To be more specific, most tests are designed around venous blood samples. Theranos' finger-prick would gather blood from capillaries in the finger, where the ve

To be more specific, most tests are designed around venous blood samples. Theranos' finger-prick would gather blood from capillaries in the finger, where the vessels are very narrow (changing the balance between blood cells and plasma) and also where diffusive processes into and out of cells are extremely active - and so assays like blood glucose level may be significantly off from what you would see in a venous draw, if for example your finger is curently burning a lot of energy ...