I am a registered nurse and I work in an ER.  ( I also have a 5y/o with SWCAH).  Anyway,  with a heelstick, the potassium is usually hemolysed (or broken down by squeezing the heel to bleed).  Hemolysed blood gives a false "high" potassium reading.  Sometimes with a difficult venipuncture, hemolysis of the blood can occur as well.  Hope this helps.