The FAST exam: overused and overrated?

FAST exam overused overrated
Cite this article as:
Morgenstern, J. The FAST exam: overused and overrated?, First10EM, August 30, 2021. Available at:

Might the FAST exam be the most overused test in emergency medicine? If not the most overused (it does have to compete with the white blood cell count after all), perhaps it is the most overrated? Considering the sacrosanct position ultrasound holds in emergency medicine, even asking this question might get me in trouble. However, all tests come with risks and benefits. All tests have false positives and false negatives. I have seen numerous patients harmed by seemingly unnecessary FAST exams, and it makes me wonder whether this test (as it is currently being used) is truly providing a net benefit in our patient population. 

The official role of the FAST exam in trauma is to identify patients with hemorrhagic shock who require immediate transfer to the operating room.1,2 “The aim is to identify life-threatening intra-abdominal bleeding or cardiac tamponade with a view to expediting definitive surgical management. It does not aim to exclude abdominal or thoracic injury.”1

Despite this clear vision from ultrasound experts, this is not how the FAST exam is actually being used. We don’t reserve it for the sickest patients; we use it on everyone. We scan patients with normal vital signs. We scan patients without abdominal symptoms. The vast majority of patients being scanned clearly don’t require a surgical procedure, let alone “immediate transfer to the operating room”. In contrast to the clearly stated aims, we are scanning to rule out disease, not rule it in. 

Unfortunately, the FAST exam is an imperfect test. The sensitivity is reported to be between 41% and 95%, so the FAST exam cannot be used to rule out intra-abdominal injuries.2,3 Furthermore, although the FAST exam is very specific, in hemodynamically stable patients, most trauma experts suggest performing a CT after a positive FAST scan to confirm and further delineate injuries. Thus, in hemodynamically stable patients, a CT scan is recommended whether the FAST scan is positive or negative, and therefore “the use of focused assessment with sonography for trauma in hemodynamically stable blunt trauma patients seems not worthwhile. It should be reserved for hemodynamically unstable patients with blunt trauma.”2 Other experts agree, stating that “FAST is a rule-in triage tool in patients with [blunt abdominal trauma], not a true diagnostic tool…. and we must question whether FAST represents a productive use of health resources in terms of equipment and training.”3

In keeping with its poor diagnostic characteristics, there is no evidence that the FAST exam improves clinical outcomes. A Cochranre review identified 4 studies, and the FAST exam was not associated with any improvement in mortality (RR 1.00; 95% confidence interval 0.5 to 2.0). They conclude that “the experimental evidence justifying FAST‐based clinical pathways in diagnosing patients with suspected abdominal or multiple blunt trauma remains poor.”4

The FAST exam has not been well validated in a pediatric population, but appears to be even less accurate than in adults.5 A 2007 meta-analysis found that the sensitivity of ultrasound for intra-peritoneal fluid in children was only 66%.6 Almost all of the included studies involved formal ultrasounds read by radiologists, so the sensitivity is likely to be much worse in the hands of a community emergency physician. Studies of the FAST exam performed by emergency physicians in a pediatric population reveal a sensitivity between 28% and 52%.7–9 Furthermore, it is not clear that identifying free fluid is helpful in the pediatric population, as children can have free fluid without any injury, and also frequently present with injury in the absence of free fluid.10 

In keeping with the poor diagnostic accuracy of the FAST exam in a pediatric population, the clinical evidence does not support a benefit. There is a randomized clinical trial looking at the use of the FAST exam in a pediatric population, and they conclude that “among hemodynamically stable children treated in an ED following blunt torso trauma, the use of FAST compared with standard care only did not improve clinical care, including use of resources; ED length of stay; missed intra-abdominal injuries; or hospital charges. These findings do not support the routine use of FAST in this setting.”9

Real world experience seems to support this conclusion. One large observational study in pediatric trauma concludes that “true positive FAST exams are uncommon and would rarely direct management. While the negative FAST would have potentially reduced CT use due to practitioner reassurance, this reassurance may be unwarranted given the test’s sensitivity.”11

With such underwhelming evidence, how did the FAST exam come to be thought of as a “standard of care”? Aside from the cool factor of being able to peer inside patients’ bodies, I think the overuse of ultrasound arises from our general misunderstanding about the harms of tests. We are all keenly aware of direct harms, such as the radiation from CT, but massively underestimate the harms of being misled by our tests. Humans are generally bad at grasping probabilities, and we don’t properly account for the effects of false positives and false negatives on our patient’s outcomes.

“This is a 16 year old who fell off her bike. Her vitals are normal, and she has no pain on exam, but I think I saw some free fluid on the FAST exam, so we are waiting on the CT.” Over the past few years, I have received this general hand-over many times. The CT has been negative in every single case. Clinically, it was obviously that imaging was unnecessary, but indiscriminate use of the FAST exam generates false positives. Young children received unnecessary radiation. Eventually, incidental findings will lead to unnecessary invasive procedures and even more harm. Ultrasound may not directly harm our patients, but our failure to understand its test characteristics, and adjust for pretest probability, certainly does. 

What about the FAST exam in non-trauma situations? I recently cared for a woman in her 20s who presented with severe pelvic pain, a positive home pregnancy test, a heart rate of 140, and a systolic blood pressure below 90. Within 30 seconds of arrival, an ultrasound allowed me to identify free fluid in her abdomen, and obstetrics was involved in her care less than 5 minutes after arrival to the hospital. It certainly felt like the FAST exam might have saved her life.

However, the more I reflect on this case, the more I realize that I might be trapped in a technological mirage. Imagine I was caring for this same patient in the era before ultrasound. Would anything have changed? Even before I put the ultrasound on her belly, I had a young pregnant female with severe pelvic pain and hemodynamic instability. I already knew the diagnosis. The ultrasound might have made me or the obstetrician more comfortable, but did it really change the patient’s management? I am fairly certain that even without the ultrasound, I still would have called the obstetrician within 5 minutes.

Many ultrasound results fall into these classic diagnostic ranges of futility. Positives that were obviously positive based on clinical findings before ever turning on the ultrasound machine. Negatives were obviously negative. This exposes the limits of anecdote, and the need for high quality science, when assessing the value and impact of bedside ultrasound in emergency medicine.

Ultrasound is an amazing tool in modern emergency medicine. I use it almost every shift. I think it has made me a better diagnostician. The procedures I perform are quicker, more successful, and safer with an ultrasound in my hand. But I think we have gone too far. When I discuss these cases with colleagues, ultrasound is assumed to be a universal good. When I suggest ultrasound might not be appropriate in hemodynamically stable, asymptomatic patients, I encounter surprise and disbelief. I have been told ultrasound is the standard of care in such patients, despite the clear lack of evidence and potential for harm. 

Bottom line for the FAST exam

The evidence of benefit is non-existent, but I think there is clearly a role for the FAST exam in hemodynamically unstable patients. However, even among the hemodynamically unstable, the role may be more limited than you think. It is essential to consider your resources, and how the test will change your management. In a trauma center, when you are deciding between the operating room and interventional radiology, the FAST exam may provide invaluable information. In the community, where most of us work, such treatment options don’t exist, and the FAST exam may just be delaying the transfer the patient needs.

In hemodynamically stable patients, I think the available data is pretty clear: the FAST exam shouldn’t be used. The sensitivity is not high enough to rule out intra-abdominal injuries. If you are concerned about a patient, CT is the imaging of choice, and a negative ultrasound may just falsely reassure you. The specificity of the FAST exam is excellent, but many hemodynamically stable patients will be managed non-operatively, so essentially all of these patients are going for CT as well. In other words, whether your FAST exam is positive or negative, the patient still needs a CT, which is the definition of a useless test.  

Other FOAMed

Tagg, A. Thinking FAST, and slow, Don’t Forget the Bubbles, 2018. Available at:

Carley, S. On a FAST track to nowhere at St.Emlyn’s. St. Emlyn’s Blog, 2013. Available at:

An opposing view is available in this talk from the amazing Vicki noble:


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Cite this article as:
Morgenstern, J. The FAST exam: overused and overrated?, First10EM, August 30, 2021. Available at:

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