Limited value of zinc protoporphyrin as a marker of iron status in chronichemodialysis patients

Background: In an attempt to find new parameters able to evaluate the actua l iron availability by bone marrow cells, zinc protoporphyrin (ZnPP), a met abolic intermediate generated in the red blood cell by the incorporation of zinc instead of iron, has been proposed. ZnPP is a good marker of iron-def iciency anemia in non-uremic people, as red blood cell ZnPP concentration r ises specifically (except for lead intoxication) in this condition. Existin g data on ZnPP as a marker of iron deficiency in uremic patients comes main ly from cross sectional studies on chronic hemodialysis and has produced co nflicting results. Subjects and methods: Therefore, we prospectively studie d 42 HID patients, 28 - 88 years old, 13 - 346 months of dialysis age, begi nning from a period of maximal iron deficiency, due to the lack of parenter al iron compounds (TO) up to the end of more than one year of follow-up wit h continuous parenteral iron supplementation (T4). ZnPP, hemoglobin, transf errin saturation and ferritin were serially determined before and after six weeks (T1), four months (T2), seven months (T3) and 14 months (T4) of pare nteral iron supplementation at a maintenance dose of 0.5 - 1 mg/kg/week. Re sults: In comparison with baseline values (95 +/- 37 mu mol/mol heme) there were no significant changes in ZnPP levels at T1 and T2 despite a continuo us increase in both transferrin saturation and ferritin values, while ZnPP significantly decreased at T4 (63 +/- 37 mu mol/mol heme, p < 0.001). There was no correlation between ZnPP and both transferrin saturation and ferrit in at any time during the study, the same was true for ZnPP and zinc and le ad serum concentration, fibrinogen and reactive C protein levels at T1 and T4, respectively. At T4, only 2/10 patients who still showed ZnPP levels > 80 mu mol/mol heme had absolute or functional iron deficiency, when the per centage of hypochromic red cells were measured. Conclusion: We conclude tha t ZnPP untimely parallels a change in iron balance in only a proportion of uremic people, in as much as confounding factors, such as chronic inflammat ion and uremia in itself may obscure its relationship with iron status. The refore, ZnPP cannot be assumed to be a first-line diagnostic marker of iron balance in uremic patients.