Rare case of atypical crescentic glomerulonephritis and interstitial lung disease with negative anti-GBM antibody and positive anti-MPO antibody

A 70-year-old man presented with 1 month of haematuria and mild right-sided flank pain with no other symptoms. Diagnostic workup included serum studies which showed the presence of antimyeloperoxidase antibodies, a kidney biopsy which demonstrated necrotising crescentic glomerulonephritis with linear immunofluorescence of the basement membrane, and electron microscopy which exhibited thickening of the glomerular basement membrane. Incidentally, the patient was discovered to have a latent hepatitis B infection, which complicated immunosuppressive therapy. He was treated with a course of plasmapheresis and methylprednisolone, followed by entecavir for hepatitis B prophylaxis, and finally by rituximab. This case of glomerulonephritis was notable for its resemblance to the better known Goodpasture's disease. Typically, Goodpasture's syndrome exists on a spectrum from seronegative disease to double-positive disease that presents with both anti-glomerular basement membrane (anti-GBM) and cytoplasmic-antineutrophil cytoplasmic antibodies/antiproteinase 3 antibodies (c-ANCA/anti-PR3). However, this patient's glomerulonephritis was unique because he presented negative for anti-GBM antibodies and positive for perinuclear-antineutrophil cytoplasmic antibodies/antimyeloperoxidase antibodies (p-ANCA/anti-MPO).