Urinary Kim-1 Correlates with Interstitial Nephritis Activity in Patients with Microscopic Polyangiitis.

Background: Microscopic polyangiitis (MPA) is a type of necrotizing vasculitis that primarily affects small vessels and belongs to the spectrum of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitides (AAVs). While previous studies have identified potential prognostic biomarkers, further research is needed to validate a reliable marker for risk stratification in clinical practice. Kidney injury molecule-1 (Kim-1), a transmembrane protein expressed on proximal tubular epithelial cells, has been implicated in tubular damage. This study investigated the potential of Kim-1 as a biomarker in MPA.

Methods: Kidney biopsy tissues, along with urine and blood samples, were retrospectively analyzed from 52 MPA patients and compared to urine samples from 7 healthy controls. Global disease activity was assessed using the Birmingham vasculitis activity score (BVAS) and vasculitis damage index, while renal disease activity was evaluated using renal BVAS (BVAS-R).

Results: Urinary Kim-1 levels were significantly elevated in MPA patients compared to healthy controls. Urinary Kim-1 was positively correlated with the Mayo Clinic Chronicity Score (MCCS) but not with the ANCA Kidney Risk Score (AKRiS), whereas tubular Kim-1 was associated with AKRiS but not with MCCS, indicating their distinct pathological significance. Higher tubular Kim-1 expression was observed in patients with elevated BVAS-R. Urinary Kim-1 levels correlated with proteinuria and were associated with the Mayo Clinic Chronicity Score (MCCS) and ANCA Kidney Risk Score (AKRiS) but not with glomerular lesion severity. Unlike C-reactive protein (CRP), neither urinary nor tubular Kim-1 predicted MPA recurrence.

Conclusions: Urinary Kim-1 reflects histopathologic findings and renal impairment but does not predict systemic disease activity or recurrence in MPA, demonstrating its potential clinical utility as a biomarker for assessing chronic renal damage.