Approach to Lymphocytosis

Peripheral blood smear. A population of mantle cells with a cleaved nucleus and paucity of cytoplasm

Diagnostic Approach to Lymphocytosis: 

Lymphocytes are white blood cells that serve primarily as the body’s adaptive immune system and provide humoral or cell-mediated immunity against a variety of bacterial, viral, or other pathogens. They are comprised mainly of T, B, and natural killer (NK) cells, and the body typically maintains the absolute lymphocyte count (ALC) in a range of fewer than 4,000 lymphocytes per uL. Elevation of the lymphocyte count above this level is most commonly due to a reactive lymphocytosis, the body’s normal response to an acute infection or inflammatory condition.

You can have a higher than normal lymphocyte count but have few, if any, symptoms. It usually occurs after an illness and is harmless and temporary. But it might represent something more serious, such as blood cancer or a chronic infection.

Your doctor might perform other tests to determine if your lymphocyte count is a cause for concern.

If your doctor determines that your lymphocyte count is high, the test result might be evidence of one of the following conditions:

  1. Infection (bacterial, viral, other)
  2. Cancer of the blood or lymphatic system
  3. An autoimmune disorder causing ongoing (chronic) inflammation

Specific causes of lymphocytosis include:

  • Acute lymphoblastic leukemia
  • Chronic lymphocytic leukemia
  • Lymphoma
  • Infectious mononucleosis
  • Cytomegalovirus (CMV) infection
  • Hepatitis A
  • Hepatitis B
  • Hepatitis C
  • Other viral infections
  • Syphilis
  • Tuberculosis
  • Whooping cough
  • Hypothyroidism (underactive thyroid)

A 71-year-old man with a history of atypical chronic lymphocytic leukemia (CLL) last treated in 2007 with a rituximab and chlorambucil-based regimen presents with an increasing M protein of 4.1 g/dL (IgG κ). The laboratory findings were as follows: WBC, 7.4 × 109/L with 29 percent neutrophils, 66 percent lymphocytes, and 5 percent monocytes; RBC, 3.94 × 1012/L; hemoglobin, 11.0 g/dL; mean corpuscular volume, 83 fL; platelets, 91 × 109/L. The patient’s bone marrow was hypercellular (90%) with a marked lymphoid infiltrate present in nodular (paratrabecular and interstitial) and focal diffuse patterns involving 75 percent of bone marrow cellularity. Lymphocytes were small and round with condensed chromatin and occasional plasmacytoid lymphocytes were also observed. The karyotype of the bone marrow was 46,XY,add(9)(p24),der(11)del(11)(p13)del(11)(q23) in four cells with a sideline containing all of these abnormalities and +13 in two cells, and an unrelated clone showing 45,X,–Y in six cells, with 46,XY in seven cells. Fluorescence in situ hybridization (FISH) found a deletion of the 13q14.3 region and was negative for deletions of TP53ATM, and LAMP1, and aneuploidy for chromosome 12.

Case response by Dr. Tracy I. George, MD, Professor of Pathology; Director of the Hematopathology Fellowship Program, University of New Mexico School of Medicine, Albuquerque, New Mexico:

Examining the Blood Smear

A slide review is appropriate in all patients with an unexplained lymphocytosis in order to confirm the automated cell counts or to perform a manual differential for leukocyte classification. In manually prepared blood smears, larger white blood cells tend to collect at the edges of the smear and in the feathered edge. Good practice for slide review requires assessments of all cell types (leukocytes, red blood cells [RBCs], and platelets) in both quantity and quality. It is not uncommon for fragile leukocytes such as in CLL, infectious mononucleosis, or acute leukemia to smudge on blood smears. In these situations, a few drops of albumin can be added to peripheral blood before preparing the blood smear. These “albumin smears” allow for proper identification of leukocytes and reduce the number of “smudge” or “basket” cells. However, the examination of RBCs and platelets should still be performed on the original blood smear because the albumin can affect platelet and erythrocyte morphology.

Approach to Lymphocytosis

CLL – small lymphocytes and “smudge” or “basket” cells (arrows). Note also the paucity of platelets; platelet count was 52,000/uL.

Reactive Lymphocytosis

Separating a monomorphic lymphocytosis from a pleomorphic lymphocytosis can help distinguish a lymphoproliferative disorder from a reactive lymphocytosis, respectively. Most reactive lymphocytoses show a wide range of sizes and shapes in lymphocytes. The classic example of a pleomorphic lymphocytosis is infectious mononucleosis, where the lymphocytes range in size from small and round, to intermediate with abundant cytoplasm (reactive lymphocytes), to frank immunoblasts. It is this spectrum of morphology that points to a greater likelihood that a patient has a reactive lymphocytosis; younger age is also a helpful clue. The causes of a reactive lymphocytosis are extensive and include infections (viral, bacterial, and parasitic), autoimmune disease, vaccination, drug hypersensitivity, endocrine disorders, stress (trauma, cardiac, extreme exercise), smoking, and malignancy.

Atypical Mononuclear Cells

Large irregular atypical lymphocytes seen in the peripheral blood of a patient with infectious mononucleosis.

While most of these reactive lymphocytoses are pleomorphic, a few important exceptions are worth mentioning. The first is Bordetella pertussis, the causative agent of whooping cough. The lymphocytes of B. pertussis are small and deeply clefted with mature chromatin.

Lymphocytosis in Bordetella Pertussis infection

Bordetella pertussis infection in a child in a child showing three reactive lymphocytes that are small with deeply cleaved nuclei and scant cytoplasm, in contrast with the monocyte in the lower right-hand corner. Reprinted with permission from I Pereira, TI George, DA Arber.

As this is commonly seen in the pediatric and pregnant populations, the clinical correlation will readily separate this from lymphomas, which can show similar morphologic features (e.g. follicular lymphoma or Sézary syndrome).

The second exception is polyclonal B-lymphocytosis, which typically shows lymphocytes with distinct nuclear clefts but will demonstrate a spectrum of morphologic changes including nuclear lobation and binucleate forms. This uncommon disorder is found in young to middle-aged female smokers with a high association with human leukocyte antigen DR7, and several genetic abnormalities have also been documented.

Approach to Lymphocytosis

polyclonal B-lymphocytosis

The final exception is a large granular lymphocytosis. Increased numbers of large granular lymphocytes (reactive lymphocytes with scattered azurophilic granules) are commonly seen with viral infections, malignancy, after bone marrow transplantation, and following chemotherapy. These populations of large granular lymphocytes will wax and wane.

Approach to Lymphocytosis

large granular lymphocyte

However, the persistence of a large granular lymphocytosis with accompanying neutropenia and variable anemia should raise suspicion for large granular lymphocytic leukemia. This is typically T cell in origin, though a chronic lymphoproliferative disorder of natural killer cells is also well described. Flow cytometry is recommended in these cases, followed by either T-cell clonality or KIR analysis, if involving T cells or natural killer cells, respectively.

Neoplastic Lymphocytosis

Lymphoma cells tend to be monomorphic in appearance. While a blood smear may contain a subset of lymphoma cells, these cells will resemble one another and stand out against a background of normal bland lymphocytes. While CLL is the most common leukemia in adults in the western world and is frequently seen in peripheral blood (or its monoclonal B-cell lymphocytosis counterpart), peripheral blood involvement by bone marrow lymphoma is found in up to 30 percent of subjects in some studies. Lymphoma cells will show a wide variety of morphologic appearance, and this appearance raises a differential diagnosis as shown in Figure 2.

Approach to Lymphocytosis

Further identification of the type of lymphoproliferative disorder typically proceeds with flow cytometry. While each laboratory has its own cocktail of antibodies used for flow cytometry, consensus guidelines have been published. While the results from flow cytometry narrow down one’s differential diagnosis to a shortlist, additional genetic or other ancillary studies are typically needed for confirmation, such as FISH for CCND1/IGH to evaluate for mantle cell lymphoma. Additionally, bone marrow biopsy or biopsy of another involved site is necessary for a final diagnosis.

fluorescence in situ hybridization

Fluorescence in situ hybridization (FISH) assays designed for del (7q) detection uncover more complex rearrangements in myeloid leukemia cell lines.

A common question is when should flow cytometry be performed? Some studies have looked at this question in adults. In one study, the authors retrospectively reviewed flow cytometry results of 71 patients 50 years of age and older with an absolute lymphocyte count of 4 × 109/L or greater that had been called suspicious for a lymphoproliferative disorder after smear review by a pathologist. Using receiver operating characteristic (ROC) analysis, they found that an absolute lymphocyte count greater than 6.7 × 109/L for patients 50 to 67 years of age, and 4 × 109/L or greater for patients older than 67 years had a 95 percent sensitivity and 76 percent specificity for predicting an abnormal flow cytometry phenotype. A more recent retrospective single-center study examined 71 adults with newly detected lymphocytosis greater than 5 × 109/L in a consecutive three-month period and found that 6.8 × 109/L was the best cut-off value for predicting a lymphoproliferative disorder with ROC analysis (sensitivity 90%, specificity 59%). In my own practice, other triggers for flow cytometry include a persistent unexplained lymphocytosis or a morphology that does not correlate with the diagnosis.

Patient Follow-up

Review of the patient’s original diagnostic material confirmed that the atypical CLL diagnosis was given based on immunophenotypic expression of FMC7, in addition to the usual phenotype for CLL (CD20+, CD5+, CD23+). Morphology in the current blood and bone marrow showed lymphocytes and plasmacytoid cells and not the usual small round lymphocytes with coarsely clumped chromatin (“soccer balls”) of CLL; the plasmacytoid cells were not readily identified on the earlier bone marrow smears. The performed flow cytometry on the patient’s bone marrow identified a κ light chain–restricted B-cell population that expressed CD19, CD20, CD10, and CD23, and lacked expression of CD5 and CD200. Additionally, a κ light chain–restricted plasma cell population was identified.

Approach to Lymphocytosis

Immunohistochemistry was performed on the bone marrow clot section. Cyclin D1 and SOX-11 were negative in the B-cells, excluding the diagnosis of mantle cell lymphoma; SOX11 is a newer marker for mantle cell lymphoma that has been found to be expressed even in mantle cell lymphomas that lack overexpression of cyclin D1. LEF1 was negative, providing no support for a diagnosis of CLL. Markers of follicle center cell origin, BCL6 and LM02, were also negative, providing no support for a lymphoma of follicle center cell origin.

The cytogenetic karyotype, while abnormal, was not specific for any particular B-cell lymphoma; the lack of t(11;14) and t(14;18) argued against both mantle cell lymphoma and follicular lymphoma. Molecular testing for MYD88 L265P mutation was performed and was negative. DNA polymerase chain reaction analysis for immunoglobulin heavy chain gene (IGH) was performed on the 2007 bone marrow and on the current 2015 bone marrow. A clone was detected in both samples that was identical in amplicon size.

This case was presented at a multidisciplinary tumor board conference. While the immunophenotype of the lymphocytes switched from CD5 to CD10 expression, the IGH data supported that the same neoplastic clone was present in both the 2007 and current bone marrow. Thus, a low-grade B-cell lymphoma with plasmacytic differentiation was found, raising a differential diagnosis of lymphoplasmacytic lymphoma, versus a marginal zone lymphoma with plasmacytic differentiation. While the lack of a MYD88 L265P mutation argues against lymphoplasmacytic lymphoma, my colleagues have found this to be 96 percent sensitive for a diagnosis of lymphoplasmacytic lymphoma in a study of 317 cases of low-grade B-cell lymphomas. A diagnosis of atypical CLL seemed less likely, as the lymphocytes lacked LEF1 expression – a marker identified based on gene expression profiling data that has been described as nearly 100 percent sensitive and specific for CLL. The lymphocytes also lacked expression of CD200 by flow cytometry, another marker overexpressed in CLL as well as lymphoplasmacytic lymphoma, though few cases of the latter were tested. While the patient lacked splenomegaly, a diagnosis of marginal zone lymphoma with plasmacytic differentiation was considered. The patient is scheduled to receive ibrutinib.


Tracy I. George, MD. Diagnostic Approach to Lymphocytosis

John Lazarchick. Chronic Lymphocytic Leukemia: Thrombocytopenia.–2

Approach to Lymphocytosis (high lymphocyte count).

Jennifer R. Brown and Matthew S. Davids. Approach to lymphocytosis

Mossafa H, Tapia S, Flandrin G, et al. Chromosomal instability and ATR amplification gene in patients with persistent and polyclonal B-cell lymphocytosis (PPBL). Leuk Lymphoma. 2004;45:1401-1406.

George TI. Malignant or benign leukocytosis. Hematology Am Soc Hematol Educ Program. 2012;2012:475-484.

Dhodapkar MV, Li CY, Lust JA, et al. Clinical spectrum of clonal proliferations of T-large granular lymphocytes: a T-cell clonopathy of undetermined significance?. Approach to lymphocytosis. Blood. 1994;84:1620-1627.

Arber DA, George TI. Bone marrow biopsy involvement by non-Hodgkin’s lymphoma: frequency of lymphoma types, patterns, blood involvement and discordance with other sites in 450 specimens. Am J Surg Pathol. 2005;29:1549-1557.

Chabot-Richards DS, George TI. Leukocytosis. Int J Lab Hematol. 2014;36:279-288.

Béné MC, Nebe T, Bettelheim P, et al. Immunophenotyping of acute leukemia and lymphoproliferative disorders: a consensus proposal of the European LeukemiaNet Work Package 10. Leukemia. 2011;25:567-574.

Creative Bioarray: Biobank – Human& Animal Tissue Bank, Research Use Cells – Primary cells, Tumor cells, Stem cells, CABR™ Probes. Approach to Lymphocytosis.

Kamel, Yasser Mostafa, et al. “Fluorescence in situ hybridization assays designed for del (7q) detection uncover more complex rearrangements in myeloid leukaemia cell lines (2014). Approach to Lymphocytosis.

Andrews JM, Cruser DL, Myers JB, et al. Using peripheral blood smear review, age and absolute lymphocyte count as predictors of abnormal peripheral blood lymphocytoses diagnosed by flow cytometry. Leuk Lymphoma. 2008;49:1731-1737. Approach to Lymphocytosis.

Sun P, Kowalski EM, Cheng CK, et al. Predictive significance of absolute lymphocyte count and morphology in adults with a new onset peripheral blood lymphocytosis. J Clin Pathol. 2014;67:1062-1066.

Soldini D, Valera A, Solé C, et al. Assessment of SOX11 expression in routine lymphoma tissue sections: characterization of new monoclonal antibodies for diagnosis of mantle cell lymphoma. Am J Surg Pathol. 2014;38:86-93.

Insuasti-Beltran G, gale JM, Wilson CS, et al. Significance of MYD88 L265P mutation status in the subclassification of low-grade B-cell lymphoma/leukemia. Arch Pathol Lab Med. 2015;139:1035-1041.

Tandon B, Peterson L, Gao J, et al. Nuclear overexpression of lymphoid-enhancer-binding factor 1 identifies chronic lymphocytic leukemia/small lymphocytic lymphoma in small B-cell lymphomas. Mod Pathol. 2011;24:1433-1443.

Challagundla P, Medeiros LJ, Kanagal-Shamanna R, et al. Differential expression of CD200 in B-cell neoplasms by flow cytometry can assist in diagnosis, subclassification, and bone marrow staging. Am J Clin Pathol. 2014;142:837-844.

Peter Maslak and Susan McKenzie. Infectious Mononucleosis – 1. | ASH Image Bank | American Society of Hematology–1?type=upload #ASHImageBank

Approach to Lymphocytosis
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Approach to Lymphocytosis
Lymphocytes are white blood cells that serve primarily as the body’s adaptive immune system and provide humoral or cell-mediated immunity against a variety of bacterial, viral, or other pathogens.
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  • Reda Nofal

    Thanks, sir .very interesting information

  • Troy Burklow

    Hey BW,
    My original comment was flagged as spam so I’ll be more concise in this next one. Would you find 5% Large granular lymphocytes significant in a patient receiving treatment for essential thrombocythemia? NRBCs and elevated IGs were also noted, RBC count decreased, PLT count is normal w/ large platelets noted.

    • Hi Troy,
      Thank you for your comment.
      No, 5% LGLs in the peripheral blood is normal.
      LGLs comprise 10 to 15 percent of normal peripheral blood mononuclear cells.
      The absolute number of LGLs in the peripheral blood of normal subjects is 0.2-0.4×109/L.

  • Jessica Fuxa

    If someone was post splenectomy 9 years and currently has chronic absolute lymphocytosis of 4956-5000+ range and 59% lymphocytes for past year is this indicative of a malignancy or is this normal for post splenectomy state?

    • Hi Jessica,
      Thanks for your comment.
      Mild lymphocytosis could be reactive but if persistent I would suggest blood flow cytometry to out rule possible underlying chronic lymphoproliferative disorder?Monoclonal B Lymphocytosis/CLL.

  • Bahman

    Hi, thanks for your answers.
    Please tell me how calculate segmented neutrophils precent and what normal range

    • Hi Bahman,
      Thanks for your comment.
      Neutrophils are counted as part of the Complete Blood Count (CBC).
      If you have the absolute neutrophil count e.g. 2.5 and your total white cell count is 5.0 then your neutrophil percentage is 50%, and so on.
      Normal range is:
      Segmented neutrophils 2,500-6,000 40-60%

  • Betty

    Hi There,
    What does this mean: An abnormal population of lymphoid cells (50% of total cellular events) positive for CD5, CD19, CD20 and CD23, negative for CD10 and FMC7, showing dim kappa light chain specificity is present. T-cells are antigenically normal.

    • Hi Betty,
      Thank you for your comment.
      If this is your blood flow cytometry report it would fit with a lymphoproliferative disorder e.g. CLL.
      You should discuss it with your hematologist.

  • Suzanne Babin

    Have just received 9 pages of blood tests results. Until I read your well thought out and clearly written article I didn’t have a clue how to start absorbing the information.
    Thank you for taking the time to simplify the complexities of blood cells.

    Suzanne November 17, 2022

  • Erika acevedo

    Hi there,

    My lymphocyte count has continuously come back between 4.5-4.9 xE9/L for the past two years. All other cell counts are within normal range therfore doctor does not want to recommend me to hematologist (I’ve seen 2!) is this normal?

    • Hi Erika,
      Thank you for reaching out to me at
      If you are not a smoker and do not suffer from a persistent infection or autoimmune disorder, and are not on steroids, you should consider doing a blood flow cytometry to rule out possible Monoclonal B cell lymphocytosis (MBL).
      Best regards,
      Dr M Abdou

  • Sabry

    Thank you so much for your interesting,clear article..Is there a count limit for extreme reactive lymphocytosis(lymphocyte leukomoid reaction)?

    • Hi Sabry,

      Thank you for reaching out.

      There isn’t a specific count limit for extreme reactive lymphocytosis, which can occur in response to an infection, inflammation, or certain medications. However, if the lymphocyte count is exceptionally high or persists for more than a few weeks, I would recommend conducting a blood film and a blood flow cytometry to rule out malignancy.

      Best regards,
      Dr. M Abdou

  • Sabry

    Much obliged 🙏🙏🙏

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