A 25-year-old woman is evaluated during a follow-up visit for systemic lupus erythematosus. She was feeling well until 2 weeks ago when she developed increased fatigue and diffuse arthralgia. Medications are hydroxychloroquine and ibuprofen as needed.
On physical examination, temperature is 37.2 °C (99.0 °F), blood pressure is 140/80 mm Hg, pulse rate is 80/min, and respiration rate is 16/min. There is diffuse alopecia of the scalp. Malar erythema is noted. Heart sounds are normal, and the chest is clear. Examination of the abdomen is normal. Tenderness with minimal swelling of the proximal interphalangeal joints is present bilaterally. Small effusions on both knees with pain on range of motion are noted.
Leukocyte count | 3000/µL (3.0 × 109/L), with 900 lymphocytes |
Creatinine | Normal |
Electrolytes | Normal |
Urinalysis | 2+ protein; trace blood |
Which of the following tests should be obtained next?