A rare case of sero-negative anti-glomerular basement membrane disease combined with diabetic nephropathy and literature review.

We report a case of a 59-year-old male who exhibited an insidious onset of hematuria, nephrotic syndrome, and mild renal dysfunction, with negative serum anti-glomerular basement membrane antibodies. The initial renal biopsy diagnosed diabetic nephropathy. As the disease progressed, the clinical manifestations evolved into rapidly progressive glomerulonephritis, and a second kidney biopsy ultimately diagnosed anti-glomerular basement membrane disease in conjunction with diabetic nephropathy. Furthermore, we incubated normal renal tissue with both the patient's serum and serum from an anti-GBM antibody-positive patient, and observed identical linear IgG deposition along the basement membrane. This result indirectly suggests the presence of anti-GBM antibodies in the patient's serum. Despite receiving immunosuppressive treatment, renal function did not improve, and the patient ultimately required maintenance renal replacement therapy. This case highlights the importance of closely monitoring disease progression in atypical presentations of rare diseases and emphasizes the necessity of repeat renal biopsies for the accurate diagnosis of sero-negative anti-glomerular basement membrane disease. We diagnosed the anti-GBM disease using an indirect immunofluorescence antibody method and we need more detection methods to determine the target antigen.