Would you pursue a kidney biopsy in a patient with stable stage 1 AKI, bland urine sediment, and a positive MPO titer without systemic signs of vasculitis?

PR3-ANCA and MPO-ANCA are associated with substantially higher specificities and positive predictive values for ANCA-associated vasculitis (AAV) than the immunofluorescence patterns to which they usually correspond (C-ANCA and P-ANCA, respectively). However, false-positive results remain a concern.

For instance, in a retrospective analysis of 614 patients at University College London Hospital, the ANCA-MPO was tested as a predictor of AAV or drug-induced vasculitis as assessed through receiver operating characteristic curve (ROC) analysis. The area under the curve (AUC) was only 0.59 (Abrantes et al., PMID 37137279). Similar results were obtained in another retrospective study from the Royal Free Hospital in London, in which the incidence o...

In general, I would monitor the patient in the absence of hematuria or proteinuria, as necrotizing GN is unlikely. However, in ANCA, arteritis can occur without GN, though this presentation is uncommon.