A client experiences an ABO incompatibility reaction after multiple blood transfusions. Which finding should the nurse report immediately to the healthcare provider?

Lower back pain and hypotension are symptoms of an ABO incompatibility reaction, which is a serious complication of blood transfusion. This reaction occurs when the client receives a blood type that is incompatible with their own. It can cause a rapid and severe response, including back pain, hypotension, fever, and chills. This should be reported immediately to the healthcare provider.