On very rare occasions, sFLCs may be undetectable by immunoassay but detectable in the serum by IFE. In such cases, further investigation is always warranted. Possible explanations include: 1) antigen excess (Section 7.5); and 2) failure of the sFLC immunoassay to recognise a particular patient's FLC epitopes. This is highly unlikely. There has been only one reported case in light chain MM, in 2004 , where κ sFLCs were underestimated during a period of follow-up. The authors supplied Binding Site with urine from the patient, and purified FLCs were incorporated in the antigen pool used to produce subsequent batches of Freelite antisera .