Our expert says:
If they always put a stretchy band around your arm when they draw blood, ask them to use a blood pressure machine cuff next time and to pump it up between systolic and diastolic pressure - if the pressure is too high around the arm especially in someone who tends to have low BP it is difficult to get blood out, as the inflowing blood is blocked. You may also have narrow veins, so the negative suction pressure of the blood tube could be too high and the vein collapses - then it may help to draw blood with a needle and syringe instead of using a "bulldog" (the plastic thingy you put the tube in) and the tube.
Veinous blood is dark - so not to worry about this
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