This is a new series focusing on important pitfalls in using POCUS. Sometimes the most important knowledge of using a specialized skill set or tool is a robust understanding of the possible pitfalls including common false positives or negatives.
The case report here from JEM brings up a great point we often overlook. Be weary of a patient with prolonged or high volume resuscitation prior to performing a FAST exam. Transudative 3rd spacing within the peritoneum is common and can lead to a false positive FAST. Both blood (fresh, unclotted) and transudate (or ascites which could be pre-existing) appear dark, hypo echoic.
Now ... the case is clearly a devastatingly sick girl, who was persistently hypotensive so its hard to argue against taking her to the OR after a positive repeat FAST. However, in the setting of a poor neurologic exam, and an echo later revealing a poor ejection fraction (10%) one may have hesitated to take her to the OR even with positive FAST given the understanding of POCUS pitfalls in conjunction with other clinical information and the risks of operating on a profoundly ill patient.
Keep in Mind #1: FAST pitfalls. False positives after high volume fluid resuscitation. Therefore, get a good, thorough prehospital resuscitation history and ensure a detailed transfer of care if you are sending a patient from your facility to a trauma center, or if you'll be the one accepting.