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Urinary protein electrophoresis analysis (UPE) is an essential investigation for the study of abnormal proteins in urines. The interpretation of this analysis must be comprehensive and relevant. Indeed, UPE is often requested by clinicians and may have an important impact in patient's management. This paper presents two cases with free light chains showing unexpected electrophoretic migration which can lead to the misinterpretation of results. This article helps biologists to keep in mind the interest of UPE among the several analyses useful in the laboratory.

Thirteen patients with light chain (LC) proteinuria (11 with multiple myeloma and two with monoclonal gammopathy of undetermined significance) were followed up for one to 3.5 (median 2.5) years and studied for urinary excretion of LCs, alpha 1-microglobulin (alpha 1 M), beta 2-microglobulin (beta 2M), albumin, and serum creatinine concentration at intervals of 6 +/- 2 months. At the beginning of the follow-up, urinary alpha 1M excretion correlated with that of beta 2M (r = 0.81, p = 0.0007) and LCs (0.69, p = 0.0084), and with the serum creatinine level (r = 0.56, p = 0.047). During the follow-up period, renal function remained stable in eight patients despite high or fluctuating LC excretion. In seven of them, urinary alpha 1M was repeatedly below 150 mg/24 h and in one it transiently exceeded that level. The remaining five patients had an unstable renal function (deterioration in four, improvement in one) although their urinary LC and albumin excretion during the study period was comparable to those in patients with stable renal function. In the five patients with unstable renal function, high (> 150 mg/24 h) urinary alpha 1M excretion was associated with a rise in the serum creatinine level. alpha 1M excretion was thus found to be a useful indicator of renal damage caused by LCs.