Extravascular fluid will accumulate in soft tissues. It tends to be pulled by gravity into dependent parts of the body.
The volume of extravascular fluid required to produce detectable dependent edema in the average adult is about 4.5 liters (associated with about a 10 pound weight gain).
To measure dependent edema the thumb is firmly pressed against skin over underlying bone for several seconds.
Grading is based on:
(1) the depth that the thumb can depress the area, measured in mm or finger diameter
(2) the length of time that the thumb imprint ("pit") remains after pressure is released
In severe ("pitting") edema the "pit" is deep and persists for some time.
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Degree of Edema
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Grade
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none
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0
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minimal
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+/-
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mild
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1+
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moderate
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2+
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severe (marked)
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3+
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extreme (very marked)
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4+
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The degree of edema can be more accurately followed by measuring the circumference of a limb at various points. However, this requires measurements taken at the same spot each time.
The degree of edema will tend to increase the longer that a part is dependent. It tends to decrease the longer that the edematous body part is elevated.
If the lower extremities are being examined then it is important to determine if the edema is unilateral or bilateral. This can help direct the investigation into the cause of the edema.
Prolonged edema may be associated with subcutaneous and dermal fibrosis, which makes the area resistant to thumb pressure. This is described as "brawny" (muscular) edema.
