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Leukemoid Reaction : Unmasking a Rare Clinical Mimic of Leukocytosis

In the realm of medical diagnostics, distinguishing between various conditions is essential for accurate treatment and management. Leukocytosis, a condition characterized by an elevated white blood cell (WBC) count, is a common finding in numerous infections and inflammatory disorders. However, in some cases, an unusual phenomenon known as “leukemoid reaction” can masquerade as leukocytosis, leading to potential misdiagnosis and unnecessary concerns. This article delves into the intriguing and rare clinical scenario of leukemoid reaction, exploring its unique features, diagnostic challenges, underlying causes, and how to differentiate it from true leukemia.

Understanding Leukemoid Reaction:

Leukemoid reaction is a condition where the WBC count in the blood rises significantly, often mimicking leukemia. However, unlike leukemia, leukemoid reaction is a reactive response rather than a malignant disorder of the bone marrow. This reaction typically involves an increase in neutrophils, although other types of WBCs may also be elevated. The distinguishing feature of leukemoid reaction is that it arises as a response to severe infections, inflammatory conditions, or certain medications, whereas leukemia is characterized by uncontrolled proliferation of abnormal WBCs.

Differentiating Leukemoid Reaction from Leukemia:

Given their overlapping presentations, differentiating leukemoid reaction from leukemia can be challenging. However, several key factors aid in distinguishing the two: 

  • Peripheral Blood Smear: A careful examination of the peripheral blood smear can reveal important clues. In leukemoid reaction, the WBCs usually appear morphologically normal, whereas leukemia shows abnormal and immature cells.
  • Bone Marrow Aspiration and Biopsy: If suspicion of leukemia remains, a bone marrow aspiration and biopsy can be performed. In leukemoid reaction, the bone marrow shows a reactive pattern with no signs of malignancy, while leukemia reveals abnormal cells and disturbances in the bone marrow architecture.
  • Clinical History: A comprehensive patient history, including recent infections, inflammatory conditions, or medication use, can provide valuable context for diagnosing leukemoid reaction.
  • Serial WBC Counts: Monitoring the WBC count over time can also be helpful. Leukemoid reaction typically shows a transient increase that resolves with the resolution of the underlying cause, whereas leukemia presents with a persistent and progressive elevation.

Causes of Leukemoid Reaction :

Leukemoid reaction is a reactive response of the body to certain conditions and factors, leading to a significant increase in white blood cell (WBC) count. The condition can be triggered by various underlying causes, including: 

  • Infections: Severe bacterial infections, such as those caused by Escherichia coli, Klebsiella pneumoniae, or Streptococcus species, are common triggers of leukemoid reactions. Additionally, viral infections like infectious mononucleosis, cytomegalovirus, and hepatitis can also induce a reactive increase in WBCs.
  • Inflammatory Conditions: Leukemoid reactions can occur in response to various inflammatory disorders, such as severe burns, pancreatitis, and rheumatoid arthritis. Inflammation in the body prompts the release of cytokines and growth factors, leading to the mobilization and proliferation of WBCs.
  • Medications: Certain drugs can cause leukemoid reactions as a side effect. For example, corticosteroids, which are anti-inflammatory medications, and granulocyte colony-stimulating factors (G-CSFs) used to boost white blood cell production, can induce an abnormal increase in WBCs.

Management of Leukemoid Reaction:

The management of leukemoid reaction primarily revolves around addressing the underlying cause that triggered the reactive increase in WBCs. This often involves the following approaches: 

  • Infection Treatment: If an infection is identified as the cause, appropriate antimicrobial therapy is administered to target and eliminate the infecting pathogen. Timely and effective treatment of the underlying infection can lead to a resolution of the leukemoid reaction.
  • Inflammatory Disorder Management: In cases where inflammation is the driving factor behind the leukemoid reaction, managing the underlying inflammatory condition becomes essential. Controlling inflammation through appropriate medications and therapies can help normalize the WBC count.
  • Discontinuation of Causative Medications: If certain medications are identified as the cause of the leukemoid reaction, the healthcare provider may consider discontinuing or adjusting the dosage of these drugs. However, this decision should be made carefully, taking into account the potential benefits and risks of the medication in the overall treatment plan.
  • Supportive Care: In mild cases of leukemoid reaction, supportive care and close monitoring of the patient’s condition and WBC count may be sufficient. In such cases, the leukemoid reaction may resolve on its own as the underlying cause is treated.


Leukemoid reaction is a rare clinical scenario that can present as leukocytosis, raising concerns of leukemia. Understanding the distinctive features and underlying causes of this reactive phenomenon is essential to avoid misdiagnosis and unnecessary anxiety for patients. By differentiating leukemoid reaction from leukemia through meticulous examination, serial WBC counts, and appropriate investigations, healthcare professionals can provide accurate diagnoses and implement targeted treatments. Further research and awareness about this rare condition are crucial for enhancing diagnostic precision and delivering optimal patient care in clinical settings.


  1. Pahuja M, Mehndiratta S, Bhalla P. Leukemoid Reaction – A Reactive Cause of Leukocytosis. Mediterr J Hematol Infect Dis. 2016;8(1):e2016016. doi:10.4084/mjhid.2016.016
  2. Mayorga C, Mendonça FM, Nóbrega JPR, et al. Leukemoid Reaction: A Systematic Review. Am J Med Sci. 2018;356(4):332-341. doi:10.1016/j.amjms.2018.07.009
  3. Nigam A, Sood R, Awasthi M, et al. Diagnostic and prognostic importance of bone marrow examination in patients with leukaemoid reaction. Hematology. 2019;24(1):107-112. doi:10.1080/16078454.2018.1501508
  4. Hsu YJ, Chen TC, Huang TY, et al. Clinical presentations, prognostic factors, and therapeutic outcomes of cefepime-induced neutropenia and comparison with β-lactam antibiotic-induced neutropenia. J Microbiol Immunol Infect. 2017;50(5):650-656. doi:10.1016/j.jmii.2015.11.002
  5. Ortel TL, Mercer MC. Clinical and Laboratory Evaluation of Leukocytosis. Mayo Clin Proc. 2000;75(3):309-314. doi:10.4065/75.3.309
Team PainAssist
Team PainAssist
Written, Edited or Reviewed By: Team PainAssist, Pain Assist Inc. This article does not provide medical advice. See disclaimer
Last Modified On:July 25, 2023

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