Clinical Laboratory - Hematology
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Complete Blood Count (CBC)
Measured and Calculated Indices
A CBC is performed using an automated instrument that measures the numbers of RBCs, WBCs, and platelets per unit volume. In addition, the RBC indices are generated. A standard complete blood count is performed on an automated laboratory instrument that quantitates the amount of Hgb as well as the size, shape, and number of RBCs. A variety of calculations are performed to produce indices that provide information about RBC disorders. The standard indices are:
RBC Count: The number of RBCs per unit volume is measured directly and given in millions per microliter.
Mean Corpuscular Volume (MCV): The MCV is measured directly. The unit is a femtoliter (fL), or it can be given as cubic millimeters (mm3). The MCV measures the size of RBCs and is the most important index for classification of anemias into "macrocytic" with higher than normal MCV and "microcytic" with low MCV.
Hemoglobin (Hgb): The Hgb content is measured directly and given in grams per deciliter (g/dL). This value, along with Hct, provides the most useful measure of the oxygen carrying capacity of the blood.
Hematocrit (Hct) in % = (RBC count in millions X MCV) ÷ 10: The Hct is the packed cell volume and is a calculated value and provides a measure of the amount of oxygen carrying capacity in relation to blood volume.
Mean Corpuscular Hemoglobin (MCH) in pg = (Hgb X 10) ÷ RBC count in millions:
The MCH is calculated and gives the average mass of Hgb in an individual RBC; the unit is a picogram (pg).
Mean Corpuscular Hemoglobin Concentration (MCHC) in g/dL = (Hgb X 100) ÷ Hct:
The MCHC is calculated and provides a measure of the concentration of Hgb in the cells in g/dL.
Red Cell Distribution Width (RDW) = standard deviation of MCV: The RDW is calculated to provide a measure of the anisocytosis, or variation in size of the RBCs.
Automated vs. Manual Differentials
Some laboratory instruments can also generate an automated WBC differential count, based upon size, shape, and biochemical properties of different leukocytes in the blood. The advantages of the automated WBC differential are speed and low cost per test while the disadvantages include the initial cost of buying the instrument and inability to distinguish subtle differences in morphology. Abnormalities in numbers or shapes, or clumping, of WBCs or platelets may cause the instrument to 'flag' the result because a true automated count may not be possible. In those cases, a peripheral blood smear is made and the count estimated visually.
A manual WBC differential count is performed by having a trained person visually review the stained blood smear and manually count 100 white cells (or 50 cells in the case of severe leukopenia). The major advantage is that the trained observer can note subtle differences in WBC morphology and report additional changes in RBC morphology and platelets. The major disadvantage is the need for a trained person to spend increased time (with increased cost) to scan the smears. Also, there can be some inter-observer variation.
The absolute white blood cell counts are most useful, and are calculated by multiplying the % of each cell type counted by the total WBC count. Simple percentages may be misleading, since an apparent percentage increase in one constituent may actually be due to a significant and absolute decrease of another type of WBC. For example, 99% lymphs with an absolute WBC count of 1300/microliter means neutropenia, not lymphocytosis. Always consider the WBC percentages in the context of the total WBC count.
When tests are ordered too frequently, such as daily, then test results may only reflect biologic and test method variability, and nothing of clinical significance. Then monetary and personnel resources are wasted.
Without a change in the patient's status, a daily complete blood count is not necessary. Don't perform repetitive complete blood count testing in the face of clinical and lab stability (from the Society for Hospital Medicine - Adult Hospital Medicine) Before ordering a test, consider what you would do differently because of the test result.