What is your approach to a patient with IgG4RD with past pulmonary involvement (biopsy proven) managed with steroids alone, now with new hematuria/proteinuria, but stable renal function?

Significant hematuria is not a typical feature of IgG4-related kidney disease. IgG4-RKD most commonly presents as tubulointerstitial nephritis (TIN), which presents as mild, non-nephrotic range proteinuria. In fact, urinalysis in the context of TIN is often normal, as the proteinuria is largely non-albumin in nature and therefore not identified by standard urinalysis (as opposed to urine protein: creatinine ratio). Radiologically, this presents numerous, bilateral, cortical lesions seen on MRI o...

New onset hematuria/proteinuria in such a patient is definitely concerning. I usually do a kidney biopsy as it would help me rule out any superimposed GN, and give me an idea about how well am I managing their IgG4RD. If there is no other etiology and interstitial nephritis is found, that would be a good reason to intensify therapy. I do not wait until renal function worsens as a lot of times it means that the patient has already accrued chronic damage.

Thanks @Dr. First Last for the wonderful response! Would also add to check complements. While the presence of low complements is of course non-specific as to etiology, hypocomplementemia has been described in renal involvement/TIN from IgG4-RD.

I would check the compliment profile, possible associated IgA nephropathy.