Pearls:

Ultrasound should be used to confirm but not definitively exclude appendicitis. The sensitivity of ultrasound is between 78 and 99%.

In patients with symptoms < 48 hours, ultrasound findings may not reveal appendicitis and inflammatory markers may not have had time to rise.

In appendicitis, ultrasound should not be seen as a test with a dichotomist result. That is, it is either “positive or negative”. It should be used in conjunction with the clinical exam and/or laboratory studies.

According to the 2010 ACEP guidelines, ultrasound (US) should be used as the initial imaging in children with suspected appendicitis. It should be noted that some children, in some centers, have no imaging before being taken to the operating room for suspected appendicitis. This is usually in situations in which clinical characteristics and laboratory findings are significant.

When do you not use ultrasound? US is not indicated in patients who are being taken to the OR emergently. Ultrasound is less accurate in patients with a BMI > 85th percentile, patients with “tip appendicitis” (ie inflammatory changes in the distal appendix), retrocecal appendicitis, those patients who have a shorter duration of symptoms (< 48 hours) and/or a < 50% pretest probability. US is also not good for determining perforation.

When is secondary imaging recommended? To answer this question, it is important to remember that what is defined as a positive, negative and/or equivocal US is not well defined in the literature. Generally, however, an accepted definition is that a positive US for an appendicitis is a tubular, non-compressible structure > 6 mm in diameter. In a patient with these findings and a clinical exam consistent with appendicitis, no further imaging is indicated. One uses the US and pretest probability to decide on further imaging.