Complete Blood Count Test (CBC) in details

Complete Blood Count Test (CBC)

What is the complete blood count test (CBC)? 

Many blood tests measure the amount of a particular chemical or protein in your blood, but a complete blood count checks the blood cells themselves. It measures the numbers of different types of blood cells, their sizes, and their appearance. It is a very common and useful blood test. Any irregularities can then be observed and used as a basis for follow-up check-ups.

In general, the test measures 3 main components of blood: 

• Red blood cells (also called erythrocytes or RBCs). The test measures the number, size, shape, and appearance of the RBCs, and also the amount of hemoglobin in them. Hemoglobin carries oxygen from the lungs to the rest of the body. The part of the test called a hematocrit measures the percentage of your blood that is red blood cells.

The erythrocytes are the most numerous blood cells i.e. about 4-6 millions/mm3.

In the red cells of the mammalians, the lack of nucleus allows more room for hemoglobin and the biconcave shape of these cells raises the surface and cytoplasmic volume ratio. Thesecharacteristics make more efficient the diffusion of oxygen by these cells. In so-called "sickle-cell anemia", erythrocytes become typically sickle-shaped. With the electron microscope, biologists saw that red cells can have different shapes: normal (discocyte), berry (crenated), burr (echinocyte), target (codocyte), oat, sickled, helmet, pinched, pointed, indented, poikilocyte, etc. The mean life of erythrocytes is about 120 days. When they come
to the end of their life, they are retained by the spleen where they are phagocyted by macrophages

• Hematocrit Count
  The hematocrit count calculates the percentage of red blood cells per micro liter of blood. The range for an average male is from 42%-52% while the range for an average female would be from 37%-47%.
  This test is highly similar to the red blood cell count and has similar implications if the count is either too high or too low.

•   Hemoglobin Count
    Hemoglobin is found within the biconcave red blood cells and it combines readily with the oxygen molecule to form the unstable compound, oxy-hemoglobin. Blood with oxy-hemoglobin is observed to be bright red.
  The hemoglobin test should be coherent with the red blood cell and hematocrit test; otherwise, further testing may be needed to confirm the irregularities. Implications of the hemoglobin tests are in sync with that of the red blood cell count.
  The normal hemoglobin count is 12-16 grams of hemoglobin per deciliter of blood in females and 14-18 grams of hemoglobin per deciliter of blood in males.
• White blood cells (also called leukocytes or WBCs). The total count of white cells is measured. White blood cells help the body's immune system fight infection. When the amounts of each of the different types of white blood cells are also measured, the test is called a differential. The most common types are neutrophils (also called polymorph nuclear cells, PMNs, polys, or granulocytes) and lymphocytes.

• Platelets (also called thrombocytes). Platelets are not actually blood cells. They are fragments of large blood -forming cells. These fragments are essential for normal blood clotting. Even if platelets appear roundish in shape, they are not real cells. In the smears stained by Giemsa, they have an intense purple color. Their diameter is 2-3 µm about; hence they are much smaller than erythrocytes. Their density in the blood is 200000-300000 /mm³.

• Differential Blood Count

         As mentioned earlier in the site, there are five different types of white blood cells which serve different functions in our body. The five different types of white blood cells are the neutrophils, lymphocytes, monocytes, eosinophils and the basophils.

The differential blood count is done to calculate the percentage of each type of white blood cell there is in relation to each other. It also gives a clearer picture in a  diagnosis for the cause of a disease.

Fig 2

White blood cells, or leukocytes, are classified into two main groups: granulocytes   and nongranulocytes (also known as granulocytes).

• The granulocytes, which include neutrophils, eosinophils, and basophiles, have granules in their cell cytoplasm. Neutrophils, eosinophils, and basophiles also have a multilobed nucleus. As a result they are also called polymorphonuclear leukocytes or "polys." The nuclei of neutrophils also appear to be segmented, so they may also be called segmented neutrophils or "segs."
• The nongranuloctye white blood cells, lymphocytes and monocytes, do not have granules and have nonlobular nuclei. They are sometimes referred to as mononuclear leukocytes. .

Two measurements of white blood cells are commonly done in a CBC:

• The total number of white  blood cells in a micro liter (1x10^-9 liters) of blood , reported as an absolute number of "X" thousands of white  blood cells, and
• The percentage of each of the five types of white blood cells. This test is known as a differential or "diff" and is reported in percentages.

Normal values for total WBC and differential in adult males and females are:

• Total WBC: 4,500 - 10,000
• Bands or stabs: 3 - 5 %
• Granulocytes (or polymorphonuclears)
• Neutrophils (or segs): 50 - 70% relative value (2500-7000 absolute value)
• Eosinophils: 1 - 3% relative value (100-300 absolute value)
• Basophils: 0.4% - 1% relative value (40-100 absolute value)
• A granulocytes (or mononuclear)
• Lymphocytes: 25 - 35% relative value (1700-3500 absolute value)
• Moncytes: 4 - 6% relative value (200-600 absolute value)

Each differential always adds up to 100%. To make an accurate assessment, consider both relative and absolute values. For example a relative value of 70% neutrophils may seem within normal limits; however, if the total WBC is 20,000, the absolute value (70% x 20,000) would be an abnormally high count of 14,000.

The numbers of leukocytes changes with age and during pregnancy.

• On the day of birth, a newborn has a high white blood cell count, ranging from 9,000 to 30,000 leukocytes. This number falls to adult levels within two weeks.
• The percentage of neutrophils is high for the first few weeks after birth, but then lymphocyte predominance is seen.
• Until about 8 years of age, lymphocytes are more predominant than neutrophils.
• In the elderly, the total WBC decreases slightly.
• Pregnancy results in a leukocytosis, primarily due to an increase in neutrophils with a slight increase in lymphocytes.

Leukocytosis, a WBC above 10,000, is usually due to an increase in one of the five types of white blood cells and is given the name of the cell that shows the primary increase.

• Neutrophilic leukocytosis = neutrophilia
• Lymphocytic leukocytosis = lymphocytosis
• Eosinophilic leukocytosis = eosinophilia
• Monocytic leukocytosis = monocytosis
• Basophilic leukocytosis = basophilia

Leukocytes: critical low and high values

• A WBC of less than 500 places the patient at risk for a fatal infection.
• A WBC over 30,000 indicates massive infection or a serious disease such as leukemia.