The nurse is caring for a male client who reports blood in their stool and has a Hgb 5.6 g/dL (reference range 13.5-18 g/dL), Hct 16.8% (reference range 40-50%), and a WBC 3,400/mm3 (reference range 4500-11,000/mm3). The provider orders 2 units of packed red blood cells (PRBC). While monitoring the patient 10 minutes after starting the blood transfusion, the patient reports a sudden onset of chills, back pain, and appears anxious. What is the nurse's first action?