Complete Blood Count Health Article

Definition

A complete blood count (CBC) is a series of tests used to evaluate the composition and concentration of the cellular components of blood. It consists of the following tests: red blood cell (RBC) count, white blood cell (WBC) count, and platelet count; measurement of hemoglobin and mean red cell volume; classification of white blood cells (WBC differential); and calculation of hematocrit and red blood cell indices. The hematocrit is the percentage of blood by volume that is occupied by the red cells (i.e., the packed red cell volume). Red blood cell indices are calculations derived from the red blood cell count, hemoglobin and hematocrit that aid in the diagnosis and classification of anemia.

Purpose

The CBC provides valuable information about the blood and to some extent the bone marrow which is the blood-forming tissue. The CBC is used for the following purposes:

• As a preoperative test to ensure both adequate oxygen carrying capacity and hemostasis.
• To identify persons who may have an infection.
• To diagnose anemia.
• To identify acute and chronic illness, bleeding tendencies, and white blood cell disorders such as leukemia.
• To monitor treatment for anemia and other blood diseases.
• To determine the effects of chemotherapy and radiation therapy on blood cell production.

Precautions

The CBC requires a sample of blood collected from a vein. The nurse or phlebotomist performing the venipuncture should observe universal precautions for the prevention of transmission of bloodborne pathogens. The collection tube must be filled completely, as under-filling increases the anticoagulant (EDTA) to blood ratio, which will crenate red blood cells. The tourniquet should be removed from the arm as soon as the blood flows to prevent hemoconcentration. If a fingerstick is used to collect the blood, care must be taken to wipe away the first drop, and not to squeeze the finger excessively as this causes the blood to be diluted by tissue fluid. The tests should be performed within four hours of collection or the sample must be refrigerated. Samples stored at 35-46°F (2-8°C) may be measured for up to 18 hours. Samples must be thor- oughly mixed prior to measurement. Many drugs affect the results by causing increased or decreased RBC, WBC, and/or platelet production. Medications should be taken into account when interpreting results.

Description

The CBC is commonly performed on an automated hematology analyzer using well mixed whole blood anti- coagulated with EDTA. A CBC is a group of tests used to quantify the number of RBCs, WBCs, and platelets, provide information about their size and shape, measure the hemoblobin content of RBCs, determine the percentage and absolute number of the five white blood cell types, and identify early and abnormal blood cells. These tests are performed simultaneously, (usually in less than one minute), using an automated hematology analyzer. When the performance limit of the automated hematology analyzer is exceeded, sample dilution or pretreatment, manual smear review, or manual cell counts may be required. Such conditions include very low or elevated cell counts, and the presence of cold agglutinins, lipemia, and cell fragments. For example, a manual WBC count may be performed when the automated WBC count is below 500 per microliter. A manual microscopic evaluation of a stained blood film is performed when an abnormal cell population is encountered. Each laboratory has established rules for determining the need for manual smear review based upon specific CBC parameters. For example, a manual differential is always performed when nucleated red blood cells are found on an electronic cell count.

Electronic cell counting

Electronic blood cell counting is based upon the principle of impedance (i.e., resistance to current flow). Some hematology analyzers combine impedance counting with light scattering to measure platelets. A small sample of the blood is aspirated into a chamber (the WBC counting bath) and diluted with a balanced isotonic saline solution that is free of particles. The diluted blood sample is split into two parts, one for counting RBCs and platelets and the other for counting WBCs. The RBC portion is transferred to the RBC/platelet counting bath where it is diluted further. The other portion remains in the WBC bath and a detergent (lysing agent) is added to destroy (hemolyze) the red blood cells. A small portion of the diluted fluid in each bath is allowed to flow past a small aperture. An electrical current is produced in each aperture by two electrodes, one on the inside and the other on the outside of the aperture. The saline solution is responsible for conducting current between the electrodes. The cells move through the aperture one at a time. When a cell enters the aperture, it displaces a volume of electrolyte equal to its size. The cell acts as an electrical resistor, and impedes the flow of current. This produces a voltage pulse the magnitude of which is proportional to the size of the cell. Instrument electronics are adjusted to discriminate voltage pulses produced by different cells. These adjustments are called thresholds. For example, the threshold for counting a RBC is equivalent to a cell volume of 36 femtoliters or higher. Voltage pulses that are equivalent to volumes between 2-20 femtoliters are counted as platelets. This process is repeated two more times so that the RBC, WBC, and platelet counts are performed in triplicate. Each time frame for counting is several seconds and many thousands of cells are counted. The computer processes the counting data first by determining the agreement between the three counts. If acceptable criteria are met the counts are accepted and used to calculate the result. The computer mathematically corrects the count for the random chance of two cells entering the aperture simultaneously. The voltage pulses for each cell type are sorted and displayed. The RBC and platelet sizes are plotted as a histogram, and the WBC sizes are plotted as a scattergram. This process produces cell counts with coefficients of variation that are on the order of tenfold lower than can be achieved by manual cell counting.

The hemoglobin concentration is measured optically using the solution in the WBC bath. The lysing agent contains potassium cyanide that reacts with the hemoglobin to form cyanmethemoglobin. The optical density of the cyanmethemoglobin is proportional to hemoglobin concentration. Source light from a small tungsten lamp or an LED that produces monochromatic light is directed through the sample contained in a small tube behind the bath. An interference filter on the other side of the tube transmits unabsorbed monochromatic light (e.g., 525 nm) to a photodiode. The photodiode current is proportional to the light it receives. This electronic signal is converted to an inverse log voltage that is proportional to the optical density of the solution. The optical density reading for the diluent is subtracted from the sample and the value is multiplied by a calibration factor (determined by measuring a calibrating solution) in order to calculate hemoglobin concentration.

The voltage pulses produced by the white blood cells depend upon the size of the cell and its nuclear density. Therefore, the pulses may be analyzed to differentiate between the types of WBCs found. For example, lymphocytes are the smallest WBCs and comprise the lower end of the size scale. Monocytes, prolymphocytes, and immature granulocytes comprise the central area of the WBC histogram, and mature granulocytes comprise the upper end. In addition to cell sizing, automated instruments may use any of three other methods to distinguish between subpopulations. These are radio frequency conductance, forward and angular light scattering, and fluorescent staining.