Definition of Acute Anemia
Acute anemia is characterized by a rapid reduction in hemoglobin concentration, red blood cell count, or hematocrit that develops over hours to days, most commonly due to hemorrhage, but can also result from hemolysis or other acute processes. 1, 2
Key Distinguishing Features
Acute anemia differs fundamentally from chronic anemia by its rapid onset and the body's inability to compensate adequately, leading to more pronounced symptoms even at higher hemoglobin levels than would be seen in chronic anemia. 1
Clinical Presentation
Acute anemia presents with symptoms of acute blood loss including tachycardia, hypotension, altered mental status, and signs of inadequate oxygen delivery, rather than the gradual fatigue and dyspnea seen in chronic conditions. 1
Patients may be symptomatic at hemoglobin levels that would be well-tolerated in chronic anemia because compensatory mechanisms (increased cardiac output, 2,3-DPG production, plasma volume expansion) require time to develop. 1
Severity Classification
While general anemia severity is defined as:
- Mild: Hemoglobin ≤11.9 g/dL and ≥10 g/dL
- Moderate: Hemoglobin ≤9.9 g/dL and ≥8.0 g/dL
- Severe: Hemoglobin <8.0 g/dL 3
In acute settings, the absolute hemoglobin value is less important than the rate of decline and clinical signs of hemodynamic compromise. 1
Etiologic Considerations
Primary Cause
- Hemorrhage is by far the most common etiology of acute anemia, and should be the first consideration in any patient presenting with rapid hemoglobin decline. 2
Alternative Etiologies When Hemorrhage is Absent
Acute hemolysis (including microangiopathic hemolytic anemia, glucose-6-phosphate dehydrogenase deficiency crisis, or autoimmune hemolytic anemia) should be considered when acute anemia develops without bleeding. 2, 4
Look for schistocytes on peripheral blood smear, elevated LDH, decreased haptoglobin, and elevated indirect bilirubin to identify hemolytic processes. 2, 4
Thrombotic microangiopathies present with the combination of microangiopathic hemolytic anemia and thrombocytopenia, requiring urgent specialist consultation. 4
Critical Clinical Context
In critically ill patients, hemoglobin drops by a mean of 0.52 g/dL per day, with approximately 30-40% developing moderately severe anemia (Hb <90 g/dL) during their ICU stay due to phlebotomy, inflammation, haemodilution, and reduced red cell survival. 3
The decision to transfuse should not be based solely on hemoglobin thresholds but must account for the patient's tolerance of anemia, particularly in those with cardiovascular disease, active bleeding, or signs of inadequate tissue oxygen delivery. 3