How Do Doctors Tell Reactive Monocytosis From CMML?

A high monocyte count (monocytosis) alone does not mean you have chronic myelomonocytic leukemia (CMML). Doctors tell the difference between CMML and a temporary “reactive” monocytosis by tracking your blood counts over time and performing specialized tests.

Because monocytes are white blood cells that fight infection and respond to tissue damage, their numbers naturally rise when your body is dealing with an illness. If the high count is caused by an infection or inflammation, it will usually return to normal once the underlying issue resolves ^[1][2]. If the count stays high, doctors will use microscopic reviews, bone marrow examinations, and genetic testing to evaluate if a blood disorder like CMML is present ^[3][4].

Safety Note: Seek urgent medical care if you experience emergency warning signs, such as uncontrolled bleeding, extreme shortness of breath, sudden confusion, or a high fever that could indicate a serious infection.

What Causes “Reactive” Monocytosis?

Reactive monocytosis happens when your bone marrow produces extra monocytes in a normal, healthy response to an external trigger. Once the trigger is gone, the count typically drops. Common examples include:

• Infections: Bacterial infections (such as bacteremia, which is bacteria in the bloodstream) and severe viral infections (like COVID-19) can cause sharp, temporary spikes in your monocyte count ^[1][2].
• Inflammatory and Autoimmune Diseases: Chronic inflammation forces the immune system to stay active, leading to persistently elevated monocytes ^[5]. Note that a reactive condition and CMML can sometimes exist at the same time ^[6].
• Solid Tumors: Non-blood cancers can sometimes trigger an immune response that raises monocyte levels ^[7].
• Recovery and Medications: Recovering from other blood disorders, rebounding from chemotherapy, or reacting to certain medications can temporarily elevate monocyte counts.

The Diagnostic Workup

To differentiate between a reactive immune response and a blood cancer, hematologists (blood specialists) use a step-by-step approach.

1. Repeat Complete Blood Counts (CBCs) and Clinical Review

Your lab report will show two monocyte numbers: the absolute monocyte count (the total number of monocytes per volume of blood) and the monocyte percentage (the portion of total white blood cells that are monocytes). Current diagnostic criteria for CMML generally require an absolute monocyte count of at least $0.5 \times 10^9$/L and a percentage of at least 10% that persists for more than 3 months ^[3].

For a stable patient with no other concerning symptoms, doctors often repeat the CBC over a 3-month period. A temporary fall in the count makes a reactive cause more likely, though it does not completely rule out a marrow disorder ^[2]. However, if you have other abnormal blood counts (like severe anemia or low platelets), an enlarged spleen, or significant symptoms (such as unexplained weight loss or night sweats), your doctor will not simply “give it time.” Instead, they will recommend a prompt hematology evaluation to investigate further ^[8][9].

2. Peripheral Blood Smear

Doctors will examine a sample of your blood under a microscope. In CMML, blood cells often look abnormal or poorly formed, a feature known as dysplasia ^[10][11]. They will also look for immature cells, called blasts and promonocytes, which should not normally be circulating in the blood ^[10].

3. Bone Marrow Biopsy

If your monocyte count remains high and reactive causes have been reasonably excluded, a bone marrow biopsy is required to evaluate for CMML ^[4]. This involves using local anesthetic to take a small sample of fluid and tissue from your hip bone (you may feel pressure during the sampling and soreness afterward). Pathologists examine this sample for dysplasia.

For a CMML diagnosis, the total combined number of blasts and promonocytes must be less than 20% in both the peripheral blood and bone marrow ^[3][12]. If the number is 20% or higher, doctors typically diagnose an acute leukemia instead ^[13].

Advanced Testing for CMML

No single test definitively diagnoses or rules out CMML on its own. Doctors integrate the results of several advanced tests:

• Flow Cytometry: This test sorts and counts subtypes of monocytes. A pattern where “classical” monocytes make up 94% or more of the total monocytes strongly supports a CMML diagnosis ^[14][15]. However, severe inflammation can skew these results, so it is used as a supportive tool rather than a standalone diagnostic test ^[16][6].
• Molecular Testing (Next-Generation Sequencing): Doctors test your blood or marrow for acquired genetic mutations to check for clonality (evidence of a genetically related population of blood cells). Over 95% of people with CMML have mutations in genes like TET2, SRSF2, or ASXL1 ^[3]. However, having one of these mutations does not automatically mean you have cancer, as they can occur naturally as people age (a condition called clonal hematopoiesis) ^[4]. Conversely, a negative test does not completely rule out CMML because some mutations might fall below the test’s detection limit.
• Cytogenetic Testing: This looks for large changes in your chromosomes. It is crucial for ruling out other specific blood disorders that can mimic CMML, such as checking for the BCR::ABL1 fusion to rule out chronic myeloid leukemia (CML), or checking for other rearrangements like PDGFRA/B or FGFR1 ^[4][17].

Comparing Reactive Monocytosis and CMML