What risk factors might someone have that makes it more likely they would receive a blood transfusion during or right after a total joint replacement?

In a recent study by Hart et al, the authors looked into the risk factors associated with blood transfusion during total hip and knee replacements. They looked at information from the National Surgical Quality Improvement Program, which is a nationally validated, outcome based program that collects data about preoperative risk factors, variables during surgery, thirty-day postoperative complications and mortality rates. This database was used to identify patients undergoing elective primary hip and knee replacements in 2011. A total of 9362 hip replacements and 13,622 knee replacements were identified. For this set of patients the blood transfusion rate for hip replacement was 22.2 percent, and for total knee replacement 18.3 percent. Significant risk factors associated receiving a blood transfusion were similar for both knee and hip replacements and included (in order of importance) age, preoperative hematocrit (red blood cell count), BMI 2. These results were somewhat surprising, for example the authors did not expect lower BMI to be associated with increased transfusion requirement, and they also found that smoking was actually a protective factor against needing a transfusion. The authors hypothesize that with decreased BMI and lower red blood cell count before surgery, with too much loss of blood then a blood transfusion may more likely. They also hypothesize that smoking increases the red blood cell count over time to make up for less ability of the smokers red blood cells to carry oxygen, thus these patient may have more blood cells available to make up for blood loss during a surgery.