A case of hook effect in the serum free light chain assay using the Olympus AU400e

We describe a case where a woman with an IgGλ monoclonal gammopathy had an undetectable serum free λ light chain due to antigen excess. The patient had a three year history of multiple myeloma and initially presented with an elevated serum free λ light chain. The serum free λ light chain concentration increased over the course of several months to > 4000 mg/L (reference interval 5.7–26.3 mg/L) and then suddenly dropped to < 3.0 mg/L. Paradoxically, during this time the monoclonal IgGλ concentration measured using serum protein electrophoresis increased from 44 to 59 g/dL. Serial dilution of the patient's serum specimen revealed that the serum free λ light chain was actually 3130 mg/L. This case represents an example of antigen excess or ‘hook effect’ using the serum free light chain assay.