The Complete Blood Count

A complete blood count (or CBC) is a blood test that measures the levels of the major blood components. It is one of the most commonly obtained tests in the medical field and gives essential information about many different acute and chronic conditions. 

Complete blood counts measure the percentage of red blood cells, hemoglobin, hematocrit, white blood cells, and platelets.

RED BLOOD CELLS

Besides a count of the cells, the size and shape of the cells are determined, along with the amount of hemoglobin contained within them.

• A normal red blood cell count for males is 4.32-5.72 trillion cells per liter and for females is 3.90-5.03 trillion cells per liter.
• Normal hemoglobin levels for a male are 13.5-17.5 grams per deciliter (g/dl), and for a female is 12.0-15.5 g/dl.
  • Hemoglobin is a measure of the protein in the blood responsible for carrying the majority of oxygen. An insufficient amount of hemoglobin is known as anemia. Anemia that results in a hemoglobin level below 7 is considered critical anemia and is almost universally treated by giving the patient more blood.
• Hematocrit is a proportion--the proportion (%) of red blood cells contained within an amount of whole blood. It compares the red blood cell mass with the total combined red blood cell mass + the plasma.
  • Normal hematocrit levels for a male is 38.8-50.0% and for a female is 34.9-44.5%. In an amount of blood, for example, if the red blood cell mass makes up 40% of the total blood in a certain container (say, a test tube), then the HCT is 40%, with 55-56% being plasma, and another 1-3% being the white blood cell mass, seen in a spun-down tube of blood as a thin white streak between the red blood cells and plasma, called the "buffy coat" layer.
  • Here's a rapid calculation trick: To estimate plasma percentage, simply add 8 to the hematocrit. This works because blood is mainly red blood cells (measured by hematocrit) and plasma, with about 2% white blood cells. The 'Add 8 Rule' accounts for this and gives you a quick plasma estimate at the bedside without complex math. HOWEVER, while this gives you a rapid calculation, it should NOT replace more accurate methods if they are available, such as direct measurement of plasma volume or detailed laboratory analyses. Conditions like multiple myeloma or other plasma cell disorders demand precise measurements for accurate diagnosis and treatment planning, so this is definitely not a one-size-fits-all. 

WHITE BLOOD CELLS

White blood cells (WBCs) fight infections as part of the immune system. A normal white blood cell count is the same for males and females and is 3.5-10.5 billion cells per liter.

White Blood Count DIFFERENTIAL:

In a CBC, the differential refers to a separation of the different WBCs into their individual types. Since many diseases cause varying predominances of one WBC type over the others as part of the immune response to fight infection, the differential can be useful in, for example, telling a bacterial infection from a viral one, a certain type of bacterial infection from another type, or in establishing a diagnosis of allergy.

TYPES OF WBCs

POLYMORPHONUCLEAR Leukocytes (Multi-lobed nuclei--"granulocytes")

• Neutrophils (62% of total WBC count), target bacteria and fungi.
• Eosinophils (2-3%), target parasites and are active in allergies.
• Basophils (0.4%) release histamine in allergic and antigen response, and heparin, which allows WBCs to migrate more easily.

MONONUCLEAR Leukocytes (round nuclei--"agranulocytes")

• Lymphocytes (30%)

composed of B-cells, which activate T-cells, T-cells, which target viruses and tumor cells, and Natural Killer (NK) cells, targeting virus-infected and tumor cells.

• Monocytes (5.3%), differentiate into macrophages.

PLATELETS

Platelets are pale, non-nucleated cells that are a vital part of the clotting process.

A normal platelet count is the same for males and females and is 150-450 billion per liter, on a CBC, it most often labeled as 150,000 - 450,000/mL. 

If the platelet count drops below 100,000 the risk of severe bleeding greatly increases, at this level you should be concerned about potential bleeding in a given patient. If the platelet count drops below 50,000 patients are generally at such a severe risk of bleeding that even emergency procedures may be considered too risky.