Influence of monoclonal immunoglobulins in direct determinations of iron in serum

I compared seven buffers and four chromogens for determining serum iron, to evaluate the frequency of falsely high or low concentrations of iron in 59 sera containing monoclonal immunoglobulins. The results for these direct assays with untreated sera were compared with those obtained by a proposed reference method (Br J Haematol 1978;38:291-4) with protein-free filtrates of the same sera. Sera with monoclonal immunoglobulins sometimes yielded erroneous results; the frequency of errors, which could be as much as 29% (17/59), depended on the composition of buffer and color reagent. Addition of thiourea or a detergent (Triton X-405) to some of the buffers lowered the frequency of errors, but did not abolish them. In only a few of the investigated buffer/chromogen combinations were no errors found. Detection of errors by analyzing the absorbance pattern after mixing sample and buffer was not always successful. Moreover, the presence and magnitude of errors bore no relationship to the type or the concentration of the monoclonal immunoglobulins. Unfortunately, the problem cannot be solved by a simple two- or threefold dilution of the sample.