Non-
transferrin-bound
iron and its labile (redox active) plasma
iron component are thought to be potentially toxic forms of
iron originally identified in the serum of patients with
iron overload. We compared ten worldwide leading assays (6 for non-
transferrin-bound
iron and 4 for labile plasma
iron) as part of an international inter-laboratory study. Serum samples from 60 patients with four different
iron-overload disorders in various treatment phases were coded and sent in duplicate for analysis to five different laboratories worldwide. Some laboratories provided multiple assays. Overall, highest assay levels were observed for patients with untreated hereditary
hemochromatosis and β-
thalassemia intermedia, patients with transfusion-dependent
myelodysplastic syndromes and patients with transfusion-dependent and chelated β-
thalassemia major. Absolute levels differed considerably between assays and were lower for labile plasma
iron than for non-
transferrin-bound
iron. Four assays also reported negative values. Assays were reproducible with high between-sample and low within-sample variation. Assays correlated and correlations were highest within the group of non-
transferrin-bound
iron assays and within that of labile plasma
iron assays. Increased
transferrin saturation, but not
ferritin, was a good
indicator of the presence of forms of circulating non-
transferrin-bound
iron. The possibility of using non-
transferrin-bound
iron and labile plasma
iron measures as clinical indicators of overt
iron overload and/or of treatment efficacy would largely depend on the rigorous validation and standardization of assays.