Because of the prolonged survival, which was typically about 10 years in past decades, but which can extend to a normal life expectancy, the prevalence (number of people living with the disease) is much higher than the incidence (new diagnoses). CLL is the most common type of leukemia in the UK, accounting for 38% of all leukemia cases. Approximately 3,200 people were diagnosed with the disease in 2011.In Western populations, subclinical "disease" can be identified in 3.5% of normal adults, and in up to 8% of individuals over the age of 70. That is, small clones of B cells with the characteristic CLL phenotype can be identified in many healthy elderly persons. The clinical significance of these cells is unknown. In contrast, CLL is rare in Asian countries, such as Japan, China, and Korea, accounting for less than 10% of all leukemias in those regions. : 1432  A low incidence is seen in Japanese immigrants to the US, and in African and Asian immigrants to Israel.Of all cancers involving the same class of blood cell, 7% of cases are CLL/SLL.People who live near areas with considerable industrial pollution have an elevated risk of developing leukemia, particularly CLL. Research directions
In light of new therapies such as targeted agents, the role of bone marrow transplants is decreasing. Bone marrow transplants are not recommended as a front-line therapy, and only recommended in specific cases where front-line therapies have either failed or there is a lack of response to BCL-2 inhibitors.
Exposure to Agent Orange increases the risk of CLL, and exposure to hepatitis C virus may increase the risk. There is no clear association between ionizing radiation exposure and the risk of developing CLL. Blood transfusions have been ruled out as a risk factor. Diagnosis
The diagnosis of CLL is based on the demonstration of an abnormal population of B lymphocytes in the blood, bone marrow, or tissues that display an unusual but characteristic pattern of molecules on the cell surface. CLL is usually first suspected by a diagnosis of lymphocytosis, an increase in a type of white blood cell, on a complete blood count test. This frequently is an incidental finding on a routine physician visit. Most often the lymphocyte count is greater than 5000 cells per microliter (µl) of blood but can be much higher. The presence of lymphocytosis in a person who is elderly should raise strong suspicion for CLL, and a confirmatory diagnostic test, in particular flow cytometry, should be performed unless clinically unnecessary. Molecular examination of peripheral blood and flow cytometry
The combination of the microscopic examination of the peripheral blood and analysis of the lymphocytes by flow cytometry to confirm clonality and marker molecule expression is needed to establish the diagnosis of CLL. Both are easily accomplished on a small amount of blood. A flow cytometer instrument can examine the expression of molecules on individual cells in fluids. This requires the use of specific antibodies to marker molecules, with fluorescent tags recognized by the instrument.