what is erythrocyte protoporphyrin
1 The free erythrocyte protoporphyrin (FEP) test measures both ZPP, which accounts for 90% of protoporphyrin in red blood cells, and free protoporphyrin, which is not bound to zinc.2  The ZPP/heme ratio gives the proportion of ZPP compared to the normal iron-containing heme in red blood cells.PP is a substance that is normally found in small amounts in red blood cells. Most of the protoporphyrin in red blood cells combines with iron to form heme, the molecule in hemoglobin that carries oxygen.