The Coulter Model S provides an accurate measurement of total hemoglobin (Hb) content in peripheral blood even when elevated levels of carboxyhemoglobin (HbCO) are present. Commercial standards and diluents for spectrophotometric determinations of cyanmethemoglobin (HiCN) are variable in quality. All diluents studied produce turbidity in some samples, and this may explain why manual Hb determinations tend to be higher than hemoglobin measurement on the Coulter S. The most reliable diluent was the reagent described by Van Kampen and Zijlstra. Its reliability is enhanced by centrifugation of sample in diluent. The use of a narrow-band-pass spectrophotometer for routine HiCN determinations in a clinical laboratory is not recommended because of its sensitivity to turbidity and HbCO.

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.