ObjectivesTo evaluate the time course of hemoglobin concentrations in nonbleeding intensive care unit patients.DesignProspective, observational study.SettingMultidisciplinary (medicosurgical) department of intensive care.PatientsNinety-one patients with no evidence of recent or active blood loss, no history of hematologic disease or chronic renal failure, and no need for extracorporeal epuration techniques.InterventionsNone.Measurements and Main ResultsData collection included primary diagnoses, Acute Physiology and Chronic Health Evaluation II and sepsis-related organ failure assessment scores, signs of sepsis, 24-hr fluid balance, and hemoglobin concentrations. For the entire intensive care unit stay, the fall in hemoglobin concentrations (calculated from the mean of individual slopes of hemoglobin concentrations over time) averaged 0.52 ± 0.69 g/dL/day. For the 33 patients who stayed in the intensive care unit for >3 days, this decline was larger for the first 3 days than for subsequent days (0.66 ± 0.84 g/dL/day vs. 0.12 ± 0.29 g/dL/day;p< .01). After the third intensive care unit day, the change in hemoglobin concentrations was inversely related to the severity of the disease, as reflected by the Acute Physiology and Chronic Health Evaluation II and the sepsis-related organ failure assessment scores. Hemoglobin concentrations decreased by 0.44 ± 0.70 g/dL/day in the nonseptic and 0.68 ± 0.66 g/dL/day in the septic patients (p= .13). After the third intensive care unit day, hemoglobin concentrations continued to decrease in the septic patients but not in the nonseptic patients (−0.29 ± 0.19 vs. 0.006 ± 0.3 g/dL/day;p= .0016). The fall in hemoglobin concentrations was not significantly related to the fluid balance. The volume of blood drawn daily for laboratory studies was 40.3 ± 15.4 mL: 49.0 ± 11.3 mL in the septic patients and 36.7 ± 14.9 mL in the nonseptic patients (p= .04).ConclusionsHemoglobin concentrations typically decline by >0.5 g/dL/day during the first days of intensive care unit stay in nonbleeding patients. Beyond the third day, hemoglobin concentrations can remain relatively constant in nonseptic patients but continue to decrease in septic patients, as well as patients with high sepsis-related organ failure assessment or Acute Physiology and Chronic Health Evaluation II scores. These observations may help in the interpretation of hemoglobin concentrations in critically ill patients.