Interference of polyclonal free light chains with identification of Bence Jones proteins

We present a case in which kappa free light chains caused difficulty in interpreting classical urinary immunoelectrophoresis, but immunofixation electrophoresis (IFE) demonstrated the presence of a lambda-Bence Jones protein. Analysis of the urine by Ouchterlony double diffusion and IFE after gel-filtration chromatography showed that the difficulty was caused by the presence of large amounts of polyclonal free light chains. The workup also demonstrated that although IFE is the more sensitive and specific technique, IFE performed on concentrated urinary samples is especially subject to misinterpretation unless densely staining patterns are diluted and reassayed. This process of sample dilution provides a means for titrating antigen and antibody concentrations such that condition-specific patterns become visible on the gel. This workup also shows that, at some dilutions, polyclonal free light chains may migrate in the same manner as an oligoclonal band in a so-called ladder configuration. These bands were observed from both monomeric and dimeric fractions isolated by gel chromatography, consistent with reports that this pattern is largely linked to the isoelectric points of the molecules. We speculate that, in rare instances, the distinction between polyclonal and monoclonal kappa free light chains migrating as a ladder-banding pattern may be equivocal.