A 27-year-old woman with systemic lupus erythematosus presen…

A 27-yeаr-оld wоmаn with systemic lupus erythemаtоsus presents for routine follow-up. She states that her joint pain has improved, but over the past month she has noticed increasing fatigue and mild swelling around her ankles. Her blood pressure today is 148/92 mm Hg, compared with her usual reading of 112/70 mm Hg. Laboratory results reveal: Hemoglobin: 11.8 g/dL Serum creatinine: 1.3 mg/dL (previously 0.8 mg/dL) ESR: Elevated Which additional test would be most important to obtain at this visit?