Most cases of pauci-immune crescentic glomerulonephritis (PICGN) are associated with serum anti-neutrophil cytoplasmic antibodies (ANCA). This article studied the sensitivity and specificity of serum ANCA, determined by flow cytometry and indirect immunofluorescence (IIF), to identify patients with acute PICGN. 577 adults presenting for first biopsy of their native kidneys with serum taken for ANCA (flow cytometry and IIF) determination were studied. A positive ANCA was defined using a flow cytometric ANCA assay as a screening test, followed by a slide-based indirect IIF technique. Pathological confirmation of acute PICGN was used to assess the sensitivity and specificity of this combined approach and its positive predictive value (PPV) and negative predictive value (NPV) in patients presenting for renal biopsy due to abnormal urinary sediment. Forty-nine patients were found to have acute PICGN on renal biopsy. Of these 47 were ANCA positive (sensitivity 95.9%). Overall 93 of the renal biopsy patients were ANCA positive, (specificity 91.3%). A further seven patients (two ANCA positive) had advanced sclerosing disease consistent with PICGN but without evidence of current disease activity. The PPV and NPV of ANCA, assessed by flow cytometry and slide IIF, in predicting that patients presenting with undifferentiated renal disease would have acute PICGN was 50.5 and 99.8%, respectively. Flow cytometric screening of serum for ANCA in patients undergoing renal biopsy has a high NPV for determining those with acute PICGN. It may provide a rapid, simple screening test for this lesion in laboratories using diagnostic flow cytometry and may complement IIF/ELISA in evaluating ANCA positive patients.