How is blood drawn from a patient on whom the lab technician or physician cannot find a suitable vein?
Assuming, whether because of obesity or any other factor, a suitable vein cannot be accessed in the patient's arm, even with the tightening of the tourniquet, and a warm damp cloth on the site where blood would normally be drawn [heat dilates veins and makes them more prominent] has not helped, then the next option is the top of the hand, where veins are usually more easily accesible, even on overweight indivdiduals. Feet also can provide access to a suitable vein, although, again, excessive fat can make even that difficult, and use of the feet requires the physician to draw the blood rather than a technician or nurse.
In obese patients, the only vein that medical personnel can usually access is the cephalic vein, which runs the length of the arm and into the shoulder, and is one of the more shallow veins, in other words, runs closest to the surface of the skin. If this vein cannot be located, and the hands and feet are not viable options, then the patient has a difficult decision to make regarding the medical necessity for the blood specimen to be drawn.