Antigen-specific radioimmunoassays for anti-neutrophil cytoplasmic antibodies in the diagnosis of rapidly progressive glomerulonephritis.

Circulating anti-neutrophil cytoplasmic antibodies (ANCA) have been described in most patients with "pauci-immune" necrotizing and crescentic glomerulonephritis. A 29-kDa serine protease (p29 or proteinase 3) and myeloperoxidase are the two best characterized antigens recognized by ANCA. The study presented here was conducted to define the diagnostic value of assays for antibodies against these two antigens in rapidly progressive glomerulonephritis. Radioimmunoassays were developed for anti-p29 and anti-myeloperoxidase antibodies, with purified antigens, and the results of the radioimmunoassays were compared with those obtained by immunofluorescence tests for ANCA. We performed assays on serum samples from 123 patients with the syndrome of rapidly progressive glomerulonephritis, as well as from 200 blood bank donors and from 717 additional control patients. Without knowledge of the results of ANCA tests, the renal pathologic findings in the 123 patients with rapidly progressive glomerulonephritis were analyzed, and 42 were classified as pauci-immune necrotizing and crescentic glomerulonephritis, 18 were classified as anti-glomerular basement membrane nephritis and 63 were classified as other forms of renal disease. We found radioimmunoassays to be more reliable in the diagnosis of pauci-immune necrotizing and crescentic glomerulonephritis than immunofluorescence testing. By radioimmunoassay, ANCA were found in 40 of 42 patients (95% sensitivity) with pauci-immune necrotizing and crescentic glomerulonephritis (14 with anti-p29 and 26 with anti-myeloperoxidase antibodies). The tests for antibodies to p29 and myeloperoxidase were 99.9 and 99.5% specific for pauci-immune necrotizing and crescentic glomerulonephritis, respectively. In the setting of rapidly progressive glomerulonephritis, a positive radioimmunoassay for anti-p29 or anti-myeloperoxidase antibodies (together with a negative test for anti-GBM antibodies) gives a probability of pauci-immune necrotizing and crescentic glomerulonephritis of over 99%.