Secretary of State Hillary Clinton admitted to hospital with blood clot following concussion (+video)

Clinton returned to the U.S. from a trip to Europe, then fell ill with a stomach virus in early December that left her severely dehydrated and forced her to cancel a trip to North Africa and the Middle East. Until then, she had cancelled only two scheduled overseas trips, one to Europe after breaking her elbow in June 2009 and one to Asia after the February 2010 earthquake in Haiti.

Her condition worsened when she fainted, fell and suffered a concussion while at home alone in mid-December as she recovered from the virus. It was announced on Dec. 13.

Reines, her spokesman, said her doctors discovered the clot Sunday while performing a follow-up exam on the concussion. He said she was being treated with anti-coagulants and would remain at New York-Presbyterian Hospital until at least Tuesday so doctors can monitor the medication.

This isn't the first time Clinton has suffered a blood clot. In 1998, midway through her husband's second term as president, Clinton was in New York fundraising for the midterm elections when a swollen right foot led her doctor to diagnose a clot in her knee requiring immediate treatment.

Medical experts said the seriousness of a blood clot diagnosis varies widely based on where it is located.

Clots in the legs are a common risk after someone has been bedridden, as Clinton may have been for a time after her concussion in December. Those are "no big deal" and are treated with blood thinners, said Dr. Gholam Motamedi, a neurologist at Georgetown University Medical Center who is not involved in Clinton's care.

But a clot in a lung or the brain is more serious. Lung clots, called pulmonary embolisms, can be deadly, and a clot in the brain can cause a stroke, Motamedi said.

Last Thursday, before the blood clot was discovered, Reines said that Clinton expected to return to work this week after the New Year holiday.

Beyond talk of future politics, Clinton's three-week absence from the State Department has raised eyebrows among some conservative commentators who questioned the seriousness of Clinton's ailment after she cancelled planned Dec. 20 testimony before Congress on the deadly attack on the U.S. diplomatic mission in Benghazi, Libya.

Clinton had been due to discuss with lawmakers a scathing report on the attack she had commissioned that found serious failures of leadership and management in two State Department bureaus were to blame for insufficient security at the facility. Clinton took responsibility for the incident before the report was released, but she was not blamed. Four officials cited in the report have either resigned or been reassigned.

Associated Press writer Ken Thomas in Washington and AP Chief Medical Writer Marilynn Marchione in Milwaukee contributed to this report.