The seriousness of a blood clot "depends on where it is," said Dr. Gholam Motamedi, a neurologist at Georgetown University Medical Center who was not involved in Clinton's care.
Clots in the legs are a common risk after someone has been bedridden, as Clinton may have been for a time after her concussion. Those are "no big deal" and are treated with six months of blood thinners to allow them to dissolve on their own and to prevent further clots from forming, he said.
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.
Keeping Clinton in the hospital for a couple of days could allow doctors to perform more tests to determine why the clot formed, and to rule out a heart problem or other condition that may have led to it, he said.
Dr. Larry Goldstein, a neurologist who is director of Duke University's stroke center, said blood can pool on the surface of the brain or in other areas of the brain after a concussion, but those would not be treated with blood thinners, as Clinton's aide described.
Clinton, 65, was forced to cancel Dec. 20 testimony before Congress about a scathing report into the Sept. 11 attack on the U.S. diplomatic mission in Benghazi, Libya, that killed Ambassador Chris Stevens and three other Americans. The report found that 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.
Some conservative commentators suggested Clinton was faking the seriousness of her illness and concussion to avoid testifying, although State Department officials vehemently denied that was the case.
Lawmakers at the hearings — including Sen. John Kerry, D-Mass., the Senate Foreign Relations Committee chairman who has been nominated by President Barack Obama to succeed Clinton — offered her their best wishes.
Last Thursday, before the discovery of the blood clot, Reines said Clinton was expected to return to work this week.
The former first lady and senator, who had always planned to step down as America's top diplomat in January, is known for her grueling travel schedule. She is the most traveled secretary of state in history, having visited 112 countries while in the job.
Clinton is considered a front-runner for the Democratic presidential nomination in 2016, although she has not announced plans to run. Clinton has long been an advocate of healthcare reform and chaired a task force during the Clinton administration to develop a plan for universal healthcare. The plan was widely condemned by the insurance and drug industries and was eventually abandoned.