OBJECTIVE: To establish a sensitive method for measuring nontransferrin-bound iron (NTBI) in serum samples using graphite furnace atomic absorption spectrometry (GFAAS). DESIGN AND METHODS: Nontransferrin-bound iron (NTBI) was chelated using nitrilotriacetic acid (NTA) and then ultrafiltered according to the method employed by Singh et al. [1]. Serum ultrafiltrates were diluted eightfold with distilled water. NTBI from the Fe-NTA complex present in the serum ultrafiltrate was measured using GFAAS. RESULTS: Nontransferrin-bound iron (NTBI) and other parameters were measured in seven patients diagnosed with hereditary hemochromatosis by liver biopsy. Total serum iron, NTBI and transferrin saturation values (ranging from 87% to 90%) were elevated for three of the seven hemochromatosis patients tested before therapeutic phlebotomy. Six of the seven hemochromatosis patients had undergone phlebotomy and revealed normal total serum iron, NTBI and transferrin saturation values. Nine test subjects (not diagnosed with hemochromatosis) with abnormally high total serum iron and/or ferritin concentrations exhibited normal NTBI values (< or =0.14 micromol/L to 0.29 micromol/L). The detection limit was 0.1 micromol/L for a 25 microL injection volume. CONCLUSIONS: The GFAAS method presented here provides a sensitive assay to quantitate NTBI in serum samples. The method developed is 4 to 5 times more sensitive than the only other GFAAS method [2] and more than an order of magnitude more sensitive than other colorimetric methods [1,3]. Improvement in sensitivity over the other GFAAS method [2] may be accounted for by differences in sample preparation between this method and that of Nielsen et al. [2]. Serum ultrafiltrates in this study were diluted eightfold with distilled water and mixed with a magnesium nitrate matrix modifier before GFAAS analysis. NTBI results obtained from this study indicate that the plasma iron pool in hemochromatosis patients awaiting phlebotomy increases to a level at which transferrin's ability to bind iron becomes exhausted and elevated NTBI levels appear in the serum. NTBI can mediate the production of reactive oxygen species and may cause organ damage associated with iron overload.