Atypical mononuclear cells: what does the indicator mean?

Atypical mononuclear cells are not a distinct type of "special" cell in the strict immunological sense, but rather a laboratory description of mononuclear cells, most often reactive lymphocytes, that have changed their appearance in response to antigenic stimulation. In real-life practice, these are typically large, activated lymphocytes with more abundant cytoplasm and a variable nuclear shape, which a laboratory technician or physician morphologically recognizes in a peripheral blood smear. [1]

Historically, this finding has been closely associated with infectious mononucleosis, which is why the term itself has long been entrenched in clinical parlance. However, the modern understanding is more precise: atypical mononuclear cells in most cases refer to reactive lymphocytes, rather than an independent diagnosis. In a general blood test, this is one of the signs that the immune system is actively responding to an infection or other antigenic stimulus. [2]

Morphologically, such cells may be larger than normal lymphocytes, have a broader bluish cytoplasm, and irregular contours. Reactive cells are particularly characterized by heterogeneity: cells of varying sizes and shapes are visible in a single smear. It is this heterogeneity that helps distinguish the reactive process from leukemia, in which the cells often appear more uniform. [3]

From a practical standpoint, it's important to understand that the word "atypical" here is not the same as "malignant." In hematology, the term "atypia" in a blood smear can denote both reactive and neoplastic changes, so the conclusion always depends on the clinical context, complete blood count, the patient's age, symptoms, and the duration of the changes. A form with the phrase "atypical mononuclear cells detected" alone does not automatically indicate either mononucleosis or leukemia. [4]

Another important detail is that modern English-language sources more often use the term "atypical lymphocytes" or "reactive lymphocytes." In older Russian-language laboratory formulations, "atypical mononuclear cells" was more often used. Essentially, in most clinical situations, we are talking about the same reactive morphological finding. [5]

Table 1. Understanding the Basic Terms

Term	What is usually meant
Atypical mononuclear cells	an old and widely used laboratory description of reactive mononuclear cells, most often atypical lymphocytes
Atypical lymphocytes	a more modern and accurate term for most of these cells
Reactive lymphocytes	lymphocytes that have changed morphology in response to immune stimulation
Reactive lymphocytosis	an increase in the number of lymphocytes against the background of a reactive process, most often an infection
Clonal lymphocytic proliferation	suspicion of leukemia or lymphoma, rather than a common reactive infection

Sources for the table. [6]

In what conditions do atypical mononuclear cells appear?

The classic and best-known cause is a primary infection with the Epstein-Barr virus, leading to infectious mononucleosis. This disease is characterized by fever, sore throat, swollen lymph nodes, and sometimes an enlarged spleen. Lymphocytosis and atypical lymphocytes are often detected in the blood. The Centers for Disease Control and Prevention (CDC) specifically states that with infectious mononucleosis, the Epstein-Barr virus can cause abnormal-looking lymphocytes to appear in the blood. [7]

However, the list doesn't end there. Merck and StatPearls guidelines emphasize that a similar picture can occur with cytomegalovirus infection, as well as with acute human immunodeficiency virus infection. This is especially important to remember if the patient has a mononucleosis-like syndrome and Epstein-Barr virus tests do not confirm acute infection. [8]

In addition, atypical mononuclear cells can be found in viral hepatitis, influenza B, rubella, measles, adenovirus infection, mumps, and some other viral diseases. StatPearls also lists non-viral causes of reactive lymphocytosis, including whooping cough, cat scratch disease, and toxoplasmosis. In symptomatic toxoplasmosis, atypical lymphocytes may be a hematological sign of a mononucleosis-like illness. [9]

There are also non-infectious causes. A StatPearls review lists drug reactions, particularly severe drug hypersensitivity syndromes, stress, and spleen abscess, among reactive causes of lymphocytosis. This is an important reminder that laboratory findings cannot be interpreted without a thorough history and clinical examination. [10]

The highest numbers of atypical lymphocytes are most typical for primary infections with Epstein-Barr virus and cytomegalovirus. Merck specifically notes that very high atypical lymphocyte counts are typically observed in these two primary infections. However, even in these cases, a definitive diagnosis must still be confirmed not only by a blood smear but also by serology or another targeted test. [11]

Table 2. Common causes of atypical mononuclear cells

Cause	How typical is this find?
Epstein-Barr virus infection	the classic and most well-known reason
Cytomegalovirus infection	a very common cause of mononucleosis-like symptoms
Acute human immunodeficiency virus infection	an important alternative in case of negative tests for Epstein-Barr virus
Viral hepatitis B	a reactive atypical lymphocytic picture is possible
Influenza B and rubella	possible, but less specific
Toxoplasmosis	may cause mononucleosis-like syndrome
Whooping cough	may cause lymphocytosis with unusual morphology
Drug reactions	possible with a pronounced immune response

Sources for the table. [12]

What does the atypical mononuclear cell count mean and how do doctors interpret the numbers?

The mere detection of atypical mononuclear cells is important, but its strength depends heavily on their number and clinical presentation. Hoagland's criteria have long been used for infectious mononucleosis: at least 50% lymphocytes and at least 10% atypical lymphocytes, accompanied by fever, pharyngitis, and lymphadenopathy. These criteria are still cited as a useful guide, although they are neither completely sensitive nor universal. [13]

The American Academy of Family Physicians, in its 2023 review, provides useful benchmarks for the clinical probability of infectious mononucleosis. More than 10% atypical lymphocytes significantly increase suspicion, more than 20% make it even more likely, and more than 40% atypical lymphocytes strongly support the diagnosis. However, this still does not replace confirmatory testing, especially if the clinical presentation is atypical. [14]

It is also important to note that the absence of atypical mononuclear cells does not completely rule out infectious mononucleosis, especially in the early stages of the disease. Furthermore, serological tests may also be negative in the first 1-2 weeks. A review of Epstein-Barr virus serology shows that a negative heterophile test does not rule out the diagnosis if clinical suspicion is high and the smear shows lymphocytosis or atypical lymphocytes. [15]

On the other hand, a small number of atypical mononuclear cells outside of a typical clinical picture should not automatically lead to a diagnosis of infectious mononucleosis. Low percentages can be found in many viral conditions and are not highly specific. Therefore, a proper interpretation always relies not on a single number, but on a combination of symptoms, a complete blood count, liver function, age, and test results for specific infections. [16]

In practice, physicians evaluate not only the percentage but also the total leukocytosis, absolute lymphocyte count, the presence of thrombocytopenia, neutropenia, and liver enzyme abnormalities. Elevated transaminases are quite common in the case of Epstein-Barr virus and increase clinical suspicion of mononucleosis, especially if rapid tests are negative early in the illness. [17]

Table 3. How the number of atypical mononuclear cells is usually interpreted

Find	Practical significance
Single atypical cells	nonspecific reactive finding, requires clinical context
More than 10% atypical lymphocytes	significantly increases the suspicion of infectious mononucleosis
More than 20% atypical lymphocytes	makes infectious mononucleosis much more likely
More than 40% atypical lymphocytes	a very strong argument in favor of a mononucleosis-like viral infection
More than 50% lymphocytes and more than 10% atypical lymphocytes	Hoagland's classical criteria are useful as a guide but not absolute

Sources for the table. [18]

How to confirm the cause and why a general blood test alone is not enough

If the clinical picture is typical of infectious mononucleosis, a complete blood count (CBC) for atypical mononucleosis may indeed be an important first clue. However, current guidelines emphasize that a definitive diagnosis must be based on confirmation of the underlying cause. For Epstein-Barr virus, serological tests for viral antigens serve as such confirmation. [19]

The US Centers for Disease Control and Prevention does not recommend the Monospot test for general use. This is because it produces both false-positive and false-negative results, and in children, heterophile antibodies are often not detected at all. For a more accurate diagnosis, antibodies to the Epstein-Barr virus capsid antigen and the Epstein-Barr virus core antigen are used, which help distinguish between primary, past, and sometimes reactivated infections. [20]

If the heterophile test is negative and the smear reveals lymphocytosis or atypical lymphocytes, the algorithm typically continues with serology for Epstein-Barr virus. If this is also negative, other causes of mononucleosis-like syndrome should be sought, primarily cytomegalovirus, acute human immunodeficiency virus infection, toxoplasmosis, and other viral infections. This stepwise approach is described in clinical reviews on atypical lymphocytosis and Epstein-Barr virus serology. [21]

A complete blood count is also helpful because it reveals associated changes. Infectious mononucleosis is characterized by relative or absolute lymphocytosis, atypical lymphocytes, sometimes a decrease in neutrophils or platelets, and a moderate increase in liver enzymes. However, even this set of findings remains probabilistic, not definitive. [22]

Therefore, the phrase "atypical mononuclear cells were found in the blood, which means it's definitely Epstein-Barr virus" is now considered oversimplification. It's much more accurate to say, "The test shows a reactive lymphocytic pattern, which makes a viral mononucleosis-like infection probable and requires confirmation of the cause." This is more accurate, safer, and more consistent with modern laboratory diagnostics. [23]

Table 4. What is usually done after detection of atypical mononuclear cells

Situation	The next step
A typical picture of mononucleosis in a teenager or young adult	clinical assessment and confirmation by Epstein-Barr virus testing if necessary
The heterophile test is negative, but there are atypical lymphocytes	Epstein-Barr virus serology
Serology for Epstein-Barr virus is negative.	search for cytomegalovirus, acute human immunodeficiency virus infection, toxoplasmosis and other causes
There is pronounced lymphocytosis and cytopenias.	expansion of the examination, exclusion of a clonal process
The picture persists for a long time without any clear reason.	repeat smear, consultation with a hematologist, flow cytometry if indicated

Sources for the table. [24]

How do reactive atypical mononuclear cells differ from leukemia and when is it urgently necessary to delve deeper?

The main difference between a reactive and a neoplastic process is that reactive lymphocytes are usually polymorphic. A smear reveals cells of varying shapes, sizes, and degrees of activation. Merck emphasizes that such cells may resemble leukemic cells, but the reactive population remains heterogeneous, which is less typical for leukemia. [25]

Other features are alarming for clonal lymphoproliferative disorders. StatPearls describes that in chronic lymphocytic leukemia, the smear is dominated by small, mature lymphocytes with dense chromatin, as well as characteristic "smeared" cells. In other lymphomas and leukemias, blasts, hairy cells, cells with angular or excised nuclei, and other more suspicious forms may appear. [26]

Alarming clinical signs are also very important. Significant weight loss, severe night sweats, persistent anorexia, large lymph nodes, liver and spleen enlargement outside the typical viral pattern, anemia, thrombocytopenia, and persistent unexplained lymphocytosis require not only an infectious but also a hematological investigation. According to StatPearls, such signs should trigger a referral to a hematologist and further evaluation. [27]

If lymphocytosis is severe, unexplained, and persists for more than 1 month, or the absolute lymphocyte count is very high, or the smear contains suspicious forms, the next step may be peripheral blood flow cytometry. This method helps determine whether the cell population is reactive or clonal, i.e., tumor-like. However, this test is not prescribed for everyone; it is needed specifically if there are alarming signs. [28]

This is why atypical mononuclear cells cannot be dismissed as either "harmless nonsense" or "almost leukemia" indiscriminately. In most cases, they are a reactive sign of infection, but with persistent abnormalities, unusual morphology, cytopenias, and systemic symptoms, the physician must think more broadly and rule out malignant lymphoproliferative diseases. This balance is the correct modern interpretation. [29]

Table 5. Reactive atypical mononuclear cells and clonal process: main differences

Sign	Reactive process	Suspected clonal process
Cell morphology	heterogeneous, polymorphic	more uniform or clearly blastic
Duration	more often temporary	persistent
Association with infection	often eat	may be absent
Other blood cells	often without severe cytopenias	anemia and thrombocytopenia are alarming
Absolute lymphocyte count	moderate or transient	very high and persistent increases suspicion
Further test	observation or infectious diagnostics	flow cytometry and hematological examination

Sources for the table. [30]

What should a patient do after such a result and how is the cause usually treated?

Treatment depends not on the presence of atypical mononucleosis itself, but on the underlying cause. For typical infectious mononucleosis, treatment is usually supportive: rest, fluids, fever-reducing medications, and pain relievers as needed. The Centers for Disease Control and Prevention (CDC) notes that most people experience improvement within 2-4 weeks, although weakness may persist longer. [31]

Antibiotics do not treat the Epstein-Barr virus itself. Furthermore, in infectious mononucleosis, amoxicillin and ampicillin may be accompanied by a generalized rash, so antibiotics should not be prescribed without a proven bacterial infection. Corticosteroids are also not used routinely and are usually considered only for complications, such as the threat of airway obstruction. [32]

Limiting physical activity is especially important. With infectious mononucleosis, the spleen can enlarge, increasing the risk of rupture. The American Academy of Family Physicians recommends avoiding athletic activity for at least three weeks after the onset of symptoms, and the Centers for Disease Control and Prevention recommends avoiding contact sports until complete recovery. [33]

If the cause is not Epstein-Barr virus, treatment varies. For cytomegalovirus infection in immunocompetent individuals, supportive therapy is also more often used. For acute human immunodeficiency virus infection, specific diagnostics and treatment are required. For toxoplasmosis, the approach depends on the immune status and clinical form. Therefore, an accurate etiological diagnosis is more important than the presence of atypical mononuclear cells. [34]

If a patient exhibits signs of a possible hematological disorder, the primary action is not self-medication or "endless observation," but timely follow-up examination. A repeat smear, assessment of absolute lymphocytosis, flow cytometry, and consultation with a hematologist are more important in such cases than any attempt to treat the laboratory finding as an independent disease. [35]

Table 6. What to do after receiving the result

Scenario	The most sensible tactic
Typical viral picture and temporary finding	confirm the cause and treat the underlying disease
Suspected infectious mononucleosis	evaluate the clinic, conduct targeted tests, limit sports
Atypical mononuclear cells without clear cause	re-evaluation of analysis and clinical findings
Persistent lymphocytosis for more than 1 month	hematological follow-up examination
Lymphocytosis with anemia, thrombocytopenia, or large spleen	expedited referral to a hematologist
Pain in the left hypochondrium due to mononucleosis	urgent evaluation due to risk of splenic injury

Sources for the table. [36]

FAQ

Are atypical mononuclear cells a diagnosis?
No. It's a laboratory description of cells, most often reactive lymphocytes, that have changed as a result of an immune response. The diagnosis is based on their appearance, not the term itself in the analysis. [37]

Do atypical mononuclear cells always indicate infectious mononucleosis?
No. This is a classic, but not the only, cause. They also occur with cytomegalovirus infection, acute human immunodeficiency virus infection, viral hepatitis, toxoplasmosis, and other reactive conditions. [38]

Can a complete blood count (CBC) alone confirm Epstein-Barr virus infection?
No. A complete blood count (CBC) can only suggest a mononucleosis-like syndrome. Specific tests, primarily serology for Epstein-Barr virus antigens, are needed to confirm the cause. [39]

What percentage of atypical mononuclear cells is particularly suspicious for infectious mononucleosis?
The most classic guideline is considered to be at least 10% atypical lymphocytes with lymphocytosis of at least 50% in a typical clinical setting. Higher percentages increase the likelihood, but still do not replace confirmation. [40]

If Monospot is negative, is mono definitely not present?
No. The US Centers for Disease Control and Prevention does not recommend this test for general use precisely because of false-positive and false-negative results. If Epstein-Barr virus infection is suspected, specific serologic tests are needed. [41]

Are atypical mononuclear cells a blood cancer?
Most often, they are not. In most cases, they are a reactive sign of infection. However, if lymphocytosis is persistent, the cells appear suspicious, there is anemia, thrombocytopenia, weight loss, night sweats, or a large spleen, leukemia or lymphoma should be ruled out. [42]

Why shouldn't you immediately return to sports with mononucleosis?
Because the spleen can be enlarged, and the risk of rupture is especially dangerous in the first weeks of the illness. Therefore, sports restrictions after the onset of symptoms are standard advice. [43]

Should atypical mononuclear cells themselves be treated?
No. It's not the laboratory finding that's being treated, but the underlying disease. For Epstein-Barr virus, supportive therapy is usually administered, while for other causes, the treatment strategy depends on the diagnosis. [44]

When is a hematologist needed?
When lymphocytosis persists for more than 1 month without a clear cause, when there is anemia, thrombocytopenia, a very high absolute lymphocyte count, suspicious cell morphology, or severe systemic symptoms. [45]