Interpreting results of coulometry and immunoprecipitation in diagnosing iron disorders

Concentration of iron in plasma, total iron-binding capacity (TIBC), and transferrin saturation are often determined by standard spectrophotometric methods, but iron concentration may be quantified by immunoprecipitation or, electrochemically, by controlled-potential coulometry. Because these iron assays do not all measure the same form(s) of iron, we studied subjects in various states of iron nutriture: normal adults, iron-deficient patients, thalassemia patients with unsaturated transferrin or oversaturated transferrin, and patients with idiopathic hemochromatosis. The spectrophotometric and coulometric methods detected essentially all non-heme iron in plasma; results correlated well but showed a negative bias toward the coulometric method. Results by an immunoprecipitation procedure, which measures only transferrin-bound iron, correlated well with those obtained coulometrically but were slightly higher than the latter. The characteristics of the various methods for iron must be understood by the clinical laboratory if diagnosis of iron disorders is to be accurate.