Drawing blood from a foot?

Hey guys. I'm a new grad RN working on a Med-Surg unit. I was taking care of a woman on a heparin drip. She had a portal vein thrombosis and DVT in the left thigh. I needed an anti-Xa level on my patient around 2300. I get in report that she's an incredibly hard stick, she has a low IV site on the right arm for the heparin infusion (policy is that we can't draw above the IV site). I go in there searching for veins, she's bruised all over and after 1 failed attempt I get some blood from a bruised site in her left arm. Poor thing. I tube the blood to the lab around 2230, call them back around 2300 for the result and they tell me they can't find the blood specimen! GREAT. Round 2. I go back in...I fail, I get two other nurses to draw and they both fail. We call the ED to see if they can send an experienced tech up but they can't. I call the doctor and he tells me to try her feet. Fortunately, this works! But the resource nurse tells me write a verbal order from the doctor that it was OK to draw blood from her foot x 1 (on the non-DVT leg of course). I asked other nurses why this was--they said drawing blood from the foot can increase the risk of DVT? I figured the lab results might be less accurate from the feet but I hadn't considered DVT. Should it be worrisome that she already had a DVT in the other leg? Does anyone know why drawing blood from the foot is a last resort? (The patient is getting a PICC line because she's such a difficult stick!) Thanks!