CT or Ultrasound For Appendicitis – Which To Choose And Why

A recent article in JAMA Pediatrics, Effect of Reduction in the Use of Computed Tomography on Clinical Outcomes of Appendicitis, discussed the use of ultrasound versus CT in pediatric patients with symptoms of appendicitis.

In the article the authors “reviewed the Pediatric Health Information System administrative database for children who presented to the ED with the diagnosis of appendicitis or who underwent an appendectomy in 35 US pediatric institutions from January 1, 2010, through December 31, 2013.”

Their results and conclusion are as follows:

1. Use of US increased 46% (from 24.0% in 2010 to 35.3% in 2013; absolute difference, 11.3%; adjusted test for linear trend,P = .02)
2. Use of CT decreased 48% (from 21.4% in 2010 to 11.6% in 2013; absolute difference, −8%; adjusted test for linear trend,P < .001).
3. The proportion of negative appendectomy declined during the 4-year study period from 4.7% in 2010 to 3.6% in 2013 (test for linear trend,P = .002),
4. The proportion of perforations (32.3% in 2010 to 31.9% in 2013) and ED revisits (5.6% in 2010 and 2013) did not change (adjusted tests for linear trend,P = .64 and P = .84, respectively).
5. Despite the increased reliance on the diagnostically inferior US, important condition-specific quality measures, including the frequency of appendiceal perforation and ED revisits, remained stable, and the proportion of negative appendectomy declined slightly.

In the article, ultrasound is described as “diagnostically inferior”. This flies in the face of even their own study where one of their conclusions is that even with the increased use of ultrasound, critical quality measures remained stable or decreased. The issue is not that ultrasound is “diagnostically inferior”; rather, it is used incorrectly in many cases.

The issue with ultrasound is that it is organ and disease specific as opposed to CT which is anatomically specific. Anatomically specific meaning that all organs are able to be reviewed within the area imaged.

With ultrasound, we should be evaluating a specific organ or specific disease process e.g. the thyroid, gallstones, hydronephrosis, ectopic pregnancy, carotid stenosis etc. Ultrasound is weakest when it is used to review anatomy without a specific indication e.g. abdominal pain or right lower quadrant pain of uncertain etiology without utilization and appropriateness criteria being met for a diagnosis of appendicitis. CT being anatomically specific, is the examination of choice where there is an unclear etiology for a set of symptoms.

The more specific the symptoms and clinical findings i.e. where utilization and appropriateness criteria are met, the more sensitive and specific ultrasound becomes. In a 40-year-old female who is obese with right upper quadrant pain – ultrasound would be the examination of choice to evaluate for cholecystitis. On the other hand, a 40-year-old male who is thin with right upper quadrant pain and where no specific utilization or appropriateness criteria are otherwise met for cholecystitis, CT will more broadly evaluate the patient and range of potential etiologies for the cause of the symptoms.

CT is not without issues. In children and thin young patients the appendix is frequently not located. IV contrast should not be used except for specific indications; however, oral contrast is very frequently necessary to accurately locate the entire appendix. Early appendicitis or appendiceal tip appendicitis is not uncommonly seen with CT and ensuring the entire appendix is located is of utmost importance. Not locating the entire appendix results in the need to rescan the patient increasing the radiation dose or missing the diagnosis altogether and delaying treatment.

Using oral contrast does increase the time required to obtain a scan creating a potential issue for ER physicians who, like radiologists, also have to consider turnaround times.

Finally, in general practice with the usual cross-section of patients scanned at all times of day, even in the presence of classic symptoms of appendicitis, with ultrasound the likelihood of finding the appendix and making the diagnosis of acute appendicitis is probably less than 50/50. As it turns out, most appendices don’t read the book and the appendix is almost never where it is supposed to be. If on the other hand a patient finds themselves at a university where there is lots of time to spend looking for the appendix; and residents, fellows and staff hovering to scan etc., the percentage of true positives is definitely higher and ultrasound is certainly a good choice.

So, what to do…

Any recommendation for imaging should ensure the greatest likelihood of locating the target organ or structure, and with the highest degree of specificity and sensitivity determine the presence or absence of disease. This should be accomplished at the lowest cost with the highest quality.

Using clinical information to determine whether utilization and appropriateness criteria are met, a decision to use ultrasound or CT usually becomes clear. In the presence of a high pretest probability for appendicitis, regardless of age ultrasound should be considered – if ultrasound technologists are available who can accurately evaluate the patient. If the ultrasound is nondiagnostic/indeterminate, and a diagnosis continues to be sought, CT with oral contrast should be obtained.

If imaging is indicated, patients who do not fit a high pretest probability criteria should go to CT. IV contrast is frequently unnecessary unless for a specific indication. If the patient is less than 15-20 years old or thin (low body fat), oral contrast should be used. If the patient is middle-aged or older and preferably slightly to mildly overweight to obese, oral contrast can be omitted as the amount of intra-abdominal fat is usually adequate to separate the bowel loops increasing the likelihood of locating the appendix.