Most of us know that when we go to our healthcare provider with a physical complaint or for a check-up there is a good chance we will be prodded with a needle. Once the blood is drawn, the doctor usually has a lab do a CBC, or complete blood count.

One of the reasons the CBC is such a popular test is that it screens for many different kinds of medical problems by measuring types of blood cells and other components of the blood. It's also used to monitor blood counts when people are taking certain medications that affect the blood, or have blood-related conditions.

Below, Anthony Kurec, administrator for the University Pathology Laboratory at SUNY Upstate Medical University in Syracuse, explains what CBCs measure and what they tell patients and doctors about patients' health.

What is a CBC?
The CBC measures the number of blood cells and related components in circulating blood. Specifically, the CBC measures the number of red blood cells, white blood cells, and platelets present in the peripheral blood, which is the blood that travels through the veins and capillaries. They all have different functions, and in normal, healthy individuals, they are relatively constant in number.

It also measures the level of hemoglobin, which is a protein within the red blood cell that carries oxygen to tissues and the hematocrit, which is the percentage of red blood cells per total volume of blood, as well as several other measurements related to the red blood cells. These include MCV (mean corpuscular volume), which measures the average volume of the red blood cell; MCH (mean corpuscular hemoglobin), which measures the average amount of hemoglobin within that red blood cell; and MCHC (mean corpuscular hemoglobin concentration), the average percentage of hemoglobin in the red blood cell.

Why would you use this screening test?
It's probably one of the first tests that a doctor orders. What they are looking for depends on what your complaint is. A doctor might recommend a CBC because you have constant fatigue and might be anemic. It is often used to screen for any suspected disease or condition caused by dangerously abnormal changes in cells counts, such as the presence of certain cancers, exposure to certain chemicals, or infections. It's routinely used when you have a hematologic disease such as leukemia or lymphoma, or other blood-related or chronic illness.

In addition, it can be used to monitor the effects of medications that are known to alter and even suppress bone marrow activity. This is especially true for patients who have certain cancers, particularly leukemia or lymphoma, and those who are receiving chemotherapy or radiation therapy. Patients with extremely low cell counts often have to be hospitalized because they are susceptible to infections, bleeding and other life-threatening concerns.

What is measured in the white blood cell count?
The white blood cell count measures the total number of white blood cells per unit of blood (usually reported as per liter of blood). In addition, the different types of white blood cells are also measured. The five main white blood cell types are neutrophils, lymphocytes, monocytes, and a small number of eosinophil and basophil cells, though sometimes these two cell types are not present at all.

What conditions can cause a high white blood cell count?
If you have a bacterial infection, the white blood cell count may increase. In some conditions, such as appendicitis, the white blood cell count increases. In mononucleosis, the lymphocyte count tends to increase. Other diseases or conditions may reflect a similar unique presence or absence of certain cell types.

What if your white blood cell count is low?
Certain conditions can suppress the white cell production in the bone marrow. Viral infections, exposure to certain chemicals (such as benzene or other carcinogens) or radiation and chemotherapy can all reduce your white cell count. And then there are some diseases such as aplastic anemia that are associated with low white blood cell counts because your bone marrow stops producing these cells.

Why does a low red blood cell count or hemoglobin value mean?
A low hemoglobin or hematocrit might mean you are anemic. Hemoglobin is a molecule of protein, but also includes a component consisting of iron. If your iron intake is low or if you have chronic bleeding, your hematocrit and hemoglobin will decrease. This requires prompt medical attention.

You might see a decreased red count in sickle cell anemia and other hemoglobin abnormalities. These are hereditary conditions and include hemoglobin S (sickle cell anemia), hemoglobin C, and hemoglobin E (often seen in Southeast Asians). Another type of hemoglobin problem is thalassemia (or Cooley's anemia) often seen in individuals from Mediterranean heritage.

What does it mean if you have a low or high platelet count?
Platelets are cells that circulate in the blood whose main function is to initiate the clotting of blood. Abnormal platelet counts may be seen in a variety of diseases or medical conditions. Decreased platelet count may result from certain drugs or diseases that suppress bone marrow cell production. You can see decreased cell counts when cancer of any organ spreads to the bone marrow because the cancer cells begin to crowd out the cells that produce normal white, red or platelet cells. Significantly decreased platelet counts can result in serious episodes of spontaneous bleeding.

You can also find elevated numbers of platelets caused by chronic medical conditions, various bone marrow diseases or with the use of certain drugs. This can result in bleeding problems caused by large clumps of platelets blocking the flow of blood within a blood vessel.

How are normal ranges for these counts established?
The normal number of white blood cells, red blood cells and platelets vary from individual to individual, so a "reference" range has to be established. Reference ranges are determined by measuring blood specimens from a number of normal, healthy individuals. By selecting a variety of normal subjects, a lower limit and an upper limit range can be established. For instance, in healthy individuals, the reference range for white cell counts is roughly 5,000 to 10,000 white cells per milliliter of blood. While there may be slight differences in the actual ranges, a reference for your laboratory is established and serves as the guideline that you use.

One thing to keep in mind when reviewing test results from different laboratories is that there can be differences in the reported test values due to variations in the testing equipment used, the analytical test kits and other environmental factors. All clinical laboratories in the United States are subject to federal regulations to ensure these laboratories operate at a high level of quality. This is why it is important to know what the "reference range" is for that test as performed by that laboratory and to discuss the test results with your health care provider.

What factors affect reference ranges?
In very young children, the white cell count, particularly the number of lymphocytes, are a little bit higher than they are in adults. You do see a real difference between men and women as far as red cell counts, hemoglobin and hematocrits are concerned. Men tend to have higher values than women, and that range reflects that.

In what context should the results be interpreted?
Because a CBC reflects the current status of circulating blood, abnormal results are not always diagnostic of a specific disease. Unlike a pregnancy test that is either positive or negative, changes in the CBC can result from a number of diseases, conditions or other environmental situations.

Are there activities that can affect normal levels?
There are situations that can affect results. For instance, your white blood cell count and your platelet count may be slightly higher after you exercise. Hemoglobin and hematocrits may be slightly elevated in those who smoke.