Low molecular mass IgM in urine of patients with primary and secondary renal diseases

Urine specimens from 50 patients with various renal diseases and from 10 normal subjects were examined for the presence of low molecular mass (7S) IgM by double diffusion, using a specific anti-7S IgM serum. 7S IgM in urine was also quantitated by radial immunodiffusion in 5% polyacrylamide gel. It was found with high frequency in the urine of patients with secondary renal diseases associated with multi-system immunological disorders, especially in systemic lupus erythematosus. Two of the patients with lupus nephritis and positive results for 7S IgM in urine were initially diagnosed as nephrotic syndrome due to a primary renal disease; the symptoms and signs of systemic lupus erythematosus developed later. However, 7S IgM was not detected in the urine of patients with primary renal diseases (except for two cases of nephrotic syndrome), nor in normal subjects. Thus detection of 7S IgM in urine may be helpful in the differential diagnosis of primary and secondary renal diseases.