A kidney biopsy is appropriate for this patient with systemic lupus erythematosus (SLE). Kidney disease occurs in some form in up to 70% of patients with SLE, especially in those who express anti–double-stranded DNA antibodies, which typically rise and fall with disease activity. Diagnosis of lupus nephritis is suggested by proteinuria (>500 mg/24 h) or cellular casts (erythrocytes or leukocytes) in the urine sediment of patients fulfilling the formal criteria for the diagnosis of SLE. Most patients with active lupus nephritis have low serum complement levels. According to the 2012 American College of Rheumatology guidelines for evaluating and treating lupus nephritis, this patient meets the criteria for kidney biopsy. Unless contraindicated, biopsy should be done before initiating therapy. This patient will most likely have a proliferative form of lupus nephritis (class III/IV) and will require aggressive immunosuppressive therapy.

The usual therapeutic agents for proliferative forms of lupus nephritis are either cyclophosphamide or mycophenolate mofetil. There are no data to suggest that methotrexate is useful in this setting. Methotrexate is renally excreted, and toxicity increases in the setting of kidney disease.

Without adequate management, class III, class IV, or class V combined with class III or class IV generally are progressive, with a probability of end-stage kidney disease as high as 50% to 70% after 5 to 10 years of diagnosis. Therefore, there is no reason to wait another month to repeat laboratory testing in this patient given the present abnormalities.

Because the vascular lesion in lupus nephritis is at the level of the arteriole, a renal artery Doppler examination will not be useful for treatment decisions.