Rapid screening assay for anti-GBM antibody and ANCAs; an important tool for the differential diagnosis of pulmonary renal syndromes

Background. Pulmonary renal syndrome is encountered in several diseases such as Goodpasture's syndrome, antineutrophil cytoplasmic antibody (ANCA) associated systemic vasculitis, systemic lupus erythematosus (SLE) and infection-associated or drug-induced glomerulonephritis. To preserve organ function it is of vital importance to make the correct diagnosis and institute adequate therapy early, in the acute phase. Methods. An enzyme-linked immunosorbent assay (ELISA), specially designed as a rapid screening assay for antiglomerular basement membrane (anti-GBM) antibody proteinase-3 (PR3-) ANCA and myeloperoxidase-(PMPO-) ANCA were evaluated from 1060 serum samples drawn from patients with clinically suspected pulmonary renal syndrome or rapidly progressive glomerulonephritis (RPGN). Results. Of the 1060 serum samples, 142 were positive for anti-GBM antibody (n=19), PR3-ANCA (n=60), or MPO-ANCA (n=73). Of the 142 samples, 10 were double positive. Reanalysis of positive sera with a quantitative ELISA yielded results manifesting good correlation with those of the rapid screening assay. Of 918 sera found to be negative in the rapid screening assay, 105 were also tested with IIF, 11 being found to be positive. However, these 11 sera manifested no specificity for PR3 or MPO, but some were specific for bactericidal/permeability-increasing proteins, lactoferrin or elastase ANCAs. Two of the patients whose sera yielded negative results in the rapid assay had systemic vasculitis. Conclusions. The ELISA thus detects the true antibodies to PR3, MPO, and GBM, whereas IIF detects additional specificities. The findings suggest the rapid assay results to be of high positive predictive value, and the assay to be of high diagnostic specificity and sensitivity and thus useful in the diagnostic workup in suspected cases of RPGN or pulmonary renal syndrome.