Background and Aims: Double positivity of anti-GBM and ANCA serology is uncommon but may represent a distinct disease entity of small vessel vasculitis. Previous research has been challenged by low disease incidence, and conflicting results pertaining to risk of death and ESRD. Accordingly, we examined incidence and outcomes based on data from multiple Danish nationwide healthcare registries. Method: All patients with incident positive anti-GBM serology between 2013 and 2018 were identified from 4 of 5 administrative regions in Denmark. Serological positivity was defined as serum concentrations exceeding the upper reference level. Double positivity was defined by either presence of PR3-ANCA or MPO-ANCA within a margin of 30 days from inclusion. Baseline information and clinical diagnoses defined by administrative diagnoses were subsequently ascertained by cross-referencing of data from the Danish nationwide administrative registries. Risks of death or ESRD were compared based on adjusted absolute risk ratios (ARR) and cumulative incidences assessed based on the Aalen-Johansen estimator. Results: A total of 118 patients with positive anti-GBM serology (4.4 cases/million/year) were identified. Concomitant ANCA serology was tested in 104 (88.1%), with 39 patients (37.5%) demonstrating double positivity (20 and 13 patients positive for PR3-ANCA (51.3%) and MPO-ANCA (33.3%), respectively, and 6 patients positive for all autoantibodies (5.8%)). Mean follow-up for the total study population was 1.9 (SD ±1.6) years. Compared with patients positive for anti-GBM alone, double positivity was associated with female gender (61.5%, P=0.02), and more frequent employment of plasma exchange (53.8%, P=0.04). No difference was observed with regard to age (63.2 years [SD 18.5], P=0.60), and mean anti-GBM concentration (125.5 [SD 182.4] IU/L vs. 108.9 [SD 212.7] IU/L, P=0.30). One-year mortality was 17.7% (n=14) in patients positive for anti-GBM alone, and 28.2% (n=11) in patients positive for both anti-GBM and ANCA. Double positive serology was associated with increased risk of death (ARR 2.10 [CI 1.20-3.65], P=0.009) (figure); however, there was no difference in risk of ESRD (ARR 1.28 [0.66-2.50], P=0.46). Of all identified patients, only 32 (27%) were diagnosed with anti-GBM disease according to ICD10 code (1.2 cases/million/year). In patients with confirmatory serology and ICD-10 code, 13 (40%) had double positive serology (46.2% PR3-ANCA and 53.8% MPO-ANCA). In the subset of patients with confirmatory ICD-10 code, double positivity was associated with male gender (63.2%, P=0.07), numerical lower mean age (56.1 [SD 25.2], P=0.50), and increased mean anti-GBM concentration (333.3 [SD 278.7] vs 150.7 [SD 146.5] P= 0.026). There was no difference in risk of death or ESRD between the two groups. Conclusion: Double positivity of anti-GBM and ANCA serology plausibly defines a distinct group of patients and is associated with a higher risk of death. While the association between an ICD10-confirmed diagnosis of anti-GBM disease and anti-GBM serology is well established, the significance of serology alone remains uncertain. Disclosures: None Figure. Mortality in patients with single anti-GBM antibodies and double positive serology