1. Remember that patients can develop intraperitoneal free fluid from a variety of etiologies. Use the principles and concepts of the FAST exam to help you in situations where you are trying to rule-out free fluid from non-traumatic causes, too.
2. Blood appears anechoic (black) when acute and free flowing, but hypoechoic (gray) when subacute or clotted. When blood is gray it is much easier to miss, especially if you are not looking for it. Always obtain multiple views and perform serial scans when you can.
3. The sensitivity of the FAST exam improves with Trendelenberg positioning, repeat imaging, and if you know how and where to look (experience). Always image the inferior tip of the liver, as this area may be positive when Morrison’s pouch is not. Likewise, blood can also accumulate near the inferior border of the spleen, even when the splenorenal recess appears normal. Turn down the gain on the pelvic view if the bladder is full, otherwise the enhancement behind the bladder may “white out” a small pocket of fluid. If you think you see a small sliver of free fluid, reposition the patient to see if the anechoic area changes or fluctuates. The sensitivity of the FAST for blood is ~85% if serial exams are performed, but it can be as low as 24% in some studies. Usually >200ml of fluid can be detected
4. The specificity of the FAST exam is about 95%. False positives may include fluid mimics such as the prostate, psoas, a perinephric fat pad, severe hydronephrosis or a large blood vessel. They may also include other causes of free fluid such as ascites, urine, physiologic fluid, and inflammatory fluid from infection, pancreatitis or ischemic bowel. Remember that it is beyond your scope of practice to rely on ultrasound to help you determine what type of fluid you see in the peritoneum. Bedside ultrasound should only be used to tell you whether or not free fluid is present. Always correlate your bedside ultrasound findings with the physical exam and clinical picture.
Brady Pregerson manages a free on-line EM Ultrasound Image Library and is the author of the Tarascon Emergency Department Quick Reference Guide. For more information visit www.EMresource.org.
Teresa S. Wu is the Associate Residency Director, and Director of Ultrasound and Simulation Based Training for the Maricopa Emergency Medicine Program in Phoenix, Arizona.