A popular way to determine a patient’s hydration status is by comparing the patient’s hematocrit and hemoglobin level. A patient is said to have fluid loss when the hematocrit is higher than the hemoglobin level multiplied by three (Ht > Hb x ±3). The question is can hematocrit and hemoglobin level be used in that manner?
Hematocrit is the volume of blood that is occupied by erythrocytes, expressed in percentage. Hemoglobin level is the concentration of hemoglobin in blood, expressed in g/dL. Hence, hematocrit and hemoglobin level basically measure the same thing: erythrocytes. Hematocrit measures their volume and hemoglobin level measures their hemoglobin content.
If the patient has normal erythrocytes, he will have a standard amount of volume and a standard number of hemoglobin content for each erythrocyte. In such a patient, the hematocrit and hemoglobin level will always be equivalent. There is no condition where the hematocrit is elevated while the hemoglobin level is not. Hemoglobin level is a measurement of concentration. Thus, in a patient with fluid loss not only will the hematocrit go up, but also the hemoglobin level.
Hydration status can not alter hematocrit-hemoglobin level relationship. The relationship can only be altered when the erythrocytes are not normal. That means, either the volume or the hemoglobin content of erythrocytes is different from the standard value. For example, a patient whose blood is filled with spherocytes (a type of erythrocyte with less volume) will have a hematocrit value that is lower than the hemoglobin level multiplied by three (Ht < Hb x ±3). Other examples include iron deficiency anemia, thalassemia, and megaloblastic anemia.
We also have to keep in mind that measurements have coefficient of variations (CVs). If measurements for hematocrit and hemoglobin level have a CV of 3%, then a sample with 36% hematocrit and 12 g/dL hemoglobin level may be read as 37.1% and 11.6 g/dL. Thus, the patient will look like as having hypochromic erythrocytes (37.1 > 11.6 x 3) while he actually doesn't.
Finally, the only way of using hematocrit and hemoglobin level to assess a patient’s hydration status is not by comparing them to each other, but by comparing them to their own baseline values. For example, if a patient is known to have a baseline hematocrit value of 45% and now he has 50%, we can assume that there is fluid loss.
Conclusion:
1. Comparing hematocrit and hemoglobin level can not be used to assess a patient’s hydration status.
2. Hematocrit-hemoglobin level relationship (Ht = Hb x ±3) can only be altered if the erythrocytes are not normal, or look altered because of measurement CVs.
3. Hematocrit and hemoglobin level can be used to predict a patient’s hydration status by using them separately and comparing them to their own baseline values.
Note: I used the terms normal and standard more freely than I should to keep things simple.
Hematocrit is the volume of blood that is occupied by erythrocytes, expressed in percentage. Hemoglobin level is the concentration of hemoglobin in blood, expressed in g/dL. Hence, hematocrit and hemoglobin level basically measure the same thing: erythrocytes. Hematocrit measures their volume and hemoglobin level measures their hemoglobin content.
If the patient has normal erythrocytes, he will have a standard amount of volume and a standard number of hemoglobin content for each erythrocyte. In such a patient, the hematocrit and hemoglobin level will always be equivalent. There is no condition where the hematocrit is elevated while the hemoglobin level is not. Hemoglobin level is a measurement of concentration. Thus, in a patient with fluid loss not only will the hematocrit go up, but also the hemoglobin level.
Hydration status can not alter hematocrit-hemoglobin level relationship. The relationship can only be altered when the erythrocytes are not normal. That means, either the volume or the hemoglobin content of erythrocytes is different from the standard value. For example, a patient whose blood is filled with spherocytes (a type of erythrocyte with less volume) will have a hematocrit value that is lower than the hemoglobin level multiplied by three (Ht < Hb x ±3). Other examples include iron deficiency anemia, thalassemia, and megaloblastic anemia.
We also have to keep in mind that measurements have coefficient of variations (CVs). If measurements for hematocrit and hemoglobin level have a CV of 3%, then a sample with 36% hematocrit and 12 g/dL hemoglobin level may be read as 37.1% and 11.6 g/dL. Thus, the patient will look like as having hypochromic erythrocytes (37.1 > 11.6 x 3) while he actually doesn't.
Finally, the only way of using hematocrit and hemoglobin level to assess a patient’s hydration status is not by comparing them to each other, but by comparing them to their own baseline values. For example, if a patient is known to have a baseline hematocrit value of 45% and now he has 50%, we can assume that there is fluid loss.
Conclusion:
1. Comparing hematocrit and hemoglobin level can not be used to assess a patient’s hydration status.
2. Hematocrit-hemoglobin level relationship (Ht = Hb x ±3) can only be altered if the erythrocytes are not normal, or look altered because of measurement CVs.
3. Hematocrit and hemoglobin level can be used to predict a patient’s hydration status by using them separately and comparing them to their own baseline values.
Note: I used the terms normal and standard more freely than I should to keep things simple.
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