Imaging studies in the Diagnosis of Appendicitis

I am frequently asked by both patients and by Emergency physicians whether an X-ray of some kind or another is needed to confirm the diagnosis of appendicitis before going to surgery. There are many surgeons who insist on imaging, usually a CT scan, before taking a patient to surgery. There are a tiny minority of surgeons who never get scans, and rely instead on the history and physical examination. And there is the group in between, which includes me, that selectively gets imaging studies to assist in an unclear diagnosis.

In the "Do I have appendicitis" exhibit I provided a link in to the "Alvarado score." This score weighs the signs and symptoms of appendicitis to determine the likelihood that the patient has appendicitis. It is statistically validated to predict the likelihood of appendicitis. An Alvarado score greater than 7 has a 93% chance of having appendicitis at surgery. (see "How often is the Doctor wrong" exhibit). Several studies have examined how often an experienced surgeon examining a patient for appendicitis is correct, and have shown a predictive accuracy of 80-95%. Every hospital has for decades had ongoing examinations of the "negative" appendectomy rate.

The purpose of obtaining an X-ray or imaging study is to improve diagnostic accuracy before taking a patient to surgery. The goal should be to improve the sensitivity and specificity of the appendicitis diagnosis without delaying care or missing appendicitis (the "false negative rate") in other words, if a person has an Alvarado score of 10, and a surgeon feels they have appendicitis, but a CT scan does not show the appendicitis and so surgery is delayed until after rupture, the obtaining of the CT scan had a negative impact on the patients medical care by delaying surgery.

The goal of imaging studies then should be to improve diagnostic accuracy - the imaging study should have a diagnostic accuracy significantly greater than what is available without imaging. So, if the Alvarado score is high, and I, as a surgeon, am comfortable that the diagnosis is appendicitis, my diagnostic accuracy (sensitivity and specificity) is around 95%. As you will see, imaging will contribute very little to the clinical picture and expose the patient needlessly to radiation. If, however, the Alvarado score is 5 and I am uncertain, particularly if the patient is a female and still has a right ovary (whose pathology can closely mimic appendicitis), then my diagnostic accuracy is around 65%. An imaging study is very appropriate. But then which one?

There are four basic tests that can be used. CT scan is the overwhelming favorite, and Barium Enema is really of historical interest only. Le'st go through the choices:

Plain Abdominal X- Ray

xray fecalith

The use of a plain abdominal xray is the simplest imaging study available. It is inexpensive and easy to read. Its usefulness depends, for the most part, on seeing a small ball of stool, called a "fecalith" plugging the opening of the appendix. This can show a fecalith when there is appendicitis about half the time. The usefulness of the plain abdominal film is more for looking for things that are not appendicitis, but could mimic appendicitis, such as severe constipation, a bowel obstruction, or a kidney stone. For the most part, most ED physicians will obtain a CT scan instead of a plain abdominal film nowadays because the CT scan gives vastly more "bang for the buck."

Barium Enema

BaE appendicitis

The idea of a barium enema is also based upon the theory that the fecalith, or some other structure will be obstructing the opeining of the appendix. By filling up the colon with contrast, the radiologist would look for an inability to fill the appendix as well, inplying that the appendix is obstructed. This test was only about 60% sensitive, and was about 80% specific - there are a lot of reasons you couldn't fill the appendix with contrast while injecting it at the anus, more than three feet away. The barium enema for the diagnosis of appendix has, as nearly as I can tell, been completely replaced with the CT scan, and is mentioned as an item of historical interest only.

ultrasound appendicitis

ultrasound abdomen and pelvis

This is a very good test when it is done under the right circumstances, and by a radiologist and technician who are skilled and experienced in doing ultrasound examinations for appendicitis. It avoids radiation, and so can be used on young people and in pregnant women whom you want to avoid radiation exposure. In the right hands it is up to 95% sensitive and specific. The emphasis must always be "in the right hands", for the accuracy of this test decreases significantly with the experience level of the team.

CT scan

CT appendicitis

This is the "gold standard" test now for appendicitis in situations where there is uncertainty. It's drawback is radiation exposure, but this exposure is much smaller than it was. These studies are much easier to read, and much less operator dependent than is ultrasound examination. The CT scan also provides a wealth of other information about the abdomen and pelvis - ovarian cysts and kidney stones for example. The CT scan study misses appendicits (false negative) about 2% of the time, and is wrong about the appendix about 3% of the time and the appendix will be found to be normal. That places its accuracy at about 97%, or about the same as a surgeon and the Alvarado score for obvious appendicitis.

Other studies

There is no role for MRI or nuclear medicine studies in the diagnosis of acute appendicitis.