Free intraperitoneal fluid (summary)

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This is a basic article for medical students and other non-radiologists

Free intraperitoneal fluid may be termed free fluid or (less correctly) free intra-abdominal fluid. It may be seen in small volumes in female patients, particularly around the time of menses and in some healthy young men. When free fluid is present in large amounts it is usually called ascites.

Reference article

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Summary

  • pathophysiology
    • free fluid may be physiological in female patients
      • usually maximal around ovulation
    • inflammation in the abdomen
    • intra-abdominal sepsis
    • hemorrhage in trauma
  • investigation
    • US abdomen
      • free fluid can be seen at ultrasound
    • CT abdomen
      • CT is more sensitive for generalized free fluid
      • useful for assessing location 
      • associated findings may narrow differential
        • fat-stranding in inflammation

Radiographic features

Ultrasound

Ultrasound is variably sensitive depending on the size of the patient and the operator. In thin patients, relatively small volume of fluid can be found. Clear fluid without internal echoes is likely to be reactive.

Low-volume free fluid in the pouch of Douglas is often seen in female patients of child-bearing age and is often physiological and of no clinical significance.

CT

Fluid on CT is relatively hypodense (dark). It can be compared to fluid in the gallbladder or stomach. Dense fluid may suggest hemoperitoneum, especially in the context of trauma.

Fluid may sit within the peritoneal space or paracolic gutters or may be interposed between bowel loops or around solid organs, e.g. the liver.

Medical student radiology curriculum