COUNTING OF BLOOD CELLS WITH AUTOMATED WHITE BLOOD CELL DIFFERENTIAL COUNT
In most automated instruments that count cells in the blood, the red blood cells, the white blood cells, and the platelets are counted by an electrical impedance method. In addition, hemoglobin is measured directly from the whole blood specimen after lysis of the red blood cells. There are many other parameters in an automated blood cell count, which are calculated, based upon the data obtained from the direct cell counts and the hemoglobin value. Calculated parameters include the hematocrit, red blood cell count, the mean corpuscular volume (MCV), the mean corpuscular hemoglobin (MCH), the mean corpuscular hemoglobin concentration (MCHC) of red blood cells, and the mean platelet volume (MPV). The white blood cell differential count is obtained after lysis of erythrocytes, using flow cytometric technology to identify size, shape, and granularity of the cells which permits identification of the individual white blood cell populations.
PERIPHERAL BLOOD SMEAR ANALYSIS
A peripheral blood smear is prepared by adding a drop of blood to a glass slide and smearing it in a specific way to spread out the blood cells for microscopic examination. Most commonly, a small device spreads the drop of blood on the surface. The sample is stained with Wright stain, and the smear is then reviewed microscopically at low and high magnification. Certain cells tend aggregate along the edges of the smear. There is a recommended procedure for microscopic review to account for variability in distribution of the cells. The microscopic review of the peripheral blood smear provides information about the size, shape, and number of red blood cells, the different populations of white blood cells, and an approximation of the number of platelets.
SICKLE CELL SCREENING ASSAY
This test provides a rapid assessment for the presence of hemoglobin S. Further analysis of different hemoglobin types is performed by hemoglobin analysis. In one commonly used test called the sickling test, a drop of the patient’s blood is placed onto a glass slide, and a reducing agent or normal saline (as a control) is added to the drop of blood on the slide. If sickle hemoglobin is present, the red blood cells from the patient exposed to the reducing agent show sickling.
In the solubility test, a reducing agent is added to a tube containing the patient’s blood. After incubation, the tube is placed in front of a card with black lines. The solubility test is positive for sickle hemoglobin when black lines cannot be seen through the specimen. The solution is turbid as a result of insoluble crystals of hemoglobin S.