How a CBC is measured
Today's automated CBC results provide the doctor with 11 different parameters, but when doctors evaluate the results of a patient's CBC, they usually focus on four basic measurements:
White Blood Cell (WBC) count and Differential
A low WBC count (leukopenia) is the norm during chemotherapy treatments. Like all blood cells, WBCs are made in the bone marrow. Chemotherapy selectively focuses its killing power on rapidly dividing cells, such as cancer cells. That is how it kills cancer cells and not the patient. Unfortunately, the cells in the bone marrow also divide rapidly and are attacked by most chemotherapy agents and the WBC count falls. If it drops too low, chemotherapy is temporarily halted while the bone marrow recovers.
A high WBC count (leukocytosis) is often found when the body is attacked by more serious infections such as bacterial pneumonia or diverticulitis. It is not unusual to see values around 15,000 to 30,000 during a serious bacterial infection. As the infection responds to antibiotic treatment, the WBC count quickly returns to normal. An extreme elevation in the WBC count is seen when cancer of the blood cells develops, a serious condition called leukemia. In this instance the WBC may be well over 100,000.
Not all WBCs are the same. In fact, there are five different types of white blood cells that together make up the total WBC count. Some are better at fighting infection while others focus on supporting the immune system. Others may be involved in allergic reactions. If the doctor orders a CBC with differential, or smear, the lab will break down the WBC count into the different types of white blood cells. This information is sometimes useful in determining the type and severity of an infection, allergic reactions, and other blood disorders.
Red Blood Cell (RBC) level
There are different types of anemia. The most common is due to iron deficiency. More common in women, iron deficiency anemia is often due to pregnancy or heavy menstrual periods. Iron deficiency anemia can also result from slow chronic blood loss from the digestive tract. Strangely, some patients with iron deficiency anemia develop a compulsive craving to eat ice, starch and even dirt. This odd craving, called pica, disappears as the iron is replenished.
More numerous than WBCs, red blood cells are measured in millions per cubic millimeter. A normal RBC count is 3.5 - 5. However, in clinical practice, most doctors use another RBC measurement that is often referred to as the "H & H." This acronym stands for Hemoglobin and Hematocrit, two other ways to monitor the amount of red blood cells in the body.
Hemoglobin is the oxygen carrying protein that gives red blood cells their red color. Hemoglobin allows red blood cells to carry fresh oxygen to the body's cells and to transport carbon dioxide waste back to the lungs where it is exhaled. Hemoglobin is measured in grams per deciliter. A normal HBG is different between the sexes: 14 - 18 gms/dL for men and 12 - 16 gms/dL for women.
Another measurement of red blood cells is the Hematocrit which measures the amount of space that RBCs take up in the blood. This simple test is done by placing fresh unclotted blood in a narrow centrifuge tube, which is spun rapidly, forcing the red blood cells to the bottom of the tube and displacing the plasma to the top. The HCT is reported as a percentage of red blood cells to the total blood volume. A normal HCT is different between the sexes: 42 - 52% for men and 37 - 48% for women.
Mean Corpuscular Volume (MCV)
A low MCV value means that the red blood cells have shrunk somewhat and are smaller than normal. This is called microcytosis and is often seen in two circumstances - iron deficiency anemia and a hereditary anemia called Thalassemia. A low MCV caused by iron deficiency takes many months to develop and suggests that slow blood loss has been going on for at least several months. Thalassemia is sometimes seen in individuals with a Mediterranean heritage. It can be mild or severe, but taking iron supplements won't help. Less experienced physicians will sometimes mistake the small red blood cells of the person with Thalassemia as a sign of iron-deficiency anemia and incorrectly prescribe iron supplements. A high MCV, or macrocytosis, may be seen in cases of severe deficiency of vitamin B12 or Folic acid. A high MCV is also seen in alcoholics as a toxic effect of alcohol on the bone marrow.
Platelet (PLT) count
When you don't have enough platelets, you have a condition called thrombocytopenia which may lead to easy bruising and increased bleeding from the nose or gums. A count below 50,000 can result in spontaneous bleeding; below 5,000, patients are at risk of severe life-threatening bleeding. This is sometimes seen during chemotherapy. A low PLT count can also be a consequence of an autoimmune reaction when the body mistakenly attacks its own platelets which clump together and are destroyed in the spleen.
Mild elevations in the platelet count called thrombocytosis, may simply be a manifestation of iron deficiency. But, when severe, thrombocytosis can lead to unwanted blood clots inside the body.
Every year, I have my annual physical and blood work. Here are the results of my last CBC. I am pleased to report that at age 63 my blood cells (WBCs, RBCs, and PLTs) are all still in the normal range:
2. A Patient with Crohn's disease
This is a young man in his twenties who had been sick for 4 months. He came to our Emergency Department when he developed abdominal pain and a fever that would not go away. Diagnostic tests found that he had developed Crohn's disease, an inflammatory condition of the small intestine. This led to malnutrition and anemia. At surgery, he was also found to have an infection in the abdomen that required drainage and antibiotics. Here is his CBC for comparison:
Note the high WBC count which corresponded with his bacterial infection. He was quite anemic with a low Hemoglobin and Hematocrit. The MCV was also quite low signifying that iron deficiency was the cause of his anemia and that it had been present for at least several months. Fortunately, he recovered from his surgery and is now receiving maintenance therapy for Crohn's disease and doing much better. The WBC count returned to normal as the infection cleared and a course of iron supplements corrected his iron deficiency anemia.
The CBC is a common inexpensive screening test that helps the doctor determine your general health status. It is also used to diagnose and monitor a variety of conditions such as infections or anemia. Normal values can vary slightly from laboratory to laboratory. The results of a CBC must be interpreted by the doctor, taking into consideration the age and sex of the patient and any symptoms that may be present. If you have any questions about the results of your CBC, discuss them with your doctor.