No abstract

Anemic patients are at increased risk of intraoperative and postoperative transfusion, infection and mortality. Anemia is common in surgical patients, previous studies report a prevalence of 30-40%. While patient blood management programs advise screening for anemia, other perioperative initiatives advocate limiting preoperative laboratory testing. Therefore, a large number of surgical patients may not be screened, missing the opportunity for further investigation and management. In our single-center retrospective cohort study of all adult surgical patients over one year, we sought to estimate the proportion of anemia in patients without a preoperative blood test and their associated intraoperative and postoperative transfusion risk using multiple imputation to estimate missing hemoglobin values and inverse-probability-of-treatment-weighting to balance the study groups. Multiple imputation generated 39 datasets containing plausible hemoglobin values for patients missing a hemoglobin. Patients were classified as known-anemic, known-not-anemic, imputed-anemic and imputed-not-anemic if the hemoglobin was measured or imputed. We then compared red-cell transfusion yes/no by logistic regression, and number of red-cell units transfused by linear regression, pooling the results. Thirty-nine percent of 7879 patients were missing a hemoglobin, of whom 20-35% may be anemic. Only known-anemic patients had an increased intraoperative transfusion risk (odds ratio (OR) 3.13, 95% confidence interval (95%CI) 2.19 to 4.48). Both known-anemic and imputed-anemic patients had increased postoperative transfusion risk (OR 3.93 (95%CI 2.92 to 5.29), OR 2.99 (95%CI 1.61 to 5.56), respectively). Anemia may be common in patients missing a haemoglobin, and may confer an increased risk of postoperative transfusion compared to known-anemic patients. Removing them from observation studies on anemia risks sample bias. Excluding patients from pre-operative testing may miss the opportunity to identify anemia and modify postoperative transfusion risk and other adverse health outcomes. Future research could use point-of-care tests to reduce screening costs, and further investigate adverse outcomes risks in these missing patients.