Chronic Lymphocytic Leukemia (CLL) is a slow-growing type of cancer that begins in the blood and bone marrow. This malignancy involves B-lymphocytes, a specific type of white blood cell that becomes abnormal and accumulates over time. Testing for CLL is a methodical sequence of steps, moving from initial suspicion found in routine screening to definitive confirmation and molecular characterization.
Identifying CLL During Routine Blood Work
The first indication of Chronic Lymphocytic Leukemia often appears through a Complete Blood Count (CBC). This routine blood test measures the quantity of red blood cells, platelets, and white blood cells. CLL is initially suspected when the CBC reveals an abnormally high total white blood cell count.
Further analysis of the CBC usually shows absolute lymphocytosis, an excessive number of circulating lymphocytes. The threshold for CLL suspicion is a sustained count of 5,000 B-lymphocytes per microliter of blood. A peripheral blood smear may also show characteristic fragile cells known as “smudge cells,” which are lymphocytes damaged during preparation. These initial findings signal the need for more specialized testing to determine the nature of the proliferating cells.
Confirming the Diagnosis with Cell Analysis
Once the CBC suggests an overabundance of lymphocytes, the definitive step to confirm CLL is flow cytometry, also called immunophenotyping. This technique analyzes the surface markers (antigens) on the abnormal B-lymphocytes, differentiating CLL from other leukemias or lymphomas. A blood sample is passed through a laser beam, where fluorescently tagged antibodies bind to specific markers. The instrument reads these markers to create a distinct cellular profile.
The hallmark immunophenotype for CLL involves the co-expression of several specific markers. Abnormal B-cells express the B-cell markers CD19 and CD23, along with the T-cell marker CD5. These cells also show a weak, or “dim,” expression of the CD20 antigen and surface immunoglobulin (sIg), which distinguishes them from other B-cell malignancies. Confirmation that the B-cell population is clonal—meaning all cells originated from a single parent cell—is achieved by verifying that the cells express only one type of immunoglobulin light chain (kappa or lambda). This detailed analysis confirms the diagnosis and is usually sufficient, meaning a bone marrow biopsy is often not required initially.
Understanding Disease Behavior Through Genetic Testing
Beyond confirming CLL, specialized genetic tests are conducted to understand the disease’s potential behavior and guide future treatment decisions. One important method is Fluorescence In Situ Hybridization (FISH), which uses fluorescent probes to detect structural changes in the chromosomes of the CLL cells. This test identifies specific chromosomal deletions that are ranked based on their prognostic impact.
Key Chromosomal Deletions
A deletion on the long arm of chromosome 13, known as del(13q), is associated with the most favorable prognosis. Conversely, the loss of part of the short arm of chromosome 17, called del(17p), which includes the TP53 tumor-suppressor gene, carries the most unfavorable prognosis. A deletion on chromosome 11, del(11q), is also considered a marker of more aggressive disease, often linked to extensive lymph node involvement.
The IgHV mutation status is another prognostic indicator, determined by sequencing the immunoglobulin heavy-chain variable gene. Patients are classified as having either mutated or unmutated IgHV. The unmutated status signals a more unstable and aggressive disease that progresses faster and responds poorly to conventional treatments.
The status of the TP53 gene is also directly assessed, both through the FISH test for del(17p) and through gene sequencing to detect point mutations. The presence of any TP53 aberration is a major factor that directs the use of targeted therapies over traditional chemoimmunotherapy.
Determining Disease Extent and Staging
After diagnosis and genetic profiling, staging determines the extent of the disease. This involves a thorough physical examination where the physician checks for enlarged lymph nodes in the neck, armpits, and groin, and palpable enlargement of the spleen (splenomegaly) or liver (hepatomegaly). The Rai staging system relies heavily on these physical findings combined with CBC results to categorize disease risk.
Imaging tests, such as a Computed Tomography (CT) scan, may assess the size of internal lymph nodes and organs more precisely, but are not always required for initial staging. A bone marrow aspiration and biopsy is sometimes performed, generally only if blood counts are severely low (cytopenias) or if CLL is suspected of transforming into a more aggressive lymphoma. Since primary diagnostic information comes from peripheral blood, the bone marrow procedure is a secondary tool for evaluating progression or complications.