SonoStudy: False negative FAST scans: association with patient characteristics/injuries/outcomes?

A study recently published in Annals of Emergency Medicine by Laselle et al attempt to estimate associations between false negative FAST results and patient characteristics, specific organ injuries, and patient outcomes by doing a retrospective analysis of consecutive patients who had a blunt abdominal trauma with pathologic free fluid found by CT, DPL (yeah, I know, weird), laparotomy, or autopsy (ouch!). Over 300 enrolled and 162 had a false negative FAST scan.

Now before you say what I think you will say, allow me to state one thing (ok, maybe 4 things after this one thing): It’s pretty darn tough to study FAST scans and their results. Why? I do have a prior post that talks about this, but let’s state some important things here. The definition of a true positive and negative differs from study to study – for one. Secondly, the amount of fluid seen on a FAST scan is more than the amount of fluid seen on CT (we need at least 300 cc to see it – that is, if we, the sonographers, are rockstars at it too). I know some even say 200cc, but come on, let’s be real. Third, and this is in no particular order, the time to FAST may be far less than what is needed for a slow bleed to be appreciated and get up to that 300 or more cc of fluid needed, but the time to CT could be enough for it to see the 100 cc of fluid it needs. Oh yeah, and was the patient supine? were the lights off? was it a complete FAST scan or a half-ass FAST? Look out for a post coming soon on how to truly complete your FAST scan. Ok, I feel better, lets talk about the study….

What they found: “Patients with severe head injuries and minor abdominal injuries were more likely to have a false-negative than true-positive FAST result. On the other hand, patients with spleen, liver, or abdominal vascular injuries are less likely to have false-negative FAST examination results. Adverse outcomes were not associated with false-negative FAST examination results, and in fact patients with false-negative FAST result were less likely to have a therapeutic laparotomy.”

After reading through the article, my thoughts were – ok, well, this is a good thing. Not surprising that a severe head injury will bring the focus to that and it’s great that the serious abdominal injuries (that cause free fluid to develop) were not missed. Most importantly, and a point that has been studied quite well in varying ways by various people, is that the false negative FAST was not associated with adverse outcomes. It also supports the recent JAMA study that was done by James Holmes et al. to “systematically assess the precision and accuracy of symptoms, signs, laboratory tests, and bedside imaging studies to identify intra-abdominal injuries in patients with blunt abdominal trauma” that found that “Bedside ultrasonography has the highest accuracy of all individual findings, but a normal result does not rule out an intra-abdominal injury.”

Of course, there have been other studies trying to see why a FAST would be negative and whether we should even do it for the hemodynamically stable patients:

One study by Hoffman et al studied this stating : “the clinical characteristics of the 79 FAST+/Confirmatory test+ examinations were compared to those of the 53 FAST-/Confirmatory test+ examinations. The presence of a radiographically proven pelvic fracture (odds ratio 3.459; 95% confidence interval of 1.308-9.157) and a radiographically or operatively proven renal injury (odds ratio 3.667; 95% confidence interval of 1.013-13.275) were found to be significant predictors. The presence of a pelvic fracture or renal injury in adult victims of blunt abdominal trauma increases the likelihood of a US-/Conf+ examination. Patients with a negative FAST examination and pelvic fracture may benefit from additional radiographic or operative evaluations for occult injuries.”

Another study in Surgery in 2010 stated that ” It is still questionable, however, whether its use results in the underdiagnosis of intra-abdominal injury. It also remains doubtful whether a positive focused assessment with sonography for trauma affects clinical decision making in hemodynamically stable blunt trauma patients as evidenced through abdominal computerized tomography use.” Good question, so they they aimed to evaluate the results of FAST in hemodynamically stable blunt trauma patients and to determine its role in the diagnostic evaluation of these patients. They (with their biases, of course) state that “118 false negative FAST were performed, of which 44 (37.3%) subsequently required exploratory laparotomy. Five patients had false positive FAST scans. FAST had an overall sensitivity of 43%, a specificity of 99%, and positive and negative predictive values of 95% and 94%, respectively. Accuracy was 94.1%. In the hemodynamically stable blunt trauma group, there were 60 patients with true positive FAST and 87 patients with false negative FAST. In this group of patients, FAST had a sensitivity of 41%, specificity of 99%, and positive and negative predictive values of 94% and 95%, respectively. The overall accuracy was 95%. So, they concluded that: “Given the low sensitivity, a negative FAST without confirmation by computerized tomography may result in missed intra-abdominal injuries. It is also observed in all FAST positive hemodynamically stable blunt trauma patients, confirmation is preferred through the use of a computerized tomography for better understanding of the intra-abdominal injuries and to decide on operative versus no-operative management. Thus, the use of FAST in hemodynamically stable blunt trauma patients seems not worthwhile. It should be reserved for hemodynamically unstable patients with blunt trauma.”

If you wonder why I refer to this study, it’s because of what our surgeons read and what many think. To me, the more information the better with a good understanding of FAST limitations. I’d like to know if the FAST is positive (my concern goes up, my suspicion goes up, Ill order blood, I’ll make sure to obtain good IV access, I’ll watch the patient much more closely, the admission will likely be the ICU and not the floor, etc etc), and I know what to think when the FAST is negative too – as I correlate with my physical exam and my pre-FAST suspicion given the mechanism of injury – all while knowing that outcome will not change. They also “confirm a positive FAST by DPL in a few cases” which unless it’s a metastatic cancer patient or a liver patient with ascites, why? really, why? They also take into consideration the hemodynamically stable patients with positive FAST scans – if it wasn’t positive, would they have gotten the CT? Hmmmm…..I argue with my surgeon frieds all the time about it – it’s fun actually, especially when it involves good wine, but we always end with “agree to disagree.”

A great post by Jeremy Jaramillo on FAST with great references, here.

One thought on “SonoStudy: False negative FAST scans: association with patient characteristics/injuries/outcomes?

  1. Pingback: Siêu âm Choáng Chấn thương mở rộng : eFAST !!! | Cấp Cứu Amateur

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