A negative FAST in an unstable trauma patient with a pelvic binder
A 34-year-old man is brought in after a high-speed motorcycle collision. Blood pressure is 82/50 mmHg, heart rate 128/min, and he has a pelvic binder in place for a clinically unstable pelvis. He has received 2 units of packed red cells. You perform a FAST examination: there is no free fluid in Morison's pouch, the splenorenal recess, or the pelvis. The subxiphoid view is obscured by bowel gas and the bladder is empty because of a urinary catheter. The trauma team asks whether the negative FAST means they can avoid CT.
1. How do you interpret the negative FAST in this haemodynamically unstable patient?
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A negative FAST means only that no significant free intraperitoneal fluid was detected at that moment. It does not exclude haemorrhage.
Key points:
- FAST sensitivity for hemoperitoneum is commonly quoted around 60–90%, and falls for small volumes (often under 200–500 mL), retroperitoneal bleeding, and an empty bladder.
- This patient's shock may be driven by a retroperitoneal pelvic fracture bleed, which FAST cannot see.
- Clotted blood can be isoechoic and difficult to identify.
- A single negative scan in an unstable patient has a high false-negative risk; the clinical trajectory outranks the image.
So a negative FAST does not justify withholding CT in a patient who is stable enough to go, nor does it delay laparotomy or angio-embolisation in a patient who remains unstable.
2. Which limitations and technical factors could produce a false-negative study here, and how would you optimise the windows?
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Limitations in this case:
- Retroperitoneal haemorrhage from the pelvic fracture is not accessible to FAST.
- Empty bladder after catheterisation makes the pelvic view difficult and may hide small amounts of fluid.
- Bowel gas obscures the subxiphoid window.
- Obesity, subcutaneous emphysema, and abdominal tenderness limit compression.
- Clot can be isoechoic to adjacent organs.
- Operator experience and a single time-point are both limiting.
Optimisation:
- Obtain Morison's pouch and splenorenal views in the coronal plane, using the liver and spleen as windows.
- Use an intercostal or left lateral decubitus approach when the subxiphoid view is obscured.
- For the pelvis, scan in transverse and sagittal planes above the pubis; a catheter does not prevent seeing free fluid in the rectovesical pouch.
- Include the pericardial view.
- Repeat the examination after fluid resuscitation or after 30 minutes, and compare with the patient's haemodynamics.
- Do not spend so long optimising that definitive care is delayed.
3. What do you recommend to the trauma team?
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Recommend a repeat FAST and CT if the patient can be stabilised enough to leave the resuscitation room. The pelvic binder and mechanism make a retroperitoneal bleed likely, and CT will define pelvic and solid-organ injury. If he remains profoundly unstable, proceed to laparotomy or angio-embolisation and resuscitate with blood products rather than relying on the FAST result.
Document that the study was negative at a specific time, that the subxiphoid view was limited by bowel gas, that the bladder was empty, and that the pelvic fracture is a recognised blind spot. Re-examine after resuscitation. The decision should be driven by physiology and mechanism, not by a single negative image.