The AP statement is confusing.
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Update: Per Huffington Post, the clot was discovered after a follow up exam earlier today. She was admitted to NY-Presbyterian Hospital for 48-hour monitoring.
http://www.huffingtonpost.com/huff-wires/20121230/us-clinton/?utm_hp_ref=homepage&ir=homepage
Thanks for the updates.
(I should have read the whole thread from the beginning.)