Renal involvement is frequently seen in patients with systemic lupus erythematosus (SLE). The occurrence of non-lupus nephritis, and especially IgA nephropathy, in SLE patients has rarely been reported. We describe here the case of a 30-year-old woman who had systemic lupus erythematosus and nontuberculous mycobacterial lung disease, and her biopsy of a renal lesion was unexpectedly diagnostic of IgA nephropathy. Although both IgA nephropathy and lupus nephritis are immune complex mediated diseases, their laboratory and histopathologic findings and the extra-renal clinical manifestations are different and these all support a different pathogenesis for the 2 diseases. Renal biopsy plays a crucial role in identifying and diagnosing renal lesions, which may have prognostic and therapeutic implications that are distinct from those of lupus nephritis. In conclusion, performing a renal biopsy in SLE patients who have urinary abnormalities is important since a correct diagnosis would permit the most appropriate treatment to be started and so avoid unnecessary immunosuppressive treatments.

Keywords: IgA nephropathy, Systemic lupus erythematosus, Non-tuberculous mycobacterium