What serologic markers may be helpful in the distinguishing the kidney vasculitides?
Complement levels may be depressed in patients with cryoglobulinemic vasculitis and other vasculitides, including systemic lupus erythematosus (SLE) and postinfectious glomerulonephritis. Complement levels are typically normal in the other small-vessel vasculitides. Although technically difficult to measure, cryoglobulins can be directly measured in serum. Antinuclear antibodies suggest connective-tissue-related disorders, such as SLE, and may be further distinguished by extractable nuclear antigen panel testing. Patients with the small-vessel vasculitides (GPA, MPA, and EGPA) often have positive anti-neutrophil cytoplasmic antibodies (ANCAs) directed to either myeloperoxidase (MPO, MPO-ANCA) or proteinase 3 (PR3, PR3-ANCA). Anti-glomerular basement membrane (anti-GBM) antibodies are detected in patients with anti-GBM disease whether limited to the kidney or associated with diffuse alveolar hemorrhage.
There is a broad overlap in the clinical presentation of the various small-vessel vasculitides. In addition, overlap syndromes can occur. For instance, one-third to one-half of patients with anti-GBM disease will also have circulating ANCA, which is more often directed against MPO. In addition to the clinical presentation and serologic studies, a kidney biopsy is usually indicated to provide a definitive diagnosis and valuable information on the extent of glomerular injury and glomerular and interstitial scarring.
