Problems in the determination of erythrocyte protoporphyrin by ethyl acetate-acetic acid extraction

Three problems with the ethyl acetate-acetic acid method of protoporphyrin analysis were identified: (1) protoporphyrin extraction from whole blood may yield low recoveries in comparison with extraction from prediluted blood; (2) recovery of protoporphyrin is low with some batches of ethyl acetate, but such unsatisfactory reagent can be identified by an iodide oxidation test; (3) protoporphyrin standards prepared with deionised water may be unstable. However, glass-distilled water is satisfactory.