Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
POCUS for Thoracoabdominal Injury
Significant thoracic injuries or abdominal free fluid 18.5 (95% CI 10.8-40.5)

Negative Findings (Patient Doesn't Have This)

Finding Decreased Disease Probability (Negative Likelihood Ratio)
POCUS for Thoracoabdominal Injury
Significant thoracic injuries or abdominal free fluid 0.27 (95% CI 0.19-0.37)

Source: Stengel D, Leisterer J, Ferrada P, Ekkernkamp A, Mutze S, Hoenning A. Point-of-care ultrasonography for diagnosing thoracoabdominal injuries in patients with blunt trauma. Cochrane Database Syst Rev. 2018 Dec 12;12:CD012669.

Narrative: Trauma is a major cause of morbidity and mortality, representing one of the top ten causes of both death and disability-adjusted life years by the World Health Organization.1, 2 Point-of-care ultrasound (POCUS) is commonly performed during or after the primary survey to identify whether significant thoracic injuries or abdominal free fluid are present, particularly when patients are unstable or cannot receive a computed tomogram (CT).3 However, it is important to determine the accuracy of this modality to ensure proper application in trauma patients.

The Cochrane Review discussed here4 included retrospective and prospective studies assessing the diagnostic accuracy of POCUS for thoracoabdominal injuries in patients with blunt trauma (defined as any non-penetrating force). The reference standard included CT scan, magnetic resonance imaging (MRI), laparotomy or laparoscopy, thoracotomy, or autopsy. The primary outcome was the diagnosis of any thoracoabdominal injury, which was defined as: free fluid in the thoracic or abdominal cavity, retroperitoneum, pericardium, or mediastinum; organ injury (e.g. splenic, other solid organ, hollow viscera, or other organ laceration); a vascular lesion (e.g. dissection of rupture of aorta or other vessels); and other injuries (e.g. pneumothorax). Subgroup analyses were performed for pediatric patients versus adult patients, as well as abdominal versus thoracic injury.4 Approximately, half of the trials were conducted in the United States, and half of the study subjects were enrolled in level 1 trauma centers.

The authors of the Cochrane review identified 2872 records, of which 34 studies (n = 8635 patients) met inclusion criteria.4 Overall, POCUS was 74% sensitive (95% confidence interval [CI], 65% to 81%) and 96% specific (95% CI, 94% to 98%) with a positive likelihood ratio (LR+) of 18.5 (95% CI, 10.8 to 40.5) and a negative likelihood ratio (LR-) of 0.27 (95% CI, 0.19 to 0.37). Among pediatric patients, POCUS was 63% sensitive (95% CI, 46% to 77%) and 91% specific (95% CI, 81% to 96%). In adults alone, POCUS was 78% sensitive (95% CI, 69% to 84%) and 97% specific (95% CI, 96% to 99%). POCUS was 68% sensitive (95% CI, 59% to 75%) and 95% specific (95% CI, 92% to 97%) for diagnosing abdominal injuries specifically. For thoracic injuries, POCUS was 96% sensitive (95% CI, 88% to 99%) and 99% specific (95% CI, 97% to 100%). Assuming an overall baseline pretest probability of thoracoabdominal injury of approximately 28% (based on the median prevalence of such injuries in all of the included studies), POCUS would hypothetically miss injuries in 7.3% of patients and falsely suggest the presence of injuries in 2.9% of the patients. Assuming a pretest probability of thoracoabdominal injury of approximately 31% in pediatric patients (based on the median prevalence of such injuries in the included studies), the miss rate (false negative) would increase to 11.8%, and the false positive rate would be 6.2%.4

Caveats: The overall quality of the trials included in this meta-analysis was unclear due to limited reporting of the selection of participants and choice of diagnostic testing used for the reference standard. The most important limitation of the original review was the inclusion of all organ injuries (rather than free fluid) in the outcome assessment. The American College of Emergency Physicians guidelines states that the primary indication of the FAST examination is to “identify pathologic collections of free fluid or air released from injured organs or structures.”3 Consequently, assessing for the presence of any organ injury is beyond the scope of the FAST examination. Additionally, it is unclear how many injuries were significant enough to require an intervention. In this case, while a solid organ injury may have been missed with the FAST exam, it may not have been clinically significant. Therefore, it may have been preferable to focus on the identification of intraperitoneal free fluid and clinically significant organ injuries.

Furthermore, a number of different gold standards were used, which can lead to differential verification bias. There was also significant heterogeneity with regard to both the patient and study characteristics. Training protocols also varied and were not explicitly defined in most studies. Additionally, provider experience and specialty, as well as the POCUS machine utilized differed significantly between studies. These factors may have further contributed to the heterogeneity between the trials. Moreover, most studies assessed abdominal injury and only four studies assessed thoracic injury. Finally, there was limited reporting of multiple methodological parameters, including how studies accounted for inconclusive results.4

While the trials included in the meta-analysis do not report the harms associated with using POCUS for identifying thoracoabdominal injuries, false positive findings might subject the patients to unnecessary diagnostic or therapeutic procedures. False negative findings might falsely reassure the providers and cause them to miss injuries. The level of training and the experience of the operators might reduce the risk of such harms. Future trials are needed to assess the risk of such harms and identify the methods by which they could be reduced.

Based on the existing evidence, POCUS appears to be highly sensitive and specific for identifying significant thoracic injury. Additionally, POCUS appears to be highly specific but insufficiently sensitive to exclude abdominal injury. This suggests that a positive POCUS is strongly suggestive of an abdominal injury and should prompt subsequent targeted intervention, particularly in unstable patients. However, a negative POCUS examination does not exclude significant injury and should be followed by advanced imaging (e.g. CT), especially in pediatric patients. Keeping the limitation of this diagnostic modality in mind, this non-invasive and rapid test could provide useful information to the clinician and guide the proper diagnosis and treatment. Therefore, we have assigned a color recommendation of Green (Benefit > Harm) to the use of POCUS for identifying thoracoabdominal injuries after blunt trauma.

The original manuscript was published in Academic Emergency Medicine as part of the partnership between TheNNT.com and AEM.

Author: Michael Gottlieb, MD; Alex Koyfman, MD; Brit Long, MD

Published/Updated: March 15, 2019

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

(Thank you to Seth Trueger, MD for this explanation!)