Tagg, A. Thinking FAST, and slow, Don't Forget the Bubbles, 2018. Available at:
10 year old Elliott is brought into you emergency department after falling off his bike. Whilst trying to escape from a gang of bullies he went off-road, left the ground and landed awkwardly. The front wheel twisted and the handlebars hit his belly. He is complaining of pain in the left upper quadrant. He has been treated with intranasal fentanyl and is haemodynamically stable. Your registrar asks if he can do a FAST exam on him.
Basics principles of the FAST exam
The Focused Abdominal Sonography for Trauma exam superceded diagnostic peritoneal lavage in the late 1980s as a means of determining significant intra-abdominal free fluid. The actual monicker, FAST, was first used by Royzycki et al back in the mid 90s.
The FAST exam is a rapidly performed test that looks at four specific areas – RIGHT upper quadrant, LEFT upper quadrant, subxiphoid region and pelvis. The wielder of the probe is looking for free fluid rather than directly looking for solid organ injury.
It’s important to remember that the FAST exam came about as a tool to examine haemodynamically UNSTABLE patients in order to determine who needed to go to the operating theatre or needed a critical intervention (such as pericardiocentesis).
According to Rippey and Royce, the sensitivity of FAST in adults ranges from 64-98%. But…
What about in kids?
CT is considered the gold standard for the examination of intra-abdominal injury in children but it is not without risk. As clinicians we are reluctant to expose kids to needless radiation and try and act within the ALARA (As Low As Reasonably Possible) principle.With an increased focus on the use of point of care ultrasound throughout paediatrics it can be tempting to translate the adult approach of using the FAST scan, in kids.
A couple of concerns have been raised regarding the use of FAST in children:
Not all children with abdominal injuries have free fluid
A number of studies in haemodynamically stable children have found significant solid organ injuries (liver, spleen or kidney lacerations) on CT with a normal bedside ultrasound. Whilst 22% of abdominal injuries in adults are not associated with free fluid this rises to a whopping 37% in children.
A 2007 meta-analysis by Holmes et al found an 80% sensitivity for detecting intra-peritoneal fluid via sonography. When the authors only looked at the more methodologically rigorous studies the sensitivity dropped to 66%.
The management of solid organ injuries in the paediatric population is different
Nearly all intra-abdominal injuries in children are managed conservatively and so accurate delineation is important. Finding free fluid on sonographic assessment does not mandate them going to theatre, even in the setting of haemodynamic instability. Operative management of hepatic injuries in children has been associated with higher mortality than a conservative approach.
So what does this all mean?
CT scanning does have its drawbacks – it involves ionising radiation, IV contrast and is time and money intensive in comparison with the FAST scan. But if ultrasound cannot tell us what we need to know then there is no comparison. A number of studies that have shown better correlation between CT and US do not use the FAST scan but a modified form or even complete abdominal sonography by qualified sonographers. Given that US is very much an operator-dependent imaging modality it is vital that anyone using it has been trained (and accredited) in its use.
Emergency physicians may think they are amazing at performing a focused abdominal assessment and wield the probe at every given opportunity ‘for practice’. This will skew the accuracy of the test. If the pre-test probability of a positive result is low in the first place then the number of true negatives will, of course, be higher and the accuracy of the test will appear to be higher than it actually is.
In my attempt to trawl through some of the data I have consistently come across the idea that FAST is a great because it is so accurate. The only way of knowing this is to look at the studies that compare it with a CT. Just because you do not pick up an injury immediately does not mean that one is not there. For example, in the Soudack et al. paper they described three negative FAST, positive CT cases – a haemo-peritoneum, one splenic laceration and one hepatic laceration. Because the CT did not show free fluid these did not count as false-negatives!
A positive FAST is helpful but a negative one…not so much.
What do I do?
What I am really interested in is the Negative Predictive Value of the test i.e. the chance that if my scan is NEGATIVE there is NO free fluid. Unfortunately a negative scan, in isolation does not tell me that there is not a significant intra-abdominal injury. In the setting of a worrying mechanism (e.g. handlebar versus spleen) with bruising and tenderness to the left upper quadrant and a NEGATIVE fast I cannot say that the child is okay and send them home. This is the concern that I have. That the test will stop the less astute clinician from thinking.
One has to be be very wary when interpreting the literature surrounding FAST scans in paediatrics. All the scan tell you is that there is no free fluid. If the patient is haemodynamically stable and there is suspicion of an intra-abdominal injury then the patient should have a CT.
Haemodynamically stable patients
In the haemodynamically stable patient with an unconcerning physical exam, good quality images on a comprehensive abdominal ultrasound and the ability to serially examine the patient then a CT may not be warranted. A comprehensive abdominal ultrasound is NOT the same as FAST.
One might think that use of ultrasound might have other benefits but a large study by Holmes et al. in 2017 showed no alteration in the number of CT scans requested, number of patients hospitalized or requiring surgery.
Haemodynamically unstable patients
These patients need resuscitation, often with blood products, until they are stable enough to enter the CT scanner/IR suite. A FAST scan is likely to be positive but given that over 90% of intra-abdominal injuries in children are managed without going to theatre it is unlikely to change my management.
Whilst this is clearly not a comprehensive review, any collection of data that has such a wide range of specificity needs to be considered. I could add another 10 studies and they might tighten up my spread but in the largest trials, involving ED physicians we are just not that great.
So the bottom line, when taken in isolation, as I see it is this best case/worst case…
Thanks to Arun Ilancheran and Ross Fisher for pushing me down this rabbit hole.
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