An abnormal blood count can look like leukemia for reasons that have nothing to do with cancer, most often an infection, a vitamin shortage, or even how the sample was handled. The body’s normal response to illness or stress can push blood cells into the same unusual numbers and shapes that leukemia produces. Still, an extreme count is often cancer, so the resemblance is a reason for follow-up tests, not for panic or for dismissal.
Infections and Inflammation Are the Most Common Look-Alikes
Your white blood cell count (WBC), listed on a complete blood count report, measures the infection-fighting cells in your blood. A leukemoid reaction is a rise in the white count big enough to mimic leukemia. It is usually caused by infection or inflammation, and it is not leukemia.1MedlinePlus. Leukemoid reaction The count settles once the infection or inflammation is treated.
People rarely feel the high count itself. They feel the illness behind it, such as fever, aches, cough, or pain, and the abnormal number turns up when blood is drawn for that illness. Reports often show many young-looking neutrophils, sometimes called a “left shift,” which is the typical pattern of the body ramping up production to fight infection. When mostly one cell type is rising and the other counts are relatively preserved, doctors lean toward a reaction, though that is one clue among several, not a test.
At very high counts, though, the odds are not reassuring on their own. Among people whose white count topped 50,000 per microliter, about 60% had a blood cancer and 40% had a true leukemoid reaction.2PubMed. Leukemoid reaction: A 21st-century cohort study Of those leukemia-like reactions, infection caused more than half, with cancers outside the blood and other causes making up the rest.3PubMed. Leukemoid reaction: A 21st-century cohort study
Some infections also produce cells that look abnormal. Epstein-Barr virus, the cause of mononucleosis, targets white blood cells and can produce atypical cells that have led young adults to be wrongly diagnosed with acute leukemia.4PubMed Central. Epstein–Barr Virus Infection Masquerading as Acute Leukemia: A Report of Two Cases and Review of Literature Long-lasting tuberculosis can raise certain white cells in patterns that resemble chronic leukemias.5BioRes Scientia Publishers. Insight into Leukemia-like Presentations in Tuberculosis Patients Tuberculosis that spreads through the body can lower all blood counts and imitate acute leukemia or marrow failure.6BioRes Scientia Publishers. Insight into Leukemia-like Presentations in Tuberculosis Patients
That is why a report may mention “atypical lymphocytes” or “blasts” during an infection. Blasts are immature blood cells, normally found mostly in the marrow, and leukemia floods the blood with them. Virus-fighting lymphocytes swell and change shape, and under a microscope they can resemble blasts. The word “atypical” describes appearance, not a diagnosis. A doctor will often repeat the count after the infection clears to see whether the odd cells disappear.
A recent sore throat, swollen glands, and fatigue fit mononucleosis, and a known infection, injury, or flare of an inflammatory disease gives the count an explanation. None of these signs is proof, which is why follow-up tests decide it.
Vitamin B12 Deficiency Can Imitate Leukemia in the Bone Marrow
Low B12 or folate impairs the body’s ability to copy DNA, which affects every type of blood cell.7PubMed. Severe Vitamin B12 Deficiency and Pancytopenia Blood-forming cells in the marrow divide faster than almost any other cells in the body, so they are among the first to suffer. Severe deficiency can cause pancytopenia, meaning low counts of red cells, white cells, and platelets at once.8PubMed Central. Vitamin B12 deficiency mimicking acute leukemia It also makes red cells unusually large and some white cells oddly segmented.9PubMed Central. Vitamin B12 deficiency mimicking acute leukemia
The marrow can look leukemic too. It can become crowded with immature-looking cells.10PubMed Central. Vitamin B12 deficiency mimicking acute leukemia Cells that cannot finish dividing grow oversized and may die inside the marrow before reaching the bloodstream, which is why counts fall even though the marrow looks busy. The changes can be striking enough to suggest acute leukemia or a pre-leukemic marrow disorder.11PubMed. Severe Vitamin B12 Deficiency and Pancytopenia Some people have gone through extensive testing and even aggressive treatment before the deficiency was found.12PubMed Central. Vitamin B12 deficiency mimicking acute leukemia
Numbness or tingling in the feet can hint at B12 deficiency. Several groups are most at risk of a severe shortage. Older adults absorb less B12 from food. People with pernicious anemia, an autoimmune problem, cannot absorb it from the gut. Strict vegans who skip supplements get little from their diet. Long-term use of metformin or stomach-acid medicines can lower levels, and stomach or bowel surgery can cut absorption.
This is a good mimic to find. A simple blood test measures B12 and folate, and treatment with injections or high-dose tablets usually restores counts over weeks.
A Lab Quirk Can Fake a Low Platelet Count
Most blood count tubes contain an anticoagulant called EDTA, and in some people it makes platelets stick together. The counting machine misses the clumps and reports a falsely low platelet count. In one reported case, a woman with fever and cough was sent to a specialist for suspected acute leukemia because of this falsely low count and normal lymphocytes misread as blasts.13PubMed Central. A case of EDTA-dependent pseudothrombocytopenia: simple recognition of an underdiagnosed and misleading phenomenon
EDTA keeps blood from clotting in the tube so cells can be counted. In some people, their own antibodies make platelets stick together once EDTA removes calcium. The clumping happens in the tube, not inside the body, so the true count is normal.
Doctors check by looking at the blood under a microscope, where they can see the clumps directly. They may also redraw blood in a tube with a different anticoagulant, such as citrate, and a normal count in the new tube settles it.
A very low platelet count with no bleeding or bruising is a clue, as is a previous normal count followed by a sudden drop with no illness, but only the smear or a redraw confirms it.
Other Bone Marrow Disorders That Resemble Leukemia
Myelodysplastic syndromes (MDS) are disorders in which the marrow makes blood cells poorly, leading to low counts.14PubMed Central. Myelodysplastic Syndromes and Acute Myeloid Leukemia in the Elderly MDS can be slow or fast-moving, and it carries a risk of turning into acute myeloid leukemia.15PubMed Central. Myelodysplastic Syndromes and Acute Myeloid Leukemia in the Elderly The main dividing line from leukemia is the proportion of immature blood cells in the marrow, with acute leukemia generally having a much higher share. MDS is also defined by malformed-looking cells and by certain chromosome or gene changes found on lab testing, though some gene changes define acute leukemia regardless of the count. Because the line rests on counting cells and testing, not symptoms, people can feel similar with either condition.
Aplastic anemia is a condition in which the marrow stops making enough blood cells. When the marrow has very few cells (under 20% filled), it can be very hard to tell apart from low-cell forms of MDS or leukemia using standard microscope checks.16Haematologica. Diagnostic criteria to distinguish hypocellular acute myeloid leukemia from hypocellular myelodysplastic syndromes and aplastic anemia: recommendations for a standardized approach Doctors often need extra tests on the marrow to decide.17Haematologica. Diagnostic criteria to distinguish hypocellular acute myeloid leukemia from hypocellular myelodysplastic syndromes and aplastic anemia: recommendations for a standardized approach
Primary myelofibrosis is a slow blood cancer in which the marrow scars and the spleen enlarges, causing anemia and fatigue.18PubMed. Primary myelofibrosis: 2017 update on diagnosis, risk-stratification, and management It often carries a gene change in JAK2, CALR, or MPL, which a lab can test for.19PubMed. Primary myelofibrosis: 2017 update on diagnosis, risk-stratification, and management It can progress to leukemia.20PubMed. Primary myelofibrosis: 2017 update on diagnosis, risk-stratification, and management
The exact name matters because treatment differs. Aplastic anemia is often treated with immune-suppressing medicine or a stem cell transplant, not chemotherapy. MDS treatment ranges from monitoring to medicines to transplant depending on risk, while leukemia usually needs prompt chemotherapy. With MDS, your doctor will repeat blood counts and sometimes marrow tests over time, watching for a rising share of immature cells, which is the warning sign.
Less Common Mimics in Adults and Children
An overactive thyroid from Graves disease has been linked to low counts of all blood cells and to an enlarged spleen and swollen lymph nodes.21PubMed Central. PANCYTOPENIA AND LYMPHOID ORGAN HYPERPLASIA IN A PATIENT WITH GRAVES DISEASE: RESPONSE TO ANTITHYROID DRUG THERAPY Once other serious causes are ruled out, treating the thyroid can bring these changes back to normal.22PubMed Central. PANCYTOPENIA AND LYMPHOID ORGAN HYPERPLASIA IN A PATIENT WITH GRAVES DISEASE: RESPONSE TO ANTITHYROID DRUG THERAPY Weight loss, a racing heart, heat intolerance, and shaky hands suggest an overactive thyroid, and a simple thyroid blood test can confirm it.
Hemophagocytic lymphohistiocytosis (HLH) is a dangerous immune overreaction that causes fever, low counts of one or more blood cell types, and an enlarged liver or spleen.23Dana-Farber Cancer Institute. What Is Childhood Hemophagocytic Lymphohistiocytosis? These features overlap closely with acute leukemia, especially in children. Infections, some cancers including leukemia and lymphoma, autoimmune disease, or inherited immune gene changes can trigger it, so doctors look for a cause once HLH is found. Very high ferritin and abnormal triglycerides, together with marrow findings, help support the diagnosis, and the marrow is also checked for leukemia at the same time.
About 10% of newborns with Down syndrome have a temporary leukemia-like blood condition.24PubMed Central. Treatment and prognostic impact of transient leukemia in neonates with Down syndrome It often clears without treatment, but it can be followed later by a true leukemia in early childhood.25PubMed Central. Transient Abnormal Myelopoiesis and AML in Down Syndrome: an Update Because of the later risk, doctors keep checking counts during the first years. Pale skin, easy bruising, or tiredness should prompt an earlier check.
Recovery Cells That Look Like a Relapse
After each phase of leukemia treatment, normal regenerating young B-cells can look like returning leukemia.26PubMed. Hematogones in acute leukemia during and after therapy When these cells make up more than 10% of the marrow, they can be especially hard to tell from leukemia.27PubMed Central. Significant Haematogone Proliferation Mimicking Relapse in Acute Lymphoblastic Leukaemia on Therapy In one reported case, a child’s marrow six months into chemotherapy looked like relapse but was this healthy regrowth.28PubMed Central. Significant Haematogone Proliferation Mimicking Relapse in Acute Lymphoblastic Leukaemia on Therapy
Chemotherapy and transplant wipe out much of the marrow, and as it rebuilds, young B-cell precursors are among the first cells to reappear in large numbers. They are a healthy sign that the marrow is working again, and their numbers usually settle back down as recovery finishes.
If a marrow result raises relapse worry, the lab tests the cells’ surface markers to see whether they match the original leukemia. Healthy recovery cells show a normal mix of markers. Results often come within days.
How Doctors Tell Leukemia From Its Look-Alikes
Testing usually starts with a repeat count and a blood smear reviewed by a specialist. A specialist doctor or experienced technologist looks at the shape, size, and maturity of individual cells, which can show whether odd cells look reactive or abnormal. Immature cells, unusual granules, or many abnormal cell types help guide which tests come next. The smear is quick and inexpensive, which is why it comes early, though it rarely gives the final answer alone. Simple checks for B12, folate, thyroid, and infection may be added.
Flow cytometry is a lab test that sorts cells by the proteins on their surface. It can show that unusual cells are reactive immune cells fighting a virus rather than leukemia.29PubMed. What Conditions Mimic Acute Myeloid Leukemia (AML)? It can spot protein patterns found on leukemia cells but not in conditions like vitamin deficiency.30PubMed. What Conditions Mimic Acute Myeloid Leukemia (AML)? After chemotherapy or transplant, it can tell normal regenerating young cells from leukemic ones, picking up leukemia cells as rare as 3 in 1,000.31PubMed. Multidimensional flow cytometry of marrow can differentiate leukemic from normal lymphoblasts and myeloblasts after chemotherapy and bone marrow transplantation It can often be run on blood before any biopsy.
If the picture is still unclear, a bone marrow biopsy follows. It removes a small sample from the hip bone with a needle, usually under local numbing. The sample shows how full the marrow is and how many immature cells it holds, and it allows gene tests. Some results return in days, while gene tests can take longer.
Most people describe a brief deep pressure or pulling pain during the procedure. It takes about 20 to 30 minutes, and soreness at the site usually fades within a few days.
Questions to Ask While You Wait for Answers
Tell your doctor about recent infections, fevers, travel, diet, medicines, and thyroid symptoms, since each points toward one of the look-alikes above.
Some questions are worth bringing to the appointment:
- Was the result confirmed on a smear reviewed by a specialist?
- Have B12, folate, thyroid, and common infections been checked?
- Is flow cytometry planned, and is a marrow biopsy needed?
- When will results come, and who will explain them?
- Should I see a hematologist, a blood specialist?
Some symptoms need same-day care while you wait: a high fever, especially with very low white counts; unusual bleeding, such as nosebleeds that won’t stop or blood in the urine; sudden shortness of breath or chest pain; and severe headache or confusion. Writing down questions before appointments helps, and so does bringing someone along to remember what was said.