http://depts.washington.edu/physdx/liver/tech.html
The patient is examined in the supine position.
Direct percussion is done over the abdomen, from the umbilicus to the flanks.
The location of the transition from tympany to dullness is noted.
Positive test: Percussion note is tympanitic over the umbilicus and dull over the lateral abdomen and flank areas
Note: The tympany over the umbilicus occurs in ascites because bowel floats to the top of the abdominal fluid at the level of the fluid meniscus.

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Note added at 7 hrs (2012-07-15 04:45:01 GMT)
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Flank dullness is elicited by percussion of the abdominal wall starting at the periumbilical region and going outwards to the dependent areas of the flanks. If ascites is present, there is a change from tympany to dullness.