An accurate diagnosis of the type of leukemia is important. The exact diagnosis helps the doctor to

Estimate how the disease will progress
Determine the appropriate treatment

1. Blood Tests
After your doctor or clinician takes your blood, he or she sends it to a lab for a complete blood count (CBC), which shows the number of red cells, white cells and platelets in your blood.

A person with CLL will have increased numbers of lymphocytes. Low platelet counts and low red blood cell counts may also be present; these counts are usually only slightly decreased in the early stage of the illness.

2. Immunophenotyping
Immunophenotyping of lymphocytes is an important process used to diagnose CLL, by comparing the cancer cells to normal immune cells. The test results indicate whether or not the person’s lymphocytes are derived from a single cancer cell (leukemia) or from other noncancerous conditions. This test is especially important if the number of lymphocytes in the blood is only slightly elevated. Immunophenotyping also determines whether the abnormal cells are from a change in either B-cell or T-cell development. If the abnormal cells are of the B-cell type, the disease is CLL. If the abnormal cells are T-cells, the disease is called "T-cell prolymphocytic leukemia."

Immunophenotyping is done with an instrument called a "flow cytometer." A sample of cells from blood or marrow is tagged with an antibody that is specific for a site on the cell surface. The cells, stained with a light-sensitive dye, go through the flow cytometer, passing through a laser beam; if they have the antibody-specific surface feature, the cells light up and these cells are counted.This test can measure the number of cells in a sample and specific characteristics of the cells, including size, shape and the presence of specific markers on the cell surface. The diagnosis of CLL requires the presence of 5,000 abnormal B-cells per microliter of blood (5,000/uL).

3. Bone Marrow Tests
Generally, if the red blood cells and platelets are normal, a bone marrow aspiration and biopsy are not needed to make a diagnosis of CLL. However, these tests may be recommended before treatment begins. The test results can help rule out other diseases during the diagnostic stage and they can also be used later, during treatment, to evaluate the effectiveness of therapy.

Bone marrow testing involves two steps usually done at the same time in a doctor's office or a hospital:
A bone marrow aspiration to remove a liquid marrow sample
A bone marrow biopsy to remove a small amount of bone filled with marrow

4. Immunoglobulin Testing
The measurement of the concentration of immunoglobulins in the blood is another important test. Immunoglobulins are proteins, called "antibodies," which are made by B cells in healthy individuals to protect the body from infection. CLL cells do not make effective antibodies. CLL cells also interfere with the ability of the normal lymphocytes to make antibodies. As a result, people with CLL often have low levels of immunoglobulins, causing immune deficiency, which increases their risk of getting infections.

Fluorescence In Situ Hybridization (FISH)
This test studies chromosomes in tissue using DNA probes tagged with fluorescent molecules that emit light of different colors. The DNA probes then bind to specific genes or areas in the chromosomes within the cells and light up when viewed under a microscope with a special light.

About 80 percent of CLL patients who are tested with FISH have cytogenetic abnormalities in their leukemia cells. These cytogenetic abnormalities can help the doctor identify those people with CLL who are more likely to progress to the point of requiring therapy or those who may benefit most from use of certain types of therapy.