Thursday, June 18, 2015

Oral contrast for abdominal CT?


PO, IV contrast for abdominal CT
This material is educational, and is not intended to serve as hospital protocol.
 
Mostly, it's here to serve a quick reference for myself, and for impromptu discussions with PAs and residents.

SUMMARY

Suspected SBO
Oral contrast is contraindicated in suspected SBO.
IV contrast is preferred, but not required.

Suspected appendicitis
Oral contrast is not required, but may be beneficial in extremely thin individuals (e.g. BMI < 18), or non-obese children.

Suspected diverticulitis
Oral contrast is not required

Unclear etiology
PO contrast is suggested by some experts in undifferentiated abd pain if the patient
1)   Is very thin (e.g. < 120 pounds, BMI < 18);
2)   Has had a Roux-en-Y gastric bypass; or
3)   Has and inflammatory bowel disease that could produce a fistula

Discussion and References

SBO
Per the American College of Radiology guidelines, in a patient with suspected SBO, PO contrast is contraindicated. They explain that: “Oral contrast will not reach the site of obstruction, wastes time, adds expense, can induce further patient discomfort, will not add to diagnostic accuracy, and can lead to complications, particularly vomiting and aspiration.”    

Appendicitis
Multiple studies have shown that the use of PO contrast does not increase the accuracy of the diagnosis of appendicitis (references below). An editorial in a recent radiology journal concluded that “routine administration of oral contrast medium may not be necessary in the setting of suspected acute appendicitis.” American College of Radiology guidelines state that PO contrast “may not be needed,” leave the decision to use PO contrast to “institutional preference.”
Some have suggested that PO contrast might be needed in extremely thin individuals (e.g. BMI < 18). See the discussion below in "Unclear etiology."

A 2005 systematic review from the American Journal of Surgery concluded that: “Noncontrast CT techniques to diagnose appendicitis showed equivalent or better diagnostic performance compared with CT scanning with oral contrast.”

A 2009 randomized study in the American Journal of Radiology found that “nontraumatic abdominal pain imaged using 64-MDCT with isotropic reformations had similar characteristics for the diagnosis of appendicitis when IV contrast material alone was used and when oral and IV contrast media were used.”

A 2011 retrospective study in the Journal of Surgical Research found that oral contrast did not reach the cecum in 1/3 of patients. Furthermore, “there appears to be no diagnostic compromise in those without contrast in the terminal ileum.
A 2014 prospective study in the Annals of Surgery concluded that: “Enteral contrast should be eliminated in IV-enhanced CT scans performed for suspected appendicitis.”

Diverticulitis
The American College of Radiology guidelines state that PO contrast “may be helpful for bowel luminal visualization.” Despite this, they conclude that “regardless of the [the use or lack of IV or PO contrast], the accuracy is high for depicting findings of acute diverticulitis.”

A 2006 prospective study in Emergency Radiology looked at patients with undifferentiated abdominal pain in the ED; 1/5 of them had LLQ tenderness, and diverticulitis was the second-most common suspected diagnosis. All patients had CT scans with and without PO contrast; no IV contrast was used. Although there were discordant interpretations between the (+) PO and (-) PO CT scans, the authors found that a “significant portion of the discordance was attributable to interobserver variability.” Thus, it would appear that a CT scan without either PO or IV contrast could be accurate in the diagnosis of diverticulitis.

Unclear etiology
Oral contrast may not be needed for undifferentiated abdominal pain, as per the discussion of the study in Emergency Radiology noted above.

However, in a 2012 study in Emergency Radiology that examined the use of PO contrast in undifferentiated non-traumatic abd pain, the authors excluded 2 groups of patients.  They excluded all “subjectively thin” patients, but did not provide cut-offs. They noted the research that suggests this exclusion is unnecessary, and may be dropped in the future.  They also excluded patients at high risk for having an intra-abdominal fistula. Primarily, they targeted patients who had had a Roux-en-Y gastric bypass, or who had IBD, disposing them to fistula formation.

4 comments:

  1. ACR recommendations you cite are for "high-grade SBO". For completeness, the ACR further says that for intermittent or low-grade SBO, "in contrast to a high-grade obstruction, oral contrast is not necessarily contraindicated and may add functional information."

    As a radiologist (having seen one or two CT scans) I find the use of enteric contrast with CT to be often helpful in problem solving.

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