How would you approach a patient with axial spondyloarthritis who develops new-onset proteinuria?

I would approach the workup for proteinuria based on the differential diagnosis with leading etiologies including, acute illness, diabetes, nephrotic syndrome (minimal change disease, FSGS, membranous neuropathy), drug-induced proteinuria, vasculitis, and of course IgA neuropathy. Standard studies including renal ultrasound, urine to Cr ratio, serum lipids and albumin, and most likely a renal biopsy would be included in the workup. 

In addition to the standard workup of proteinuria, in the context of pre-existing ankylosing spondylitis, I would consider (and sensitize the nephrologist) about the association with IgA nephropathy, amyloid nephropathy, and if the patient was on an anti-TNF the possibility of drug-induced lupus with renal involvement.