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To test a local red spot for dilated
capillaries:
- For several seconds press directly on
the red area with the ball of your thumb. This forces the
capillaries to be squeezed shut, excluding blood from
them.
- Quickly pull your thumb away, and
watch the color of that spot. If the redness is being
caused by high capillary blood flow because they are
dilated, at the moment you pull your thumb away, the spot
will appear "white."
- Over a couple of seconds or less, the
redness will "flow" back to the spot.
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