solution is not FDA approved for bowel preparation prior to colonoscopy and is not considered equivalent to the isomotic, low-volume, 2-L, PEG-ELS.[3] Numerous studies have found mixed results regarding colonic cleansing and electrolyte abnormalities, such as changes in sodium, potassium, and chlorine(Cl). Some studies have shown that PEG-3350 is more likely to cause electrolyte abnormalities such as hyponatremia than low-volume, 2-L, PEG-ELS.[26]

Hyperosmotic agents include magnesium citrate, oral sodium sulfate, and sodium phosphate.

  • Magnesium citrate is a magnesium-containing, osmotically-acting saline solution that also stimulates the release of cholecystokinin, leading to intraluminal fluid and electrolyte shift in the small intestine and possibly the colon. It is not typically recommended as a bowel preparation due to potential magnesium toxicity leading to bradycardia, hypotension, nausea, and drowsiness. Moreover, magnesium is secreted via the kidneys and should be avoided in those patients with kidney disease.[3]
  • Oral sodium sulfate has not been associated with significant fluid, or electrolyte shifts thought to be due to the sulfate, a poorly absorbed anion.[3] Although there is limited research on oral sodium sulfate, one study found that oral sodium sulfate was similar to low-volume, 2-L, PEG-ELS with ascorbic acid in bowel preparation. Moreover, compared to 4-L, PEG-ELS preparation, 1-day oral sodium sulfate preparation was associated with increased GI events; however, this was not seen in split-dosed regimens.[11]
  • Sodium phosphate is no longer recommended as a bowel preparation regimen due to its serious side effects. Those with renal dysfunction, dehydration, hypercalcemia, or hypertension treated with an angiotensin-converting enzyme inhibitor or angiotensin receptor blocker have had developed phosphate nephropathy.[12] Moreover, it has been associated with hyperphosphatemia, elevated blood urea nitrogen, increased plasma osmolality, hypocalcemia, hyponatremia, and seizures.[14][15][16][17] Given many of these adverse effects, the FDA has issued a warning for the prescription tablet form of sodium phosphate.[3][15]

There are also combination agents such as sodium picosulfate/magnesium citrate and sodium sulfate/sulfate-free PEG-ELS that have been used for bowel preparation.

  • Sodium picosulfate/magnesium citrate acts dually as a stimulant laxative to increase the force and frequency of peristalsis by sodium picosulfate and as an osmotic laxative to retain fluid in the colon by the magnesium citrate component.[27] Side effects are typically GI-related, such as abdominal pain, nausea, and vomiting.[3]
  • Sodium sulfate and sulfate-free PEG-ELS combination typically consist of oral sodium sulfate and 2 L of sulfate-free PEG-ELS.[3] In a study comparing split dosed administration of this combination to low-volume, 2-L, PEG-ELS with ascorbic acid, both bowel preparation regimens had successful bowel preparation, but the combination of sodium sulfate and sulfate-free PEG-ELS was associated with higher rates of vomiting.[3][18]

The Boston Bowel Preparation Scale (BBPS) was developed to assess bowel preparation after all cleaning maneuvers. Each segment of the colon, the right colon, transverse colon, and left colon are assigned points from 0 to 3 with regards to the cleanliness of the colon.[28] A score of 0 includes an unprepared colon, 1 includes those in which only a portion of the mucosa of the colon segment is visible, 2 includes those with a minor amount of residual staining and small fragments of stool present. Lastly, 3 includes those where the entire mucosa of the colon is seen well with no residual stool. Each segment of the colon is assigned a score, and the entire colon is assigned a cumulative score.[28] Higher scores indicate better preparation.

Technique or Treatment

Bowel preparation can be given in a single dose or a split dose. A higher quality bowel preparation and increased adenoma detection rate have been demonstrated in those taking a split dosed bowel preparation.[3][29] Typically, the first dose should be taken the day before the procedure and the second dose is taken 3 to 8 hours before the start of the colonoscopy.[3][30][31]

Clinical Significance

Poor bowel preparation can be a potentially severe limitation on the usefulness of colonoscopies. Proper bowel preparation leads to clean identifiable mucosa, and as a result, there is a greater ability to detect polyps and other lesions. Contrary, poor bowel preparation can potentially cause missed identification of lesions or polyps that can have morbidity and mortality effects on the individual.

Enhancing Healthcare Team Outcomes

Interprofessional care for a patient undergoing bowel preparation is vital to ensure better preparation and colonoscopy outcomes. An interprofessional team consists of a primary care physician, gastroenterologist, nurses, and a pharmacist. With a team-based approach, a proper bowel preparation regimen can be selected based on the patient's medical history and potential side effects. A single dose versus a split-dose regimen can be selected to accommodate the patient's lifestyle. 

One study found that the utility of telephone reeducation about bowel preparation the day before a colonoscopy significantly improved the quality of bowel preparation and the rate of polyp detection.[32] [Level I]



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