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PubMed This is a summary of 15 peer-reviewed journal articles Updated
Hematology · Chronic Lymphocytic Leukemia

Biology and Diagnosis of CLL

At a Glance

Chronic Lymphocytic Leukemia (CLL) is diagnosed primarily through a blood test called flow cytometry, which identifies a specific pattern of cell markers. A definitive diagnosis requires having at least 5,000 clonal B-cells per microliter of blood, often without needing a bone marrow biopsy.

To understand Chronic Lymphocytic Leukemia (CLL), it helps to look at the “machinery” of your immune system. CLL is a cancer of mature B-cells—specialized white blood cells that are supposed to protect you by producing antibodies [1]. In a healthy body, these cells grow, fight germs, and eventually die. In CLL, a “glitch” in the cell’s internal signaling (specifically the B-cell receptor or BCR pathway) tells these cells to keep living and multiplying long after they should have stopped [2][3].

The Blueprint of a CLL Cell

Because CLL cells look very much like normal white blood cells under a microscope, doctors use a specialized test called flow cytometry to identify them [4]. This test acts like a high-tech scanner that looks for specific proteins, called markers, on the surface of the cells [5].

For a definitive diagnosis of CLL, the “barcode” on the cell surface usually looks like this:

  • CD19+ and CD20+: These confirm the cell is a B-cell [4].
  • CD5+: This is a marker normally found on T-cells; its presence on a B-cell is a hallmark of CLL [4][1].
  • CD23+: This helps confirm the cell is a “classic” CLL cell [4].
  • CD200+: This is an extremely helpful marker that is almost always present in CLL but usually absent in “look-alike” cancers [5][6].

Differentiating CLL from “Look-Alikes”

One of the most important jobs your hematologist has is making sure your CLL isn’t actually Mantle Cell Lymphoma (MCL). MCL is a different type of B-cell cancer that often requires more immediate or aggressive treatment [7].

Doctors distinguish them by looking for specific clues:

  • CD23 and CD200: These are usually “positive” in CLL but “negative” or very low in Mantle Cell Lymphoma [8][9].
  • Cyclin D1: This is a protein caused by a specific genetic swap called the t(11;14) translocation [7]. It is the “smoking gun” for Mantle Cell Lymphoma and is almost never found in CLL [7].
  • CD20 Intensity: CLL cells usually have a “dim” or weak CD20 marker, whereas MCL cells often show it “brightly” [8].

Your Diagnostic Checklist

A diagnosis of CLL is rarely based on a single blood draw. According to international guidelines (iwCLL), several criteria must be met for a definitive diagnosis [4]:

  1. The “5,000 Rule”: You must have at least 5,000 clonal B-cells per microliter (µL) of blood [4]. (“Clonal” simply means that these cells are all identical copies of the same cancerous cell.) If the count is lower and you have no other symptoms, it might be a precursor condition called Monoclonal B-cell Lymphocytosis (MBL) [3].
  2. Flow Cytometry: This is the essential test that confirms the CD5/CD19/CD23 protein “barcode” [1].
  3. FISH Testing: Short for Fluorescence In Situ Hybridization, this test looks for chromosomal “breaks” or extra pieces (like the 17p deletion) that help predict how the disease will behave [10][1].
  4. IGHV Mutation Analysis: This molecular test determines if your CLL cells are “mutated” or “unmutated,” which is one of the most important factors in long-term planning [11][12].
  5. TP53 Sequencing: A separate test from FISH that looks for specific mutations in a “guardian” gene that helps control cell growth [13][14].

In most cases, a bone marrow biopsy is not required for a diagnosis of CLL if the blood work provides clear answers [1][15]. It is generally reserved for situations where blood counts are confusing or when your doctor needs to investigate why other blood cells (like red cells or platelets) are low [1].

Common questions in this guide

What is the 5,000 rule for diagnosing CLL?
To be diagnosed with CLL, you must have at least 5,000 clonal B-cells per microliter of blood. If your count is lower and you have no other symptoms, it may be a precursor condition called Monoclonal B-cell Lymphocytosis (MBL).
What does flow cytometry do in a CLL diagnosis?
Flow cytometry is a specialized blood test that acts like a scanner to identify specific proteins on the surface of your cells. It confirms the unique pattern of markers, such as CD5, CD19, CD23, and CD200, which are classic signs of CLL.
Do I need a bone marrow biopsy to be diagnosed with CLL?
In most cases, a bone marrow biopsy is not required for a CLL diagnosis if your blood work provides clear answers. It is usually only necessary if your blood counts are confusing or if other cells like red blood cells or platelets are unusually low.
How do doctors know I have CLL and not Mantle Cell Lymphoma?
Doctors differentiate CLL from Mantle Cell Lymphoma by looking for specific cell markers. Mantle Cell Lymphoma usually tests positive for a protein called Cyclin D1 and lacks the CD23 and CD200 markers that are typically found in CLL.
What are FISH and IGHV tests for CLL?
FISH testing looks for chromosomal changes in your leukemia cells, while IGHV mutation analysis checks for specific genetic mutations. Both are critical molecular evaluations that help predict how your disease will behave and guide your long-term care plan.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does my flow cytometry report show the 'classic' CLL profile of CD5, CD19, CD23, and CD200?
  2. 2.Is my absolute clonal B-cell count above the 5,000 cells/µL threshold required for a CLL diagnosis?
  3. 3.How did you rule out Mantle Cell Lymphoma in my case—was a test for the t(11;14) translocation or Cyclin D1 performed?
  4. 4.Have we confirmed my IGHV mutation status and TP53 status yet?
  5. 5.Is a bone marrow biopsy necessary for me at this stage, or is the blood work sufficient for a definitive diagnosis?

Questions For You

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References

References (15)
  1. 1

    Composite Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma and Mantle Cell Lymphoma; Small Cell Variant: A Real Diagnostic Challenge. Case Presentation and Review of Literature.

    Ibrahim F, Al Sabbagh A, Amer A, et al.

    The American journal of case reports 2020; (21()):e921131 doi:10.12659/AJCR.921131.

    PMID: 32150530
  2. 2

    Chronic lymphocytic leukemia and mantle cell lymphoma: crossroads of genetic and microenvironment interactions.

    Puente XS, Jares P, Campo E

    Blood 2018; (131(21)):2283-2296 doi:10.1182/blood-2017-10-764373.

    PMID: 29666114
  3. 3

    Tonic signaling of the B-cell antigen-specific receptor is a common functional hallmark in chronic lymphocytic leukemia cell phosphoproteomes at early disease stages.

    Díez P, Juanes-Velasco P, García-Vaquero ML, et al.

    Molecular oncology 2025; (19(12)):3706-3728 doi:10.1002/1878-0261.70032.

    PMID: 40129242
  4. 4

    Atypical Chronic Lymphocytic Leukemia-The Current Status.

    Robak T, Krawczyńska A, Cebula-Obrzut B, et al.

    Cancers 2023; (15(18)) doi:10.3390/cancers15184427.

    PMID: 37760396
  5. 5

    Reproducible diagnosis of chronic lymphocytic leukemia by flow cytometry: An European Research Initiative on CLL (ERIC) & European Society for Clinical Cell Analysis (ESCCA) Harmonisation project.

    Rawstron AC, Kreuzer KA, Soosapilla A, et al.

    Cytometry. Part B, Clinical cytometry 2018; (94(1)):121-128 doi:10.1002/cyto.b.21595.

    PMID: 29024461
  6. 6

    Positive predictive value of CD200 positivity in the differential diagnosis of chronic lymphocytic leukemia.

    Sorigue M, Magnano L, Miljkovic MD, et al.

    Cytometry. Part B, Clinical cytometry 2020; (98(5)):441-448 doi:10.1002/cyto.b.21849.

    PMID: 31692239
  7. 7

    Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma With Secondary Acquisition of t(11;14)(q13;q32)/CCND1-IGH: A Rare Variant Of Richter Transformation to Mantle Cell Lymphoma.

    Zhao Y, McCracken J, Rehder C, Wang E

    Clinical lymphoma, myeloma & leukemia 2022; (22(5)):e310-e313 doi:10.1016/j.clml.2021.10.017.

    PMID: 34840090
  8. 8

    Clonally related composite chronic lymphocytic leukaemia and mantle cell lymphoma.

    Dong Q, Wang Y, Xiu Y, et al.

    British journal of haematology 2023; (200(5)):660-664 doi:10.1111/bjh.18565.

    PMID: 36375473
  9. 9

    The contribution of CD200 to the diagnostic accuracy of Matutes score in the diagnosis of chronic lymphocytic leukemia in limited resources laboratories.

    Jalal SD

    PloS one 2021; (16(2)):e0247491 doi:10.1371/journal.pone.0247491.

    PMID: 33606850
  10. 10

    The Dohner fluorescence in situ hybridization prognostic classification of chronic lymphocytic leukaemia (CLL): the CLL Research Consortium experience.

    Van Dyke DL, Werner L, Rassenti LZ, et al.

    British journal of haematology 2016; (173(1)):105-13 doi:10.1111/bjh.13933.

    PMID: 26848054
  11. 11

    NCCN Guidelines Insights: Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma, Version 1.2017.

    Wierda WG, Zelenetz AD, Gordon LI, et al.

    Journal of the National Comprehensive Cancer Network : JNCCN 2017; (15(3)):293-311 doi:10.6004/jnccn.2017.0030.

    PMID: 28275031
  12. 12

    Chronic lymphocytic leukaemia.

    Hallek M, Shanafelt TD, Eichhorst B

    Lancet (London, England) 2018; (391(10129)):1524-1537 doi:10.1016/S0140-6736(18)30422-7.

    PMID: 29477250
  13. 13

    Evaluation of 230 patients with relapsed/refractory deletion 17p chronic lymphocytic leukaemia treated with ibrutinib from 3 clinical trials.

    Jones J, Mato A, Coutre S, et al.

    British journal of haematology 2018; (182(4)):504-512 doi:10.1111/bjh.15421.

    PMID: 29873072
  14. 14

    NCCN Guidelines® Insights: Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma, Version 3.2022.

    Wierda WG, Brown J, Abramson JS, et al.

    Journal of the National Comprehensive Cancer Network : JNCCN 2022; (20(6)):622-634.

    PMID: 35714675
  15. 15

    "Accelerated" chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL): unraveling the biological gray zone of CLL/SLL in the era of novel therapies.

    Vadasz B, Zak T, Aldinger J, et al.

    Virchows Archiv : an international journal of pathology 2025; (486(4)):739-750 doi:10.1007/s00428-024-03920-7.

    PMID: 39243299

This page explains the biology and diagnostic criteria of CLL for educational purposes only. Always consult your hematologist or oncologist for an accurate diagnosis and interpretation of your specific laboratory results.

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