Kidney Damage from Sodium Phosphate Bowel Preps

Background

More than 15 million colonoscopies are performed in the U.S. each year.¹ Properly cleansing the bowel prior to these procedures is important in optimizing their ability to detect colon cancer.¹ Oral sodium phosphate products (Fleet Phospho-soda, OsmoPrep [U.S. only], Visicol [U.S. only], etc) are frequently used for bowel prep because of their convenience. However, it's known that sodium phosphate bowel preps can cause up to five cases of nephropathy (a type of acute renal failure) for every one million doses used.¹ The high phosphate content can potentially harm the kidneys. In 2006, the FDA issued a warning about the use of oral sodium phosphate bowel preps in patients with impaired renal function. This document reviews new evidence that suggests sodium phosphate bowel preps may lead to an accelerated decline in renal function in older patients with normal renal function.²

Evidence

The retrospective study by Khurana et al included 286 cases and 125 controls. The mean age of study subjects was around 68 years. All subjects had serum creatinine levels £ 1.5 mg/dL within the six months prior to data collection. Demographics and comorbidities were similar in both groups. Patients underwent either colonoscopy or flexible sigmoidoscopy after receiving an oral sodium phosphate bowel prep. The primary endpoint of the study was degree of decline in renal function for one year following administration of oral sodium phosphate solution.²

At one year, there was an 8% difference between the mean initial and mean final glomerular filtration rate (GFR) in patients who had taken oral sodium phosphate bowel preps. The difference in mean initial and mean final GFR in the control group was about 1%. (This is what would be expected for the age group and comorbidities.)¹ Factors such as diabetes, use of an ACE inhibitor or ARB, and a lower baseline GFR were significantly associated with a more rapid decline in GFR at six months.²

Commentary

The use of oral sodium phosphate bowel preps can cause electrolyte disturbances (hypokalemia, hyperphosphatemia, hypernatremia, hypocalcemia) and dehydration. Sodium phospate bowel preps can also cause a high concentration of phosphate in the renal tubules, which can result in precipitation of dicalcium phosphate and subsequent renal injury.¹ Since colonoscopy is recommended from every five to every ten years, many patients could be exposed to this potential hazard multiple times in their lives.²

One potential shortcoming of the Khurana et al study is that a control group of patients undergoing colonoscopy after bowel prep with a PEG solution might have been more appropriate than an age-matched control group. Another retrospective study failed to show a difference in chronic renal failure between a group that used oral sodium phosphate solution and a group that used PEG. In addition, less than 10% of the 3,000 potential subjects screened met criteria for inclusion in the study via the availability of lab data in the medical records. This might suggest that patients who had labs reported over the course of the year were less healthy. The actual clinical relevance of the modest changes in GFR might also be questioned.^1,2

Oral sodium phosphate solutions are available OTC as compared to polyethylene glycol (PEG) solutions, which are prescription products. Oral sodium phosphate tablets also require a prescription. According to guidelines for bowel prep prior to colonoscopy, oral sodium phosphate has been shown to be more than or equally as effective (depending on the study) compared to PEG.^3,4 Oral sodium phosphate is generally better tolerated than PEG solutions (Colyte, GoLytely, etc. [U.S.] and PegLyte, etc. [Canada]).^1,3,4 Whereas patients may be required to ingest a total of 4 L of PEG solution, only 90 mL of sodium phosphate solution is required.¹ A 2 L volume of PEG (HalfLytely, Miralax [both U.S. only]) can be used for bowel cleansing prior to colonoscopy. (Note: Miralax given as one capful per 240 mL [8oz] of clear liquid every ten minutes until 2 liters are consumed.)³ When used with a stimulant laxative (bisacodyl 10 mg to 20 mg), 2 L of PEG is as effective as 4 L, and better tolerated.^3,4 Regardless of the product used, compliance is key to adequate cleansing of the bowel.

Conclusion

It's suggested that the use of oral sodium phosphate for bowel prep may be associated with an accelerated decline in renal function in older patients with normal renal function, especially those with diabetes, those taking an ACE inhibitor or ARB, and those with reduced GFR [Evidence Level B; case-control study].² Patients taking NSAIDs or diuretics could also be at higher risk. Be cautious about using sodium phosphate bowel preps in these types of patients. Consider using PEG instead.^1-4

According to the FDA warning in 2006, sodium phosphate should not be used for patients who are dehydrated, or those who have electrolyte imbalances or renal dysfunction.⁵ The Health Canada warning in late 2005 also advised against the use of sodium phosphate in patients with renal dysfunction. Health Canada advocated caution when using sodium phosphate in patients who are elderly, debilitated, dehydrated, or have electrolyte disturbances. These individuals are at increased risk for kidney damage with sodium phosphate bowel preps.^4,5

Urge patients to drink at least eight ounces (240 mL) of clear liquids with each dose of sodium phosphate, and plenty more throughout the day. This will help prevent dehydration and minimize the insult to the kidneys from the phosphorus load.^2-4

For patients who use PEG, the 2 L regimen plus 10 mg to 20 mg of bisacodyl given several hours beforehand works as well as the 4 L regimen and might be better tolerated.^1,3,4 Chilling PEG solutions or flavoring them with Crystal Light, etc. might improve palatability.^3,4 Tell patients to avoid red- or purple-colored liquids.

Levels of Evidence

In accordance with the trend towards Evidence-Based Medicine, we are citing the LEVEL OF EVIDENCE for the statements we publish.

Adapted from Siwek J, et al. How to write an evidence-based clinical review article. Am Fam Physician 2002;65:251-8.

Project Leader in preparation of this Detail-Document: Stacy A. Hester, R.Ph., BCPS, Assistant Editor

References

1. Roy HK, Bianchi LK. Purging the colon while preserving the kidneys. Arch Intern Med 2008;168:565-7.
2. Khurana A, McLean L, Atkinson S, Foulks CJ. The effect of oral sodium phosphate drug products on renal function in adults undergoing bowel endoscopy. Arch Intern Med 2008;168:593-7.
3. Wexner SD, Beck DE, Baron TH, et al. A consensus document on bowel preparation before colonoscopy: prepared by a task force from the American Society of Colon and Rectal Surgeons (ASCRS), the American Society for Gastrointestinal Endoscopy (ASGE), and the Society of American Gastrointestinal and Endoscopic Surgeons (SAGES). Gastrointest Endosc 2006;63:894-909.
4. Barkun A, Chiba N, Enns R, et al. Commonly used preparations for colonoscopy: efficacy, tolerability, and safety - a Canadian Association of Gastroenterology position paper. Can J Gastroenterol 2006;20:699-710.
5. Acute phosphate nephropathy and renal failure associated with the use of oral sodium phosphate bowel cleansing products. Pharmacist's Letter/Prescriber's Letter 2006;22(6):220614.

Cite this Detail-Document as follows: Kidney damage from sodium phosphate bowel preps. Pharmacist's Letter/Prescriber's Letter 2008;24(5):240505.

May 2008

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