Polyethylene glycol (PEG), the standard bowel cleansing preparation for conventional colonoscopy, did just a little better than magnesium citrate at virtual colonoscopy, according to researchers writing in Radiology.
The University of California, San Francisco (UCSF) study prospectively compared the effectiveness of cleansing and stool and fluid tagging, as well as patient acceptance, between the two preparations. Results showed that the amount of residual stool and fluid was substantially equivalent between the two -- but also that reduced-volume PEG had a slight edge in fluid and stool tagging, along with shorter interpretation times (Radiology, August 24, 2011).
"Investigators in previous studies have found improved patient tolerance with reduced-volume PEG preparations compared with 4-L PEG and sodium phosphate preparations," wrote Dr. Alexander Keedy, Dr. Judy Yee, and colleagues from UCSF. "However, to our knowledge, reduced-volume PEG and magnesium citrate have not been directly compared. Magnesium citrate preparations are generally very well tolerated by patients, so it is encouraging to find that the reduced-volume PEG was equally well tolerated in our study."
In the randomized study, 50 patients (96% male) referred for conventional colonoscopy underwent virtual colonoscopy (also known as CT colonography or CTC) following cathartic bowel cleansing with 2L of PEG or a magnesium citrate solution, tagging with oral contrast agents, and automated bowel insufflation with CO2 (ProtoCO2l, Bracco Diagnostics).
Patients avoided solid food on the day before the exams. A barium sulfate suspension (Tagitol V, Bracco Diagnostics) was used for fecal tagging, and residual fluid tagging was achieved with diatrizoate meglumine (Gastrografin, Mallinckrodt). Patients were randomized to receive one of two bowel preparations:
- 2-L PEG solution (100 g of PEG-3350) ingested in eight 473-mL aliquots, taken over two hours on the evening before the examination. Each liter of PEG solution was followed by 473 mL of clear liquid (MoviPrep, Salix Pharmaceuticals).
- 237-mL magnesium citrate solution consisting of magnesium carbonate (31%), citric acid (65%), and potassium citrate (3%), yielding 18 g of magnesium citrate (LoSo Prep, Bracco Diagnostics).
Four 5-mg enteric-coated bisacodyl tablets (Bracco Diagnostics) were taken two hours after ingesting the magnesium citrate solution. A 10-mg bisacodyl suppository was administered two hours before the CT colonography examination.
All CTC exams were performed on a 64-detector-row scanner (LightSpeed VCT, GE Healthcare), at 120 kVp, 70 mAs, 0.625-mm section thickness, and pitch of 1.375. One of three abdominal radiologists experienced in CTC interpretation read the images.
The authors compared residual stool burden and tagging adequacy, and patients completed a tolerance questionnaire within two weeks of their exams. Bowel preparations were compared for residual stool and fluid by using generalized estimating equations, and the Mann-Whitney test was used to compare the qualitative tagging score, mean residual fluid attenuation, and adverse effects assessed on the patient experience questionnaire, Keedy and colleagues reported.
Sixteen polyps 6 mm or larger were detected. CTC depicted 12 true-positive lesions and four false-negative lesions measuring 7, 8, 9, and 10 mm.
The researchers found similar amounts of fluid and stool for both preps. However, fecal and fluid tagging scores showed significantly better results for PEG than magnesium citrate. For PEG, the mean fecal tagging score on a scale of 0 (best) to 4 (worst) was 0.48 ± 0.83. For the magnesium citrate group, the mean fecal tagging score was 1.52 ± 1.42 (p = 0.006). The qualitative mean fluid tagging score was 0.28 ± 0.68 for the PEG group and 1.28 ± 1.51 (p = 0.009) for the magnesium citrate group.
Better tagging should lead to better polyp detection, the authors noted, and although the small number of clinically significant polyps in the study precludes a meaningful statistical analysis of this trend, "it is noteworthy that there was only one false-negative lesion and one false-positive lesion in the PEG group, compared with three false-negative lesions and three false-positive lesions in the magnesium citrate group," they wrote.
In addition, quantitative analysis showed higher attenuation of residual fluid in the PEG prep group for all colonic segments combined (765 HU ± 491 and 443 HU ± 393 for the PEG and magnesium citrate groups, p = 0.01), as well as in the cecum, descending colon, and sigmoid colon.
"One possible explanation for this finding is that the osmotic laxative or the stimulant laxative (bisacodyl) used in the magnesium citrate preparation interferes with the mixing process required for homogeneous tagging," the authors wrote. "Our study was not designed to address this question; therefore, additional validation is needed."
Finally, mean interpretation time was shorter for PEG (14.8 minutes) than for magnesium citrate (18.0 minutes, p = 0.04).
Patient tolerance ratings were not significantly different between preparations (p > 0.05). However, 19 patients stated a preference for CTC over optical colonoscopy (48%) versus only three patients who preferred conventional colonoscopy (8%). Eighteen patients (45%) had no preference for either exam (p < 0.001).
"There were several advantages for the PEG preparation, which required a shorter study interpretation time and promoted improved tagging of residual solid stool and fluid," Keedy and colleagues wrote.
The PEG solution used in the study was half the volume of the standard 4-L PEG solution. "The smaller volume is easier for patients to ingest but maintains the advantages of an osmotically neutral electrolyte lavage solution, which has fewer deleterious effects on serum electrolytes than do osmotically active preparations," the authors wrote.
The PEG solution also includes high doses of ascorbic acid (21.2 g), which improves the taste and provides an additional laxative effect. On the other hand, the citric acid in the magnesium citrate preparation also improves taste but does not offer a laxative effect. And although magnesium citrate's volume is lower, it is "an osmotic laxative that can potentially cause electrolyte disturbances," the group noted.
As for study limitations, there were only 50 subjects, and the group consisted almost entirely of men. The study also was probably underpowered to detect subtle differences in preparation adequacy, and there were few clinically significant polyps. In addition, the inclusion of both average- and increased-risk patients may preclude comparison of the results with completely asymptomatic populations undergoing screening only. Finally, only one radiologist interpreted each exam.
There were no significant differences in the amount of residual solid stool or fluid or patient acceptance when comparing reduced-volume PEG and magnesium citrate preparations, the authors concluded. Both were effective for bowel cleansing and both maintained adequate polyp detection.
"Improved solid stool and fluid tagging, as well as a shorter interpretation time, were important advantages observed in the reduced-volume PEG group," Keedy and colleagues wrote.
