Several retrospective studies in adults have shown that if a patient is preoperative anemia predicts worse postoperative outcome. Is anemia the reason for poor outcome, or is anemia a surrogate measure for some other measure of patient disease? Dr. Leif Saager, Department of Outcomes Research, Cleveland Clinic, Cleveland, Ohio, and coauthors used the used the American College of Surgeons National Surgical Quality Improvement Program (NSQIP) to assess whether preoperative anemia was related to 30-day mortality or some other 30-day morbid outcome. Their work was published in the article “The Association Between Preoperative Anemia and 30-Day Mortality and Morbidity in Noncardiac Surgical Patients” that appeared in the current issue of Anesthesia & Analgesia.
The authors defined anemia as a hematocrit <36% for female patients and <39% for male patients. They used data on patients treated between 2005 and 2009. Utilizing logistic regression analysis, they evaluated the total association and the direct effect of preoperative anemia for each of 9 outcomes (mortality, cardiac, central nervous system, respiratory, systemic, urinary, thrombotic complications, return to the operating room, and wound infection). Using propensity matching, the authors adjusted for a series of variables and primary procedures as confounders by exact matching of 119,298 anemic patients to nonanemic controls.
The unadjusted odds ratio of 30-day mortality with anemia was 4.69; using propensity matching, the odds ratio decreased to 1.59; after adjusting for mediator variables, the odds ratio decreased to 1.24, a weak association. The total effect odds ratio for the effect of anemia on systemic, respiratory, urinary, thrombotic, and cardiovascular complications, return to the operating room and mortality ranged between 1.2 and 1.3. The odds ratios for systemic complications, respiratory complications, and cardiovascular complications were no longer significant once such events as intraoperative wound contamination, intraoperative RBC transfusion, duration of surgery, functional dependence before surgery, open wound preoperatively, and dyspnea were considered.
It is unclear if individuals with anemia who then received a blood transfusion would have improved outcome, particularly since other studies have shown that transfusion is associated with increased morbidity after surgery. Would there be a difference in outcome if the transfusion was performed before surgery or during surgery? Though the confounders the authors used showed a change in association, there might be other unrealized confounders that were not known or recorded. Finally, this was a retrospective analysis. It should probably be considered hypothesis generating, and used to design a prospective study to confirm this retrospective analysis.
Nonetheless, the report demonstrates that anemia was an independent predictor of postoperative 30-day morbidity and mortality. We should consider this in our overall assessment of the patient’s perioperative health.