I do believe that getting stuck in the heel does give a higher potassium reading than from the vein.  We were told never to have our baby stuck in the heel for the renin tests because of it.  Something I believe having to do with potassium being a healing agent and when you are pricked the potassium runs to the sight to help heal and you get a large jump of it in your reading.  In the vein you are getting the circulating blood with the child’s  normal potassium level.  Thats how I understood it anyway. 

Take care,

RebeccaM