Final answer:
The nurse's first action should be to stop the blood transfusion, closely monitor the patient, and notify the healthcare provider when the client shows signs of a transfusion reaction.
Step-by-step explanation:
In the scenario presented, the client showing symptoms such as chills and back pain during a blood transfusion may be experiencing a transfusion reaction. The immediate first action the nurse should take is to stop the blood transfusion to prevent any further possible adverse reactions. Once the transfusion is stopped, the nurse should then continue to monitor the patient's vital signs closely, check the blood product and patient identification, maintain venous access with saline, and notify the healthcare provider of the situation.