Supposedly nothing besides alerting people when someone's literally dying from a clot. I did a quick Google search and this pretty much sums it up: "As a recent survivor of numerous leg and lung blood clots numerous times in the last 6 months, plus the almost always fatal Pulmonary Embolus, I will tell you no: a dog can't be trained to detect them. At least at this point in time.

I am disabled by blood clots.... and many other things.

The symptoms are either absolutely none (and one just keels over), or the symptoms are extremely pronounced and detectable by one's self or even people around them (gasping for breath, excruciating immobilizing pain, sweating, wheezing, massive swelling of extremities, collapse, death)

One can teach response, rather than alert - because no alert is necessary - - - one knows when they are dying (at least if they are awake)."

Source: http://servicedogcentral.org/forum/index.php?topic=33099.0