MPO and anti-neutrophil cytoplasmic antibody-associated vasculitis: Although historically, the prevalence of kidney involvement did not differ in adults stratified by ANCA specificity, more recent studies describe significant differences12 25 26 that are aligned with our findings in this paediatric AAV cohort of more frequent kidney involvement at diagnosis in individuals with MPO-ANCA relative to PR3-ANCA seropositivity.