The most likely diagnosis is transfusion-associated circulatory overload (TACO). TACO is an underdiagnosed condition that occurs in 0.3% to 8% of patients undergoing transfusion in the hospital and is associated with increased inpatient mortality and a longer length of hospital stay. TACO is defined as the new onset or exacerbation of at least three of the following findings within 6 hours of completing a transfusion: acute respiratory distress, elevated B-type natriuretic peptide (BNP) level, elevated central venous pressure (CVP), evidence of left heart failure, evidence of positive fluid balance, and radiographic evidence of pulmonary edema. Risk factors include age older than 60 years, chronic kidney disease, chronic heart failure, number of blood products transfused, and the volume transfused per hour. Preventive measures include a slower rate of infusion for those at risk (1 mL/kg/hour) and diuretic therapy to maintain euvolemia.

An acute hemolytic transfusion reaction (AHTR) is unlikely. The patient's hemoglobin level increased appropriately after the transfusion (1 g/dL/U [10 g/L/U]), and the lactate dehydrogenase and total bilirubin levels are not elevated. No protein was seen on urinalysis, kidney function is unchanged, and the patient is not hypotensive. Additionally, pulmonary opacities as seen on the radiograph would not be expected with AHTR.

Distinguishing TACO from transfusion-related acute lung injury (TRALI) can be challenging. Fever and hypotension occur in only one third of patients with TRALI. However, TRALI is not associated with signs of volume overload (increased jugular venous pressure, lower extremity edema, elevated CVP). BNP and N-terminal proBNP levels have not been studied in TRALI but are elevated with TACO.

Transfusion-associated sepsis is highly unlikely in the absence of fever and hypotension.