OBJECTIVES: Red blood cell (RBC) transfusions are commonly administered in the care of critically ill patients. However, it is less clear how transfusion practice varies with hemoglobin (Hgb) level and with presence of heart and chronic kidney disease. We sought to describe the frequency of RBC transfusions among critically ill patients, how it varies with Hgb level, and to identify other associated patient characteristics. METHODS: This was a retrospective observational study involving secondary analysis of administrative data of patients admitted to Veterans Affairs (VA) intensive care units (ICU) from 2001 to 2005. First ICU admissions of the year for unique patients were included. The outcome of interest was RBC transfusion during the first 30 days of ICU admission. Explanatory variables include demographic and admission-related information, Hgb level, co-morbid conditions, ICU admission diagnosis, and laboratory test results. Logistic regression modeling quantified associations between these variables and transfusion. RESULTS: The data of 259,281 admissions were analyzed. Overall incidence of RBC transfusion was 12.5%, varying from 0.6% for Hgb above 12g/dl, to 68.0% for Hgb below 5g/dl. Increased age (OR 1.05, 95%CI 1.03-1.06 every 10 years), male gender (OR 1.19, 95%CI 1.08-1.30), admission for acute myocardial infarction (AMI) (OR 1.28, 95%CI 1.21-1.36), and co-morbid heart disease (OR 1.05, 95%CI 1.02-1.09) were independently associated with transfusion, but chronic kidney disease was not. Significant interactions between Hgb level and 3 admission diagnoses (AMI, angina, and congestive heart failure) resulted in even higher likelihood of transfusion among patients admitted for these conditions and had lower Hgb levels. CONCLUSIONS: Independent of Hgb level, ICU patients admitted for AMI, angina or congestive heart failure had higher likelihood of RBC transfusion, with this likelihood increasing even further among those with lower Hgb levels. Further research is needed to determine how these practices influence outcomes.