From the Guidelines
Hemodilution and hemoconcentration can significantly impact hemoglobin measurements, potentially causing misleading clinical interpretations, with hemodilution decreasing hemoglobin concentration by 1-2 g/dL and hemoconcentration increasing it by 1-3 g/dL. These effects are crucial in clinical settings where hemoglobin guides transfusion decisions, as seen in critical care patients where anemia is common, affecting about two thirds of patients on admission, with a mean haemoglobin level of 11.0 g/dL 1. The severity of anemia on admission is associated with increased morbidity and mortality in critical care patients.
Key Considerations
- Hemodilution occurs with IV fluid administration, pregnancy, and certain medical conditions, reducing hemoglobin values by increasing plasma volume relative to red blood cells.
- Hemoconcentration, on the other hand, increases hemoglobin concentration by reducing plasma volume, potentially elevating hemoglobin during dehydration, burns, or severe vomiting/diarrhea.
- These changes are relative concentration changes rather than true alterations in red cell mass, as they involve changes in the ratio of red blood cells to plasma volume without altering the absolute number of red blood cells.
Clinical Implications
- Clinicians should consider the patient's volume status when interpreting hemoglobin values, potentially repeating measurements after volume status normalization for accurate assessment.
- The interpretation of hemoglobin thresholds as transfusion triggers should also consider the dynamic nature of hemoglobin concentration, which is affected by changes in plasma volume and does not always reflect the degree of anemia or reduced red cell mass, as noted in studies comparing restrictive and liberal transfusion strategies 1.
Evidence-Based Practice
- Recent studies, such as those comparing restrictive and liberal transfusion strategies, highlight the importance of considering individual patient factors and clinical contexts when making transfusion decisions, rather than relying solely on hemoglobin thresholds 1.
- The mechanism of hemodilution and hemoconcentration underscores the need for a nuanced approach to interpreting hemoglobin values, taking into account the patient's overall clinical condition and volume status.
From the Research
Hemodilution and Hemoconcentration Impact on Hemoglobin
- Hemodilution after initial treatment in acute decompensated heart failure patients was associated with severe pulmonary edema at admission and higher readmission rates 2.
- A study on crystalloid hemodilution in a prospective randomized control trial with blood donation as a model for hemorrhage found that hemodilution after crystalloid administration resulted in a significant drop in hemoglobin levels 3.
- Hemoconcentration, or the relative increase in the cellular elements in blood, was consistently associated with markers of aggressive fluid removal and improved short-term mortality and rehospitalization in patients hospitalized for heart failure 4.
Mechanisms of Hemodilution
- A study on high-resolution dynamics of hemodilution after exercise-related hemoconcentration found that hemodilution was guided independently by plasma- and erythrocyte-related effectors, and a critical hemoconcentration zone may be formed during the recovery period in trained elite athletes 5.
- Blood volume analysis can distinguish true anemia from hemodilution in critically ill patients, and the use of peripheral hematocrit alone may overestimate anemia, potentially leading to unnecessary interventions 6.
Hemoglobin Level Changes
- Hemoglobin levels decreased less in the hemodilution group (-0.16 ± 0.98 g/dL) than in the hemoconcentration group (-0.88 ± 1.11 g/dL) from before admission to admission in acute decompensated heart failure patients 2.
- A formula was created to predict hemoglobin levels from a given estimated blood loss and volume replacement: Hemodilution Hgb = (mean pre-donation Hgb - hemorrhage Hgb drop - equilibration hemoglobin drop - resuscitation Hgb drop) 3.
- Hemodilution after exercise-related hemoconcentration was maintained for 7 to 10 minutes and then diluted back to baseline 30 minutes after exercise 5.