Double dose of oral sodium phosphate unnecessary for VC

Feb 18, 2007

Sometimes it pays to mess with success. In virtual colonoscopy, the double-dose (90 mL) sodium phosphate bowel cleansing regimen, despite its proven effectiveness, may be too much of a good thing.

The double-dose technique has not only produced thousands of clean, readable virtual colonoscopy studies, it was the method used in the most successful VC trial to date: the multicenter U.S. Department of Defense trial that beat conventional colonoscopy in a screening cohort of 1,132 (New England Journal of Medicine, December 4, 2003, Vol. 349:23, pp. 2191-2200).

There have been concerns, however, about potential side effects of the double split-dose regimen, particularly in vulnerable patients. And it does constitute off-label use, according to the U.S. Food and Drug administration.

"In addition, morbidity concerns exist in patients with significant cardiac and renal conditions, precluding its use in these populations," wrote Dr. David Kim, Dr. Perry Pickhardt, Dr. J. Louis Hinshaw, and colleagues at the University of Wisconsin School of Medicine and Public Health in Madison. "Although no significant complications have been encountered, if a single-dose administration were adequate, it would be advantageous because of an improved therapeutic index and overall simplified cathartic regimen" (Journal of Computer Assisted Tomography, January-February 2007, Vol. 31:1, pp. 53-58).

In their study, the researchers prospectively compared the efficacy of the 90 mL (double) sodium phosphate cleansing regimen against that of a 45 mL (single) regimen, and essentially found no difference.

One hundred consecutive patients undergoing sodium phosphate cleansing for screening CT colonography (CTC or virtual colonoscopy [VC]) were randomly given either a single dose of 45 mL or two doses of 45 mL separated by three hours' time.

The mean age and sex distribution of the single-dose group (59.6 ± 8.9 years, 20 men and 30 women) were similar to the double-dose group (56.5 ± 7.7 years, 24 men and 26 women), the authors noted. Eleven patients with significant cardiac or renal conditions were steered to polyethylene glycol cleansing instead, and these patients were not included in the study.

All patients underwent the sodium phosphate regimen with a single or double dose, beginning the day before CT imaging. The patients were limited to a clear liquid diet. They drank a diluted barium mixture of 250 mL to tag residual stool, and 60 mL of water-soluble iodinated contrast to tag residual fluid.

Colonic distension was achieved with manual room-air insufflation, or by automated CO₂ insufflation (ProtoCO₂l, E-Z-EM, Lake Success, NY) to tolerance. Scout views were examined to ensure adequate colonic distension. All exams were performed on an eight- or 16-slice CT scanner (LightSpeed series, GE Healthcare, Chalfont St. Giles, U.K.) using 1.25-mm collimation, 1-mm reconstruction intervals, 120 kVp, and 50-75 mAs, Kim and colleagues wrote.

Two very experienced VC readers scored each dataset for residual stool and fluid using a four-point scale.

The results showed excellent cleansing in both groups of patients, demonstrated in the low residual stool and fluid scores.

"A stool score of one or 2 (indicating a stool-free segment or only minimal particles < 5 mm) was seen in 90.3% (271/300) of segments in the single-dose group and 87% (522/600) in the double-dose group," the team wrote. "There was no significant association for sodium phosphate dosage level and residual stool for the cecum (p = 0.27), the ascending colon (p = 0.74), the transverse colon (p = 0.86), the descending colon (p = 0.67), or the sigmoid colon (p = 0.10)."

There was a significant association between the sodium phosphate dose level and residual stool in the rectum (p = 0.0056). A score of two or greater was 4.5 (95% CI, 1.61-12.54) times more likely for the double-dose group compared to the single-dose group.

"Similarly, there was minimal residual fluid with a score of 1 or 2 (indicating < 25% of lumen occupied by fluid) in 82% (488/600) of segments in the single-dose group and 87% (522/600) in the double-dose group," the group reported. "There was no significant association between the sodium phosphate dose and residual fluid score (supine and prone) for the cecum (p = 0.45 supine, 0.41 prone), the transverse colon (p = 0.43, 0.36), the descending colon (p = 0.78, 0.41), the sigmoid colon (p = 0.053, 0.43), or the rectum (p = 0.52, 0.14)."

No significant attenuation values were seen between the two dosage groups for either the right or left colon, and no immediate complications were seen in either group.

"A cornerstone of effective polyp detection at CTC involves adequate colonic cleansing," Kim et al wrote. "Ideally, a cleansing regimen would result in a colon free of stool and fluid. Residual fluid and stool significantly increase the difficulty of detecting polyps, as they may be obscured by residual material. In addition, untagged adherent stool may be mistaken for polyps, potentially leading to unnecessary optical colonoscopy."

Sodium phosphate is recommended over other agents due to its increased cleansing ability and drier preparation, along with the fact that most patients are able to comply with the regimen by ingesting the entire dose. With polyethylene glycol, many patients are unable to drink the large volume of fluid required.

"It is important that no significant differences were seen in the characteristics of the residual stool and residual fluid attenuation values between the single- and double-dose cleansing regiments," Kim and colleagues wrote. "If differences existed, suggesting an effect on the stool tagging and fluid tagging techniques, these differences could theoretically impact on both the sensitivity and the specificity of polyp detection."

Given the effectiveness of the single-dose regimen, the use of double-dose sodium phosphate has been discontinued at the University of Wisconsin. "This immediately changed our practice," Dr. Perry Pickhardt told Auntminnie.com.

By Eric Barnes
AuntMinnie.com staff writer
February 19, 2007

New method corrects for hyperattenuation surrounding tagged VC data, August 8. 2006

Reduced prep, low-residue diet, and mild tagging suffice for accurate VC, July 13, 2005

Prepless VC yields high sensitivity in average-risk subjects, June 10, 2005

Iodine tagging regimen yields best VC results, January 27, 2005