Which statement regarding blood transfusion reactions is true?

The statement regarding blood transfusion reactions that is true is that urticaria can indicate an allergic reaction. Urticaria, commonly known as hives, is characterized by raised, itchy welts on the skin and can occur as part of a mild allergic reaction to the blood product being transfused. Recognizing urticaria is important for healthcare providers as it helps to identify potential allergic reactions early and assess whether the transfusion should be continued or stopped.

In the context of the other statements, it's essential to understand their limitations. Not all transfusion reactions manifest within the first hour; some may occur later, making a strict time frame inappropriate. Furthermore, fever is just one of many signs indicating a transfusion reaction, which can also include chills, back pain, or respiratory distress, hence not only fever defines a reaction. Lastly, while allergic reactions can be severe, they are not always so; they can range from mild symptoms like urticaria to more severe and life-threatening reactions, emphasizing the importance of evaluating each case on its own merits.