Why is the serum FLC measure normal when there is a band visible in urine?
This is rare but can occur for three reasons. If there is a very minor level of production of monoclonal FLC that does not lead to any serum FLC abnormality it is possible it may be detectable as a weak band on an IFE of a highly concentrated urine. Arguably such a minor band would be irrelevant. Other urinary proteins (eg transferrin) may be found at the same point on a gel as a free light chain band. Bands from IFE of urine samples are often diffuse and poorly defined, occasionally an unusually strong (or weak) batch of kappa or lambda antibody used in IFE can lead to uneven staining and give the appearance of monoclonality.
